Download Exploring the Possibility of Using Used Car Batteries as Residential

EXPLORING THE POSSIBILITY OF USING USED
CAR BATTERIES AS RESIDENTIAL ENERGY STORAGE
A Thesis
Presented to the
Faculty of
California State Polytechnic University, Pomona
In Partial Fulfillment
Of the Requirements for the Degree
Master of Science
In
Regenerative Studies
By
Ben S. Yoon
2014
SIGNATURE PAGE
THESIS:
EXPLORING THE POSSIBILITY OF USING
USED CAR BATTERIES AS RESIDENTIAL
ENERGY STORAGE
AUTHOR:
Ben S. Yoon
DATE SUBMITTED:
Spring 2014
College of Environmental Design
Dr. YamLee
_________________________________________
Thesis Committee Chair
Chemical and Materials Engineering Department
Dr. Denise Lawrence
Architecture Department
_________________________________________
Dr. Lin Wu
_________________________________________
Geography and Anthropology Department
ii
ACKNOWLEDGEMENTS
Mom, dad, and my sister Anna, I love you and I respect you. Thanks for being
my inspiration and the drive to continue to live each day in a good spirit. If I can only
live half as hard as you all do each and every day, I know that I will be all that I can be.
The past two years would not have been possible without my loving family.
I would like to extend my gratitude to all my teachers and mentorswho have led
me here. Dr. Landry, thank you for being the only UCSD professor that wrote a letter of
recommendation for me. Your kind words of encouragement meant the world to me.
Retired Master Sergeant Luna and Retired Colonel Rees, thank you for your help as well.
My professors and teachers of Cal Poly Pomona, you were all instrumental in my
pursuit of learning and my Master’s Degree in Regenerative Studies. My most sincere
gratitude goes out to Dr. Brown, Dr. Lawrence, Dr. Kristen-Gomez, Dr. Lee, Dr. Wu, Dr.
Kim, Professor Dong, Don Serio, Dr. Bobich, Dr. LaRoche, and Karen Mitchell. Also, I
would like to thank my MSRS friends for making my time very memorable. Thank you
Gabrielle, Shahrzad, Brother Ryan, Brother Brandon, Cammi, Christine, Aiden, Ariel,
Olga, Maria, Sara and others. Also, thanks to all my dearest friends that I have grown up
with. Thank you Peter, Richard, Hyunwoo, Byungchae, Simon, Hyunsoo, Sangmin,
Jimmy, Seojong, my Godson, Yeonkyung, and others.
Ben. Job well done. Go out and strive to be a better man each and every day.
And thank you Lord, for you have made everything possible.
iii
ABSTRACT
Distributed energy sources, such as solar and wind generated electricity, provide
intermittent, yet “free” and renewable energy. Distributed (or decentralized) energy has
the potential to empower consumers andis likely to spearhead the next energy revolution,
but only if the use of renewable energy can become more reliable, regardless of weather
conditions. Recognizing the importance of renewable energy, including energy storage,
the state of California recently set mandates on the expansion of energy storage to 1.3
gigawatts by 2020. One common form of energy storage, car batteries,are manufactured
in the tens of millions each month, and as much as 98% are recycled for lead and
plastic. These lead-acid batteries (LAB) have beenoverlooked as potential storage
devices for home energy because they are specifically designed as automobile starter
batteries. However, with so many units in circulation, used automobile LAB are an
easily accessible option that deserve to be tested for their energy storage capacity and
residual economic value.
In this study, the storage capacity and function of usedautomobile LABs with
detectable voltages were testedto check:
1)theability to be trickle-charged via 110-volt wall outlet;
2) levelof capacity by multiple cycling test under load;
3) level of functionality as potential energy storage system (ESS) in a solar
powered house; and
4)level of efficiency by measuring energy input and output.
At the beginning of the experiment, the three used batteries each had
approximately 50%, 50%, and 25% remaining capacity. All three functioned well in the
recharge and discharge cycle when they were installed as energy storage devices of the
iv
TJ house PV power system. Due to the limitation of the equipment, no power intensive
loads could be tested, but these batteries were able to supply 4 to 5 hours of electricity to
loads consisting of two lights and a fan before completely discharging.
v
TABLE OF CONTENTS
Signature Page .............................................................................................................. ii
Acknowledgements ...................................................................................................... iii
Abstract ......................................................................................................................... iv
List of Tables ................................................................................................................ ix
List of Figures ............................................................................................................... xi
1. Introduction .............................................................................................................
1
1.1.Problem Statement ......................................................................................
1
1.2. Hypothesis and Research Objectives .........................................................
2
1.3. Energy Storage in Regenerative Studies ....................................................
3
2. Literature Review......................................................................................................
7
2.1. Lead-Acid Battery Characteristics .............................................................
7
2.1.1. Chemical Reaction ......................................................................
7
2.1.2. Battery Power and Capacity........................................................
9
2.1.3. Loss of Power and Capacity ....................................................... 10
2.1.4. Aging Factors .............................................................................. 12
2.1.5. Temperature ................................................................................ 13
2.1.6. Discharging of Battery ................................................................ 14
2.1.6.1 Discharge rate of lead-acid battery ................................. 15
2.1.6.2 Depth of discharge and life cycle.................................... 16
2.2. Case Studies ............................................................................................... 17
2.2.1. China: Remote Area Equipped with PV Generation equipped with
lead-acid batteries ................................................................................. 18
vi
2.2.2. Zambia. Application of lead-acid batteries in a developing African
nation..................................................................................................... 19
2.2.3. Battery behavior prediction and battery working states analysis of a
hybrid solar-wind power generation in China ...................................... 20
2.2.4. “My Modest Solar Setup”by Prof. T. Murphy of UCSD............ 21
2.2.5. Field performance of lead-acid batteries in Photovoltaic Rural
Electrification Kit.................................................................................. 25
2.2.6. On the storage batteries used in solar electric power systems and
development of an algorithm for determining their ampere-hour capacity
...........................................................................................................27
3. Methodology ............................................................................................................. 30
3.1. Recharge and Voltage Test ........................................................................ 30
3.2. Specific Gravity Test ................................................................................. 32
3.3. Load Test ................................................................................................... 33
3.4. Mahmoud’s Algorithm for calculating Amp-Hour Capacity of Batteries . 34
3.5. Field Test: The TJ House Photovoltaic Power System .............................. 36
3.6. Equipment .................................................................................................. 40
3.6.1. Batteries ...................................................................................... 40
3.6.2. Hydrometer and Specific Gravity ............................................... 43
3.6.3. Onset Hobo Data Logger ............................................................ 43
3.6.4. 12V AC/DC Trickle Charger ...................................................... 43
3.6.5. Xantrex C40 Inverter .................................................................. 43
4. Resultsand Analysis .................................................................................................. 45
4.1. Recharge and Voltage Test ........................................................................ 45
4.2. Initial Load Test ......................................................................................... 47
vii
4.3. Specific Gravity ......................................................................................... 49
4.4. Amp-Hour Capacity Algorithm ................................................................. 52
4.5. TJ Power System Test................................................................................ 56
4.5.1. The ValuCraft Battery Load test ................................................. 59
4.5.2. The Battery Bank Load Test ....................................................... 60
4.6. Efficiency: Input Energy vs Output Energy............................................... 62
5. Conclusion ............................................................................................................... 66
5.1. Limitations ................................................................................................. 69
6. References ................................................................................................................. 71
viii
LIST OF TABLES
Table 1
Peukert Table for 10Ah Lead-Acid Battery ................................................ 16
Table 2
Battery Rating Factors and Corresponding Attributes ............................... 26
Table 3
Measured and Calculated Parameters of a Lead-Acid Battery at 12V/110 Ah
..................................................................................................................... 28
Table 4
State of Change as Related to Specific Gravity and Open Circuit Voltage
..................................................................................................................... 31
Table 5
Battery BCI Group and Size Specifications ............................................... 42
Table 6
Battery BCI Group and Performance Specifications ................................. 43
Table 7
Voltage Test Result: 2 Amps Recharge ..................................................... 46
Table 8
Voltage Test Result: 10 Amps Recharge ................................................... 47
Table 9
Voltage And SOC of Four Batteries .......................................................... 47
Table 10 A Load Test with Trickle Charger ............................................................. 49
Table 11 Specific Gravity and State Of Charge of Fully Charged Batteries ............ 50
Table 12 Specific Gravity of Everstart Battery ......................................................... 51
Table 13 Valucraft Amp-Hour Calculations ............................................................. 53
Table 14 Kirkland Amp-Hour Calculations .............................................................. 54
Table 15 Econopower Amp-Hour Calculations ......................................................... 54
Table 16 Range Of Recharge for 3 Batteries Based on The Algorithm’s Calculation
..................................................................................................................... 55
Table 17 Maximum Amp-Hour Capacity of Batteries .............................................. 57
Table 18 Comparison of Amp-Hour Capacity between Charge Controller, Algorithm,
and Range ................................................................................................. 58
Table 19 Amp-Hour Energy Input Vs Watt-Hour Energy Output of 3 Batteries ..... 59
Table 20 Valucraft Battery Input and Output Energy ................................................ 60
ix
Table 21 Econopower and Kirkland Battery Bank Input and Output Energy .......... 61
Table 22 System Efficiency Comparing Solar Input to Battery Output in Watt-Hour
..................................................................................................................... 65
x
LIST OF FIGURES
Figure 1
John T. Lyle’s storage in regenerative model .............................................
4
Figure 2
John T. Lyle’s regenerative system ............................................................
5
Figure 3
Charging of lead-acid battery .....................................................................
8
Figure 4
Discharging of lead-acid battery .................................................................
9
Figure 5
Starter battery (left) and deep cycle battery (right) ..................................... 11
Figure 6
Lead-acid battery capacity loss ................................................................... 11
Figure 7
Resistance buildup of a lead-acid battery ................................................... 12
Figure 8
Temperature effect on lead-acid battery life ............................................... 13
Figure 9
Temperature effects on lead-acid battery discharge ................................... 14
Figure 10 Typical discharge of lead-acid battery ........................................................ 14
Figure 11 Discharge rate effect on discharge performance ......................................... 15
Figure 12 Discharge rate characteristics: cell voltage vs discharge time .................... 16
Figure 13 Cycle life of sealed lead-acid battery .......................................................... 17
Figure 14 Wiring diagram of Dr. Murphy’s system.................................................... 22
Figure 15 Single line diagram of the Pentametric monitoring system placement....... 24
Figure 16 Energy use profile in kWh .......................................................................... 23
Figure 17 Recharge voltage of the EconoPower battery ............................................. 35
Figure 18 TJ House and photovoltaic array of five modules (in yellow).................... 37
Figure 19 Wiring diagram of the TJ House photovoltaic power system..................... 37
Figure 20 Three phase LAB recharge phases .............................................................. 40
Figure 21 Average Specific Gravity and SOC for fully charged battery .................... 50
Figure 22 Average full SOC for each battery according to the algorithm .................. 56
xi
Figure 23 ValuCraft power vs capacity graph............................................................. 60
Figure 24 Decline of input and output capacities ........................................................ 62
xii
1. Introduction
1.1. Problem Statement
The energy storage system (ESS) is an essential component of a photovoltaic (PV)
powered electrical system because of the PV power system’s fundamental energy generation
process. Sunlight powers PV systems to generate electricity but no generation occurs at night or
under cloud cover, when the sun is not accessible. When the sun is shining, the amount of power
generation depends proportionally on the amount of sunlight hitting the PV modules. Electricity
harvested by PV modules may not be completely used by the load at the time of production, in
which case extra electricity can be stored for later use by incorporating an ESS. The most
common form of energy storage is the battery. Depending on the purpose and size of the storage
need, several other forms of energy storage may be utilized such as pumped hydro, molten salt,
flywheels, etc. With the advent of distributed renewable energy generation and electric vehicles
(EV), energy storage use – including the battery –is far more prevalent, and energy storage
technology has become the subject of much new research and development.
The lead-acid battery (LAB) is a form of energy storage invented in the 18th century. It
is a very old battery type still being used today due to its low cost and well-established
manufacturing infrastructure. LAB is also a popular choice for the ESS in a PV system.
The LAB can be designed to achieve either high power or high capacity, but not both; one must
be sacrificed for the other, depending on the purpose. Among the various options, startinglighting-ignition (SLI) and deep-cycle batteries are the two most popular types of LAB. SLI
batteries are installed in automobiles to turn over the engine to power the ignition system. SLI
batteries are designed to sustain the supply of a high amount of current (+ 200 amps) in few
seconds and to complement the alternator in supplying power to onboard electrical loads (lights,
air conditioner, navigation, radio, etc.) once the engine is powering the vehicle. SLI batteries last
1
for several years under light discharge and charge conditions but cannot withstand deep discharge
and recharge cycles.
SLI batteries have been overlooked as residential energy storage due to the presence of
deep-discharge batteries, which are designed to provide moderate flow of electrical current under
higher depth-of-discharge for longer service life. Literature and information on the performance
of deep-discharge batteries as residential energy are already abundant. However, in many locales,
people need other optionsfor home energy storage solution, because they lack a dependable
energy supply and thus produce their own power using PV modules.Common car batteries,
instead of expensive and inaccessible deep-discharge batteries, are a much more attainable and
affordable option in these situations. A detailed and quantified data analysis on the energy
potential of used car batteries can be a useful guide for people in need of such knowledge,
especially in developing nations.
1.2 Hypothesis and Research Objectives
The purpose of this research is to investigate the potential performance of used
automotive batteries as a home energy storage device. The hypothesis being tested states that a
battery bank of multiple used-car batteries will be able to provide enough power and energy to
cook food, cool and heat a small space, and provide light. This type of low-cost and recycled
energy storage paired with solar panels may be enough to improve the quality to life in energy
stricken areas, such as rural regions of emerging countries. Three lead-acid car batteries were
tested to see how much energy each battery is able to store and dispense as part of a residential
PV power system. Prior to installing the batteries to the PV power system, a series of tests were
conducted to evaluate the state of the used batteries. The first test assessed whether the used
batteries were in an operational state. The batteries were recharged using a 12V car battery
trickle charger for one to two hours at two different recharge rates, 2 amps and 10 amps, then the
2
initial and final voltages were compared. A positive voltage change indicates that the batteries
are able to be recharged. The second test measured the sulfuric acid concentration, or specific
gravity, of the electrolyte in the batteries’ six cells. This test complements the voltage test
because voltage alone is insufficient to measure the battery’s state-of-charge (SOC). The
measured voltage and specific gravity data were used to calculate amp-hour capacities of the
batteries by applying the algorithm developed by Professor Mahmoud (Mahmoud, 2004).
After initial tests,the batteries were connected to the PV power system of the TJ House at
the Lyle Center for Regenerative Studies to test whether the car batteries would function properly
as a battery energy storage system. Input electrical energy produced from the PV array was
measured by the charge controller of the PV system and the energy output was measured by
applying electrical loads to the battery bank. This setup allowed the calculation of the total
system efficiency by incorporating the energy from solar irradiance, PV module efficiency,
wiring efficiency, battery efficiency, inverter efficiency, and output energy in watt-hours.
The outcome of this experiment can serve as a valuable tool for those who wish –out of
necessity or curiosity— to test and utilize common car batteries as a battery energy storage
system.
1.3. Energy Storage in Regenerative Studies
Energy storage takes many different forms: Food, running water, natural gas, battery,
and petroleum are all stored forms of energy. Storage is the also a very fundamental concept of
Regenerative Studies - “Storage lies also at the core of regenerative systems. Maintaining
adequate storage and balancing the rate of replenishment with the rate of use are important keys
to sustainability. Since rates of productivity, assimilation and use all vary, storage is the essential,
every-varying maintainer of equilibrium.” (Lyle 43)
3
Figure 1.John T. Lyle storage in regenerative model. (Lyle, 1994)
The sun is the ultimate source of all energy on Earth, and without it, there would be no
energy, and therefore no life. Energy storage is just as important as energy itself, and they both
exist in countless shapes and forms. The human race evolved around managing energy and
resources for its survival. Lyle also emphasized the importance of energy storage: “Also of
particular relevance here is the ninth strategy-managing storage for sustainability. This is
especially hard to apply to energy systems because energy use is notoriously sporadic, rising and
falling through the day and from season to season. At the same time, partly because it does not
have material substance, energy is inherently elusive in character and thus difficult to store.”
(Lyle 56)
4
Figure 2.John T. Lyle’s regenerative system. (Lyle, 1994)
As is common knowledge, the photosynthesis process in plants converts solar energy to
energy usable by plants for survival and growth. Plant biomass is the main type of stored energy
produced in this process. Humans have utilized this stored energy by burning plants, most
commonly in the form of wood, to convert biomass into thermal energy.Stored energy is also
directly consumed in the form of grains, fruits, and vegetables. Furthermore, dead biomass
processed naturally over millions of years has turned into stored energy in the form of as fossil
fuel. Today, the most common forms of fossil fuel used by people are petroleum, coal, and
natural gas, which are utilized heavily to fuel the modern industrial society.
For more immediate creation of energy, conversion devices like photovoltaic modules
process solar radiance to convert sunlight to electricity. Once solar radiation is converted to
electricity, a battery is themedium used to store the newly produced electricity. The importance
of energy storage has been heightened with the explosive growth of non-centralized, distributed
renewable energy production systems that are powered by sporadic weather events such as wind
and solar radiance. A more economical method of residential scale battery energy storage is the
lead-acid battery (LAB). The life cycle of a LAB is regenerative to minimize the financial and
environmental cost of its use. Materials used in LABs are recognized as hazardous, and are
5
actively recycled for reuse. This study focuses on automotive LABs because of their ubiquitous
nature, which makes it an easily accessible storage source. This study investigates the energy
potential of the very commonautomotive batteries by conducting a series of experiments to
determine the batteries’ potential as storage device of residential PV power systems. This study
also seeks to discover the possibility of extending the service life of LABs by upcycling used
batteries into a home energy storage option,once their utilization in automobiles is exhausted.
6
2. Literature review
This literature review covers the fundamentals of LAB technology and its uses in modern
energy storage systems via six case studies. Since 1859, lead-acid batteries have been usedwith
certain levels of reliability, but several factors are known to affect their efficacy and longevity.
The LAB technology section focuses on the basic electrochemical reactions that take place in
batteries: capacities, recharging and discharging, voltages, electric currents, battery aging, and
other key factors that affect itsuse. Then, the application of LABs as an energy storage system
for PV and/or wind power generation systems is discussed. These cases studies provide a
comprehensive technical review of LABs, including mathematical formulas, test equipment,
battery specifications, and situational problem solving scenarios that demonstrate the interactions
of batteries, PV modules and electrical components. Understanding the background of PV
electrical power systems that consist of batteries and electrical components helps to frame the
approach that is necessary to analyze the complex nature of an electrical power system.
Familiarization and comprehension of these methodologies are essential in designing and
conducting experiments that involve live electrical currents.
2.1. Lead-Acid Battery Characteristics
2.1.1. Chemical reaction. Lead-acid battery is an electrochemical device that operates
bya chemical reduction-oxidation (redox) reaction. Two dissimilar metals immersed in an
electrolyte react with the electrolyte to produce voltage. The chemical reaction formula of LAB
is:
Pb(s) + PbO2(s) + 2H2SO4(aq)  2PbSO4(s) + 2H2O(l).
The left side is the fully charged condition and the right side is fully discharged condition. This is
a reversible reduction-oxidation equation so both sides can be reactants and products, depending
on whether the battery is charging or discharging. In a fully charged state, the lead plate (Pb) and
7
lead oxide plate (PbO2) react with the sulfuric acid. The product of the reversible reaction is lead
sulfate and water. Over the discharge cycle, lead and lead oxide plates slowly turn into two lead
sulfate plates, with water as the byproduct. At the end of discharge, the sulfuric acid turns almost
completely into water and two lead sulfate plates. When charging takes place, electricity is
introduced into the system and the reaction reverses to the previous state--lead plate, lead oxide
plate and sulfuric acid electrolyte. The electrolyte is the one of the main reactant and the state of
the sulfuric acid plays a key role in the battery performance (Averill, 2011).
Figure 3.Charging of Lead-Acid Battery. (Delpierre& Sewell, 2006)
Figure 4.Discharging of Lead-Acid Battery. (Delpierre& Sewell, 2006)
8
A LAB is made up of multiple cells, which are connected and packaged in a hard plastic
case that seals and protects the internal components.Each cell produces 2.1 volts of electricity,
regardless of size. Six cells are serially connected to form a 12.6 volt car battery, while certain 6
volt deep-discharge batteries are comprised of three cells. A volt is the measurement of potential
difference between two electrodes and represents the ability to perform work.A higher voltage
means a higher capacity to perform work. Each cell consists of a cathode (positive), and an
anode (negative) electrode immersed in a liquid electrolyte. For a cell in a car battery, the
positive electrodes are thin plates of lead dioxide (PbO2) and the negative electrodes are thin
plates of lead (Pb)--typically lead sponge. The electrolyte that both electrodes are immersed is a
mixture of water (H2O) and sulfuric acid (H2SO4). The capacity of the battery depends on the
amount of surface area of the electrodes and does not affect the voltage. Between each cathode
and anode is a porous separator that is placed to prevent cathode and anode from contact (Fasih,
2006).
Figure 5.Starter battery (left) and deep cycle battery (right). (Buchmann, 2011)
2.1.2. Battery power and capacity.There are twomain barometers for measuring battery
performance: capacity and power. As mentioned earlier, capacity depends on the surface area of
the electrodes. A battery’s capacity is the measure of how much energy it can deliver (or hold)
under specific conditions, and is represented in amp-hours (Ah) or reserve capacity (RC)minutes
in 25A rate discharge. 100 RC minutes means that the battery can supply 25 amperes rate of
9
electric current for 100 minutes. Batteries sold in Europe are rated by battery capacity such as 45
amp-hours (Ah), while in the US the capacity is stated as RC minutes. Ampere (amp) is the
measurement of the electrical current that a battery provides and Ah is electrical current
multiplied by time in hour. A 45Ah battery should deliver 45 amps for one hour or one amp for
45 hours.
The second barometer of battery performance is power, the ability to provide electrical
current in amps. The most important attribute of a car battery is the ability to provide enough
electrical current, or power, to crank a heavy engine. The cranking amp and the cold cranking
amp are clearly marked on the batteries usually in the range of 500 to 1,000 amps. Heavier
engines of larger vehicles require larger batteries with more power output. The most common car
batteries are all 12-V, but the output power differs depending on the size of the battery.
2.1.3. Loss of power and capacity. Batteries experience aging by losing capacity and
power. Loss of capacity occurs when a rechargeable battery loses the ability to supply energy for
the required duration of time. Power loss occurs when the battery cannot supply enough energy
to perform a task. Numerous reasons are accountable for loss of capacities and power. To
explain the relationship between aging and power loss, the battery can be compared to a water
tank. There are three elements of the water tank that are comparable to the battery. Thefirst is
empty space, which is akin to rechargeable capacity. The second element is water, which is like
the available energy inside the battery. The third element is a crystal or rock, which represents
the portion of energy that has transformed into unusable mass from degradation of the active
material. To clarify, if a rock crystal starts to grow in a large tank that can store a certain quantity
of water, the available storage capacity for water will decrease as the crystal grows.
10
Figure 6.Lead-acid battery capacity loss.(http://batteryuniversity.com/learn/article/capacity loss)
Another reason for capacity loss is the self-discharge rate. While self-discharge is an
inherent battery characteristic, an elevated rate of self-discharge will cause a fully charged battery
to discharge quickly under no-load conditions and result in an unusable battery. Returning to the
water tank analogy, self-discharge can be compared to as holes in the bottom of the water tank
that lead to water loss without a specific purpose. As continuous outflow of water through these
holes result in less available water, similarly the battery will lose its electric charge.
The analogy of the water tank needs the addition of a faucet to explain the loss of power
in a battery. The intensity of flowing water is dependent on the resistance, which is determined
the size of the faucet pipe. A clogged faucet has high resistance and will not deliver enough
power compared to an unclogged, open faucet that can deliver high power.
11
Figure 7.Resistance buildup of a lead-acid battery.
(http://batteryuniversity.com/learn/article/rising_internal_resistance)
2.1.4. Aging factors. Batteryperformance gradually deteriorates with age and use,
eventually leading to the end of the battery’s service life. Major factors of aging include: anodic
corrosion of grids, plate-lugs, straps or posts, positive active mass degradation, irreversible
formation of lead sulfate in the active mass, short-circuits, and loss of water. According to
Ruetschi (2006), “Aging mechanisms are often inter-dependent. For example,irreversible
formation of lead sulfate in the activemass is usually the result of insufficient charge. The
lattermay arise from excessively high acid concentration, due toloss of water; but it could also be
the result of short-circuits.The latter, in turn, may result from positive active massdegradation.”
The grid in a battery anchors and holds the active mass plates. Corrosion of the positive
plate, grids, plate-lugs, and any combination thereof are probably the most common reasons for
the failure of a SLI battery. Positive active mass degradation causes the loss of contact between
the plates and grids. Sulfation is the permanent buildup of lead sulfate that does not revert back
to lead, and this causes corresponding loss of capacity. These phenomena can occur in prolonged
discharged state where the battery is either not charged frequently enough, or charged
12
insufficiently. In order to avoid sulfation, the battery should be fully recharged occasionally.
Besides sulfation, prolonged discharges can cause short-circuits across the separators that
separate positive and negative plates. The last major factor in battery failure is the loss of water
caused by gassing, which happens when the batteries are excessively charged. (Ruetschi, 2006)
2.1.5. Temperature. The operating temperature of the battery is a factor in the
performance of LABs; changes in temperature are positively correlated with performance
measures. Thus,when the operating temperature decreases, the expected service life of the battery
decreases accordingly.
Figure 8.Temperature effect on lead-acid battery life (Hansen, 1999).
The temperature also has a profound impact on the LAB’s voltage. As the operating or
environmental temperature decreases, the voltage curve experiences a drop. That means the
battery operates under lower voltage and the duration of discharge is decreased. The components
of the LAB operate with more efficiency at a higher temperature, because cold temperature slows
down the chemical reaction inside the battery.
13
Figure 9. Temperature effects on lead-acid battery discharge (Pascoe, 2004).
2.1.6 Discharging of battery. As the battery dispenses its energy, the voltage decreases
but not in a linear fashion. At the very beginning of a discharge, there is a slight voltage drop
followed by a recovery and continuous linear discharge, and eventually an accelerated voltage
drop as the battery nears depletion. The initial drop of voltage is called the coup de fouet phase.
The battery voltage quickly recovers to proceed with natural and gradual voltage drop. Towards
the end of the capacity of a battery, however, the gradual voltage drop accelerates to more drastic
voltage drop curve.
Figure 10.Typical discharge of lead-acid battery (Pascoe, 2004).
14
2.1.6.1 Discharge rate of lead-acid batteries. The rate of discharge, defined as how
much energy is drawn at a specific time, is another influential aspect of voltage and
capacity. The voltage and rate of discharge are not linearly related. A battery is drained at a
higher rate when more charge is drawn from it, meaning that capacity loss is accelerated when the
discharge rate increases. A 100Ah battery lasts 100 hours when 1 amp is drawn from it. When
50 amps are drawn, the battery should hypothetically last two hours, but it drains faster and does
not last two hours.
Figure 11. Discharge rate effect on discharge performance (Pascoe, 2004).
The non-linear discharging characteristic is known as the Peukert Effect. Peukert
Equation is Q = K * I1-n , where Q is capacity, I is current, K is battery constant, and n is
discharge rate sensitivity exponent. The table below illustrates the effect of Peukert Equation
with varying discharge rate.
15
Table 1.
Peukert Table for 10Ah Lead-Acid Battery (Picciano. 2007)
Discharge Current (A)
C-Rate
Discharge Time
Voltage @ End of Disharge
0.5
0.05C
20h
1.75V/cell
0.1
0.1C
10h
1.75V/cell
2
0.2C
5h
1.70V/cell
2.8
0.28C
3h
1.64V/cell
6
0.6C
1h
1.55V/cell
10
1C
0.5h
1.40V/cell
1C rate is the discharge rate of a complete discharge of a battery in one hour. Mathematically, 10
amp discharge of 10 Ah battery should render 1 hour of discharge time, but it only lasts 0.5
hour. The graph below illustrates the relationship between voltage and discharge time.
Figure 12.Discharge rate characteristics: cell voltage vs discharge time (Buchmann, 2001).
16
2.1.6.2. Depth of Discharge and Life Cycle. Depth of discharge (DOD) is the indicator
of how much energy is expended from the battery’s full energy potential. 100% DOD means the
battery is fully depleted and its state of charge is 0%. The number of cycles in a battery’s useful
life depends heavily on the level of each discharge. At the depth of 30% discharge, the number of
cycles hovers around 1000 to 1200, but at 50% DOD it drops to about 500 cycles. If the battery
is fully discharged each cycle, it only lasts about 200 cycles.
Figure 13.Cycle life of sealed lead-acid battery (Panasonic, n.d.).
2.2. Case Studies
Energy storage systems (ESS) are essential in renewable energy generation systems for
three main reasons: 1) to act as a power buffer 2)to store energy during times of production and 3)
to release stored energy in times of little or no production. ESS is especially indispensable in a
non-grid connected, remote PV power system. The PV system does not provide constant flow of
power nor constant voltage due to its dependence on solar insolation. By having an ESS
connected to the PV system, it provides a source of constant voltage source to act as a power
17
buffer between the PV generator and the load (Mahmoud 2003). When the energy output from
PV modules exceeds the load, the excess energy charges the ESS. Conversely, when the load
begins to exceed the energy generated from PV modules, the ESS can fulfill part of the energy
demand or act as sole provider of energy, depending on the situation.
Among many energy storage technologies, lead-acid batteries are a popular form of
energy storage for their affordable cost, low maintenance, mature technology, and well
established distribution system. The following four discourses discuss the application of deep
cycle lead-acid batteries as energy storage systems in various settings.
2.2.1. NW China: Remote area PV generation equipped with lead-acid batteries
(Hua, Zhou, Kong, & Ma, 2006). TheChinese government initiated an energy project named
“The Brightness Project of China” to mitigate energy problems for the 23 million people residing
in remote areas without access to a grid. Most of China’s PV stations built through this program
are located in the remote northwestern region, and the majority of them are stand-alone PV
systems. The environmental conditions of northwestern China include: high elevation, large
diurnal and nocturnal temperature difference, diminished atmospheric pressure, significant
seismic activity, and difficulty of access. The requirement for batteries used in this area are: long
life, wide operational temperature range, low self-discharge rate, good sealing to prevent water
and gas loss, and the ability to withstand upto 7 Richter magnitude scale earthquakes. The
remoteness of the area makes timely and proper maintenance of the batteries difficult, which can
lead to underperformance from technical problems such as operating withpartial charge and
overcharging. The batteries enjoy longer service lives when properly maintained, but the
conditions in northwestern China make it difficult to provide routine checkup.
The methods to test batteries fit for use in northwestern China are divided into three
parts: A) IEC 61427 Standard Cycling Test, B) Chinese National Standard Test, and C) Test of
cycling at low discharge rate. The IEC 61427 Standard Cycling Test tests one battery at the low
18
charge state of 20% and another at a charge of 80% to replicate conditions of seasonal
variability. Fifty cycles of 30% DOD are carried out between 5 and 35% SOC (70% charged
batteries to 5% ~ 35%) and then 100 cycles between 75% ~ 100% SOC. The Chinese National
Standard Test tests the cycling durability of the battery. The battery bank is subject to 0.2C10
discharge and 0.2C10 charge cycle for 50 cycles. The test result shows that GMFU VRLA
batteries with thick positive plates have a long life at high current density cycling. The final test
of ‘Test of cycling at low discharge rate’ subjects batteries to many cycles or recharging and
discharging under very low discharge rate.
2.2.2. Lead-acid battery capacity in solar home systems—Field tests and experiences
in Lundazi, Zambia (Gustavsson&Mtonga, 2005). 150Homes in Lundazi, a neighborhood in
Zambia, were selected to be a test bed for PV and energy storage solutions for application of leadacid battery storage systems. A group of thirteen lead-acid batteries designed for PV power
system was tested to analyze their deterioration rate. All thirteen batteries had been in use for one
year in the homes of Lundazi. The battery owners wanted to keep the batteries operating as long
as possible for cost saving purposes, so the paper discussed battery management strategies for
maximizing battery life. The batteries, called First National RR2, are manufactured in Africa and
were selected from 150 homes equipped with PV panels and solar-type batteries. The depth-ofdischarge (DOD) of 80% is equivalent to 250 cycles with nominal capacity of 96 Ah. The
batteries’ voltages were in the range of 12.12 - 12.17 V, indicating a discharge level of
60%DOD. According to the C20 discharge curve, these batteries should have approximate 70 Ah
as a reserve capacity.
Another parameter to gauge the capacity of these batteries was measuring the specific
gravity of the sulfuric acid, although this metric provides only a rough estimate of the battery’s
available energy. After conducting controlled discharge test, these one year old batteries only had
about 34 Ah compared to 61 Ah of new batteries that were tested as the control group. The July
19
2002 records show that, 45% of the batteries only had 50% SOC, with 75% having a SOC level
below 45%. Only 5% of the batteries had SOC of 95%. These results indicate the batteries were
not recharged sufficiently, for undercharging causes faster battery deterioration. The second
measurements rendered better results than the first set, because fully recharging the batteries
actually restores the health and capacity of the batteries. One of the reasons these batteries were
undercharged was having inadequate power for recharging. Although adding more PV modules
to generate energy is difficult, the residents gradually purchased appliances to increase the
usage. The initial PV systems were designed to allow only a slightly higher charging capacity
than daily usage. Thus, increased usage did not allow enough charge to fully recharge the
batteries in times of low usage.
Another reason for the undercharging is the setting of the charge controller. The charge
controller is designed to disconnect power when the battery voltage dips to 11.7V. Since the fully
charged battery voltage is 12.8, this setting only allows for a small window of 1.1V for keeping
the power connected. The reconnect reengages the system when 0.5V is recovered after 1 to 2
hours of recharging. Also, when the load is cut, the cells have time to balance the potentials
without external recharging, and the system reconnects again. The new batteries of the control
group were discharged without any interruptions and exhibited better performance. These
disconnect and connect combinations without fully recharging is unhealthy for the lifetime of the
batteries. Compared to a new battery, these one year old batteries lost a substantial portion of
their capacity. To better manage these batteries, it’s important to widen the 1.1V and 0.5V
windows to allow more charging after the disconnect due to low voltages. The company
managing the batteries was not properly equipped to perform this task.
2.2.3. Battery behavior prediction and battery working states analysis of a hybrid
solar-wind power generating system(Zhou, Yang, & Fang, 2006). This article discusses a
hybrid solar-wind project equipped with lead-acid battery storage system. Most simulations of
20
lead-acid battery storage system functions are carried out and validated in a laboratory
environment, but lead-acid storage systems in real world situations are subject to penalizing
operating conditions. The recharge is highly dependent on weather conditions due to the
intermittent nature of solar and wind power generation. The state-of-charge (SOC) model of this
paper is based on ampere hour counting method to simulate the lead-acid battery SOC
behaviors.
GFM-1000 lead-acid batteries with a capacity of 1000Ah, rated at 10hr discharge time
were used for the experiment. These batteries are deep-discharge batteries used for renewable
energy applications. The modeling process concentrates on calculating battery voltage, current,
and SOC for all the concerned time periods by measuring the voltage and current. Using
measured voltage and current, the original SOC is calculated. Using series of equations, the
intended voltage, current, and SOC for concerned time are also calculated.
The experiment set up for the solar-wind hybrid system consists of a load of 1500 watts,
PV array capable of producing 7.8 kW, wind turbine capable of producing 12 kW and battery
bank of 5000 Ah capacity. An important result of this experiment is the battery component
efficiency. The input of average annual energy was 0.82 kW and the output was 0.65 kW, which
is equal to 79% efficiency.
2.2.4. University of California, San Diego Physics Professor: “My Modest Solar
Setup” and “Blow-by-Blow PV System Efficiency.”(Murphy, 2012).Tom Murphy, a physics
professor at University of California, San Diego, utilizes a modest solar system that supplies
power to his house. The system is comprised of four lead-acid batteries connected to a 1040 Watt
PV power system. The battery bank consists of four Trojan T-1275 golf-cart batteries (12-volt,
150 amp-hour), which equals 7,200 watts. The four batteries are connected in series and in
parallel. Two batteries are connected in series first, and then in parallel (2x2 series/parallel
21
arrangement). This setup doubles the voltage from 12-v to 24-v and doubles the capacity by
bundling them in parallel.
Figure 14.Wiring diagram of Dr. Murphy’s system (Murphy, 2012)
The above diagram illustrates the grid-tied PV system. The electricity generated by PV
modules are fed to the charge controller. The charge controller regulates the electricity input
stream and performs two functions: charging the battery bank and directly powering the DC/AC
inverter to convert direct-current (DC) electricity to alternating-current (AC) for AC appliances.
The Pentametric monitoring system monitors two voltages and three currents of the entire
electrical system. The PV modules and batteries provide 60% of the household electricity
needs. The electrical load consists of a television, two laptop computers, attic fan, entertainment
components, cable modem, wireless hub, and a printer. Below is the diagram showing the
placements where voltages and currents are measured.
22
Figure 15.Single line diagram of Pentametric monitoring system placement (Murphy, 2012)
Figure: VA: the battery bank voltage, across the 2×2 series/parallel arrangement of 12 V golf-cart
batteries; VB: a mid-point voltage on one of the two battery chains, of secondary value; IC: the
current supplied by the charge controller into the rest of the system; ID: the net current into/out-of
the battery bank; IE: the net current through a single parallel chain of the battery bank. At night,
only the battery feeds the load and the current measured at Point D is all going to the load. In the
daytime, the battery bank may or may not receive charge from the PV modules depending on
whether the load exceeds the solar input.
Figure 16.Energy use profile in kWh (Murphy, 2012)
23
The PV modules perform at 16% efficiency, meaning the modules convert 16% of radiant
solar energy into electricity. Approximately 2% of this energy is lost in the delivery rates(de-rate)
of wires. The most important metric to consider is the amount of energy delivered to the AC
outlet. All energy produced by the PV modules is routed to DC/AC inverter before it is delivered
to the AC outlet. The inverter needs base power as well as other system components, in order to
operate. A battery is a net consumer of energy, even though it stores and dispenses
energy. Besides the inverter and battery, other system components such as wires and monitoring
system also consume energy to operate. Thus, before the solar-generated power is delivered to
the AC outlet, there are several components that consume energy on the way. Hence the power at
inverter equals to power from charger controller minus the power consumed by battery minus
power of system minus base power needed for the inverter. Power at charge controller equals to
solar power multiplied by the charge controller efficiency. The total system efficiency is equal to
Power delivered to the AC outlet divided by radiant solar power. Over the 30 months of data
collection, Tom Murphy’s system had total efficiency of 62.2%. During this period, the system
had input of 4.3 kWh and eventually delivered 2.7 kWh into the house.
Battery efficiency can be measured by comparing energy in and energy out of the
battery. If only the current is measured, current multiplied by time will equal amp-hour. Charge
efficiency of a battery can be calculated by measuring the amp-hour in and dividing it by the
amp-hour out. For the Trojan batteries used in this system, the charge efficiency is 92%--for
every 100 Ah recharged, 92 Ah is discharged. Energy efficiency is different since the charging
voltage is higher than discharging voltage. Putting 1 amp-hour into a battery at 27 volts is 27
Wh. When discharging 1 amp-hour of electricity, the discharge occurs at 24 volts so it only
delivers 24 Wh of energy. That’s 88% energy efficiency of the Trojan battery.
24
2.2.5. Field Performance of Lead-Acid Batteries in Photovoltaic Rural
Electrification Kit. (Huacuz, Flores, Agredano, &Munguia, 1994) A field survey was
conducted in 83 communities across seven states in Mexico to monitor the performances of rural
PV electrification kits and to follow up on performance satisfaction levels of the systems over an
18 month period. 555 samples were taken from 2512 PV electrification systems, and car batteries
represent 87% of the samples. A semi quantitative method developed by Electrical Research
Institute of Mexico was used to test the batteries. The seven states are divided into two climate
regions, hot-dry and temperate. These rural electrification kits are designed for either residential
or communal use. A residential system is made of a 51W PV module, a 20-amp max charge
controller, a lighting load of around 60 watts, and a lead-acid battery of between 50 to 104 amphour capacity. A communal system has up to six PV modules connected in parallel, a 200 amphour capacity battery bank, a charge controller, and inverter to feed AC loads. A typical load
profile for residential system is one hour before sunrise and three to four hours peak demand,
beginning around sunset. The load profiles for communal systems are not well established.
The batteries were visually inspected for following conditions: condition of the plastic
case, missing caps for open batteries, electrolyte levels, electrolyte leaks, broken or damaged
electrical terminals, corrosion at the terminals or connectors, and excessive boiling of electrolyte.
Two quantitative measures taken during inspections were electrolyte density (specific gravity)
and voltage. After all the inspections have been completed, the batteries were divided into three
categories: good, fair, and bad.
25
Table 2
Battery Rating Factors and Corresponding Attributes (Huacuz, Flores, Agredano, &Munguia,
1994)
Rating Factors
Good
Fair
Bad
Voltage
Over 12
Between 11 and 12
Under 11
Specific gravity of
electrolyte
Over 1.2
1.2
Under 1.2
Body related conditions
Free of anomalies
Electrolyte boiling,
wrong or missing
caps
Broken caps,
shorted cells
Terminal related conditions
Clean and tight with
proper connectors
Loose contacts
Corroded, broken,
or missing terminals
Operating status of
corresponding PV system
Working well
Working with
limitations
Out of order
No battery in the sample was determined to have a bad rating. Perfect batteries were
58.23%, good batteries were 37.88%, and fair batteries were 3.89%. The batteries were also
divided by four age groups: less than 6 months (55.2%), between 6 and 12 months (13.5%),
between 12 and 18 months (30.1%), and older than 18 months (1.1%). Age defined here as the
period of time between installation and inspection.
Seven different battery capacity sizes were found in this study ranging from 50Ah to
105Ah. The researchers proposed that capacity of car-type battery should be seven times the
daily load in Ah to be properly sized. Undersized batteries show faster sign of wear than properly
sized batteries. The batteries were also distributed in two different climate, hot-dry and temperate,
and the batteries in the hot-dry region performed better.
2.2.6. On the storage batteries used in solar electric power systems and development
of an algorithm for determining their amp-hour capacity. (Mahmoud, 2004) The storage
battery is a main component of a stand-alone PV power system. It may only be 8% of the initial
cost of a PV power system, but the cost rises to 23% during the lifetime of the system. This paper
26
illustrates the key characteristics of a storage battery, and most importantly discusses an
experimental method for derivation of a mathematical algorithm for calculating amp-hour
capacity of lead-acid batteries. This algorithm allows the calculation of the amp-hour energy
capacity by measuring voltage and specific gravity of the cell’s electrolyte.
For the algorithm development, Anker-Sun Power, a new 12V/110 amp-hour (AH) solar
lead-acid battery was fully recharged and discharged. Four battery parameters were measured
during the discharge: output current (I), voltage (V), acid concentration (ρ), acid temperature (T).
All four parameters were measured in 10 second intervals. The test was conducted until a depth
of discharge (DOD) level of 66.18% was reached. The measured voltage and acid concentration
values are in line with findings from another equation developed by Hermann et al. , Vn= 1.85V +
0.917V(ρn− 1), for a single-cell LAB. (Hermann, 1987)
Table 3
Measured and Calculated Parameters of a Lead-Acid Battery Rated at 12V/110 AH (Mahmoud,
2004)
n
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
In (A)
–
9.19
9.34
9.67
9.82
9.79
9.76
9.69
9.67
9.67
9.7
9.61
9.69
10.08
9.99
9.91
Vn (V)
12.76
12.57
12.52
12.47
12.42
12.38
12.34
12.29
12.24
12.19
12.15
12.1
12.06
12.01
11.95
11.9
Pn (kg/l)
1.24
1.24
1.24
1.227
1.2075
1.2025
1.2013
1.1975
1.18125
1.175
1.169
1.151
1.1425
1.13
1.1175
1.1175
Tn (◦C)
27
27.5
28
28.5
28
27.625
27.75
27.75
27.75
27.575
27.3
27.35
27.4
27.325
27.2
27.35
27
DODn
(%)
0
4.18
8.45
12.82
17.27
21.73
26.18
30.55
35.00
39.36
43.73
48.18
52.55
57.09
61.64
66.18
Cn (Ah)
(measured)
Cn
(calculated)
110.0
105.4
100.7
95.9
91.0
86.1
81.2
76.4
71.5
66.7
61.9
57.0
52.2
47.2
42.2
37.2
112.400
103.540
101.200
95.250
87.465
84.200
82.002
78.610
71.736
67.660
64.116
56.750
52.500
46.690
40.395
38.064
At quasi-constant acid temperature, capacity of LAB can be represented as a linear function of
voltage and acid concentration according to equation: C (capacity)=aV + bρ + c. Using the least
square method, we can obtain
where a, b, c are constant
and E is the error. For a minimum error E will be differentiated according to a, b, c and set to
zero.
. After a, b, and c are differentiated,
the equations are:
,
, and
.
Calculating V2, Vρ, VC, ρ2, ρC and their summation from Table 3, the resulting equations are:
2410.457a + 232.575b + 196.35c = 14 595.609;
232.575a + 22.447b + 18.940c = 1414.590; and
196.35a + 18.940b + 16c = 1182.60.
When these three equations are solved for a, b, and c the result is: a = 46.614, b = 279.573, and c
= -829.069. Substituting these a, b, and c values into the original equation, C = aV + bρ + c, the
resulting final equation to solve for capacity is C = 46.614V + 279.573ρ – 829.069. This equation
allows the calculation of the amp-hour capacity of the battery and enables the user to adjust the
setting of the rest of the power system to elongate and enhance the battery’s performance.
28
3. Methodology
The methodology of this quantitative study provides background information about each
experimental setup that describes why and how each step is conducted. This section is organized
by the name of each discrete test and will mostly be a technical background overview, so there
will not be a detailed explanation of each step.
The main goal of this experiment is to perform a series of tests to evaluate the energy
potential of used batteries. It includes a section on adapting Mahmoud’s algorithm for calculating
amp-hours of lead-acid batteries to find out potential energy capacities of used automotive
batteries for home energy storage device (Mahmoud, 2003). The detail of the algorithm was
presented as a case study in the literature review section. The experimental subjects - used car
batteries- were purchased from a local junkyard. The methodology also includes a section that
lists all the major electrical devices and accessories used to conduct these tests.
3.1. Recharge and Voltage Test
One way to determine the state of charge (SOC) of a battery is the voltage. Voltage can
easily be measured by a commercial voltmeter by touching the positive and negative terminals of
a battery with the positive and negative prongs of a voltmeter. A battery with a higher SOC has a
higher voltage than that of a lower state of charge. For example, a fully charged (100% SOC) 12V battery has a voltage of 12.7, but battery with 50% charge has a 12.0 volt. The first experiment
was to charge the battery with a 12-V trickle charger to determine whether the charging will
result in a higher voltage. Surface charges must be accounted for in this test. Surface charge is a
phenomenon that occurs during the recharge process. Electrical charges that linger around the
terminals inflate the voltage, so the surface charge must be removed for the voltmeter to correctly
measure the voltage of each battery. One method to remove surface charge is to momentarily put
29
a heavy load on the batteries or wait for the surface charge to settle to their normal voltages.
(battery manufacturer PDF) Open circuit voltage is the voltage when there is no load.
Table 4
State of Charge as Related to Specific Gravity and Open Circuit Voltage
(http://www.trojanbattery.com/BatteryMaintenance/Testing.aspx)
The voltages of each battery were measured before the recharge, immediately after the
recharge, and several hours after recharge to allow the charges to settle. The trickle charger
supplies DC electricity to the connected battery of voltages between 13.8 and 14.2, which is
considerably higher than the maximum voltage of a 12-V battery. This inflow of electricity at
higher voltages at the time of recharging masks the true voltage of the battery. Measuring the
voltage of the batteries in the process of recharging is not a viable method of monitoring battery
voltages, because voltmeter displays the flow of electricity from the trickle charger instead of the
true battery voltages.
The trickle charger is equipped with a built-in analog monitoring display, but it is
incomprehensible and inaccurate in displaying the real-time charging status. The battery does not
30
have built-in device that monitors voltages, either, so real-time monitoring of voltages is difficult.
Since excessive overcharging damages battery and its performance, the first recharge test was
limited to just a few recharging cycles. This test is sufficient for the purpose of measuring initial
and final voltage to determine if the voltage increases after charging.
3.2. Specific Gravity Test
Voltage alone does not confirm a battery’s state of charge or its health. Specific gravity
is another simple test that complements the voltage test. Water and sulfuric acid mixture are
active ingredients in the electrochemical reaction of the lead-acid battery. Measuring
electrolyte’s specific gravity (SG) is an accurate and dependable method to gauge a battery’s
SOC. A hydrometer is a device that measures the SG of the battery’s water and sulfuric acid
electrolyte. The hydrometer does not directly measure acid concentration, but instead measures
the specific gravity of sulfuric acid compared to the specific gravity of water. Measuring the
specific gravity of sulfuric acid in the process of recharging does not render an accurate state-ofcharge measurement and displays less than the actual SOC. Hydrometer readings are most
accurate when the battery is nearly or completely charged (Dyke, 1920). The specific gravity test
in this experiment not only tests the batteries’ SOC, but the measurements will also be used to
calculate the amp-hour capacity of batteries based on the study by Mahmoud (Mahmoud, 2004).
The specific gravity will be measured when the battery is discharged to measure the
difference electrolyte states in fully discharged and fully recharged conditions. The test will also
measure the consistency of SG across all six cells, because the six cells should be within 25
points of each other in a healthy battery. A cell with a particularly low SG beyond 25 points may
be a sign of a bad cell that can cause malfunction of the entire battery (Dyke, 1920).
31
3.3. Load Test
Lead-acid batteries are simple in build and operation, but they are very unpredictable and
complex when they fail to perform properly. Battery voltage alone cannot be trusted to estimate
the health of the battery. This metric should be complemented with a capacity test to measure
whether the battery has enough stored electricity to provide a constant flow of electricity for
powering electrical loads. The capacity test is an industry standard test of a battery’s SOC,
making it easily comparable to a new battery. (Picciano, 2007)
For all the load tests in this study, 10.8 closed-circuit volts were the cut-off point to end
the load test. Closed-circuit voltage is the voltage with an active load drawing current from the
battery. Based on observations from this study, once the load is disconnected the voltage rises by
0.4 volt, making the open-circuit voltage 0.4 volt higher than the corresponding closed-circuit
voltage. The open-circuit voltage is 11.2 volt once the load is disconnected at 10.8 volt.
Referring to Table 4, which displays the level of SOC at various voltages, open-circuit voltage of
11.2 is 0% SOC. Therefore, ending the load test at 10.8 close-circuit volt means that the load test
ends when the battery is theoretically completely discharged.
Although the reason is not clear, even one bad cell in a six cell battery is detrimental to
the health of the total battery and can result in failed performance. However, the specific bad cell
does not easily reveal itself in a voltage test. Each cell is responsible for 2.1 volts and the six
cells are serially connected in a straight line so that eachcell is responsible for 1/6 of the battery
voltage. If one cell is not functioning, a full battery with the one bad cell should give a voltage
reading of 10.5V = 12.6 V – 2.1 V. This, however, is not manifested in reality. A battery with a
bad cell still misleadingly appears to have the same voltage as a wholesome battery without any
failing cell.
32
The experimental subjects used for this project, used car batteries, have no state of charge
indicators and no written record of their past operating conditions. The load test serves the
purpose of testing whether these batteries are useful batteries, and establishing their performance
base to determine their utility.
3.4. Mahmoud’s algorithm test for calculation of amp-hour capacity of batteries
Dr. Mahmoud’s formula can be used to calculate amp-hour capacities of LABs using two
sets of data: specific gravity and voltage. The table below displays the voltage and SG that are
measured in increments of approximately 4% depth of discharge (DOD). Only the first
measurement is an open circuit voltage, meaning there was no load to the battery. All the other
measurements were taken with a load of 9.7 amps. The batteries were discharged to 66.18%
DOD meaning a full battery would be discharged to 66.18% of its maximum capacity.
Mahmoud’s original calculation process was formulated based on an 110 amp-hour deepdischarge battery, so his calculations were modified for use on batteries with less amp-hour
capacities for this thesis. The algorithm was developed by experiment on a deep-discharge
battery; however the study states that the same algorithm may be used to determine amp-hour
capacity of any LAB used for energy storage purposes. The table below is taken from Dr.
Mahmoud’s study. All the steps of Mahmoud’s algorithm can be found in his paper “On the
storage batteries used in solar electric power systems and ampere hour calculation”, page 5.
For a 12V/110AH battery, Mahmoud’s equation to solve capacity is:
C = 46.614V + 279.573ρ - 829.069. (C is capacity, V is voltage, ρ is specific gravity)
Mahmoud focused on the fact that battery voltage and acid concentration can form a linear
function to represent amp-hour capacity. From measuring voltages and acid concentration at
different depths of discharge, Mahmoud was able to formulate an equation to calculate amp-hour
33
capacity from battery voltage and acid concentration. Please refer to Mahmoud’s paper and/or
the literature review for the complete algorithm development process.
Equation for 50AH battery is: C = 16.570 V + 159.180 ρ - 358.176
Equation for 45AH battery is: C = 14.913V + 143.262ρ - 322.358
The solar battery used in Dr. Mahmoud’s study has one figure for specific gravity of
sulfuric acid. However, each automotive LAB has six cells and these six cells are equally rated at
2.1 volts. For this calculation, an average specific gravity of the six cells was used as the only
specific gravity reading of the battery.
Voltage
Recharge Voltage of EconoPower Battery
14.8
14.6
14.4
14.2
14
13.8
13.6
13.4
13.2
13
Absorption Stage
Bulk Stage
0
20
40
60
80
100
120
Float Stage
140
160
Recharge duration in minutes
Figure 17. Recharge voltage of EconoPower battery.
The recharging cycle takes places in three stages: bulk, absorption, and float. The bulk
stage of the recharging initiated at 13.158 volts and slowly climbed up to 14.517 volts, at which
point the absorption stage begins. The entire bulk recharging stage lasted 75 minutes. During the
first 75 minutes, the amperage rises sharply to a peak level and recharges at this peak level up to
34
the absorption level. The absorption stage lasts 60 minutes till the 135th minute and during this
time, the amperage slowly drops while the voltage is maintained at the peak level. The last 20
minutes are the float stage where voltage is maintained at a stable level. The bulk stage is 48%,
the absorption stage is 39% and the float stage is 13% in terms of duration of the recharge. The
actual amperage data is not available due to a lack of equipment.
3.5. Field Test: The TJ House Photovoltaic Power System
The Tijuana (TJ) House is a low cost prototype building built at the western end of the
Lyle Center for Regenerative Studies. The main material is paper-crete, a combination of pulp
from paper and concrete. Instead of expensive sand and gravel, newspaper pulp was mixed with
cement to create a light alternative to conventional concrete. In 2010,Dr. Ritz and his students
installed a PV power system from decommissioned PV modules and other recycled materials on
the roof of the building. The system’s main components are five Siemens SM55 PV modules, a
DC combiner box, two circuit breakers, a Xantrex C40 charge controller, a salvaged fan, a car
radio, a light, and two 6 volts deep cycle lead-acid batteries. One SM55 PV module is rated at
maximum of 55 watts. The rated current is 3.15 amps and the rated voltage is 17.4 volts. The PV
modules on the TJ house are individually wired to a terminal in the DC combiner box. DC
combiner box is a device that amalgamates wires from all the modules, usually grouped in arrays,
into a single unit.
35
Figure 18.TJ House and photovoltaic array of five modules (in yellow).
The five PV modules are individually routed to the DC combiner box. This means that
these modules are connected in a parallel circuit. The other way of wiring PV modules is a serial
circuit connection. A serial circuit connection is displayed below.
Figure 19. Wiring diagram of the TJ House photovoltaic power system.
36
Electricity from PV modules connected in a series multiplies in voltages, depending on
the number of connected PV modules. One SM55 PV module is capable of producing at 17.4
volts, so two SM55 modules connected in series is capable of producing at 34.8 volts. In contrast
to the series circuit, a parallel circuit multiplies the amperage of the electricity. Amperage is a
unit designated to measure the electrical current and its symbol is “I”. One SM55 PV module is
capable of producing 3.15 amperes of current so a pair of SM55 modules can produce 6.3
amperes of electrical current.
A PV module has multiple cells inside. Each PV cell reacts with incoming light to
produce electricity and these cells are usually connected in a series. A PV module has a positive
wire and a negative wire that allows modules to be connected to each other and ultimately deliver
the PV generated electricity. Each PV module of the TJ house power system is directly wired to
the busbars of the DC combiner. This means all the positive wires are connected to the single
positive busbar of the combiner box, and all the negative wires are connected to the single
negative busbar of the combiner box. In this parallel setup, the voltage of the combined
electricity remains equal to the voltage of a single module at 17.4 volts, but the amperage of the
combined power is increased five-fold at 15.75 amperes.
The solar power rated at 274 watts (17.4 volts x 15.75 amperes) is then routed to the
charge controller via a circuit breaker. A circuit breaker (or a switch) performs these functions: 1)
senses when overcurrent occurs, 2) measures the amount of overcurrent, 3) acts by tripping
(disengaging) the circuit to prevent damage to the circuit breaker and the conductor. The
breakers also act as a switch to disconnect current flow between PV modules, charge controller,
and batteries.
A charge controller has several functions depending on the application, but this charge
controller is used to charge and regulate the batteries. The charge controller keeps the batteries
37
from being undercharged or excessively charged, the two conditions that deteriorate the health of
batteries. It regulates the voltages from the PV modules and charges the batteries at around 14.4
volts, keeping the batteries from overcharging. When the batteries are depleted, the charge
controller activates the recharging process. The LCD on the charge controller displays the
current status of the incoming PV electricity when the batteries are charging in watts, volts, and
amperes. It also tracks energy that was used to recharge the batteries in watt-hour units. The
charge controller is a device that will only work in conjunction with a healthy battery bank. A
pair of serially connected 6-V deep cycle batteries was connected to the charge controller before
the experiment commenced, but the charge controller appeared to be out of service. The
voltmeter readings of the existing deep cycle batteries determined that the voltages of each
battery were below 1 volt. The LCD display was off and the light bulb indicating the status of the
charge controller was unlit as well. When healthy batteries replaced the incumbent batteries, the
charge controller resumed its normal operation.
The LCD screen displays the voltage, current, and power of electricity feeding the battery,
as well as the accumulated amp-hours that the battery has stored. The Xantrex C40 is a three
stage type charge controller that delivers electricity to the battery in three stages. First is the bulk
stage where the voltage rises to the bulk rate level of around 14.4 to 14.6 V. During the
absorption stage, the voltage remains between 14.4 and 14.6, then the amperage tapers down
gradually in the float stage as the battery is charged.
38
Figure 20. Three phase charge phase. (http://www.freesunpower.com/chargecontrollers.php)
For this experiment, the battery bank consisting of a single or a pair of batteries was
directly connected to the charge controller, and a 12-V AC/DC inverter was connected to the
battery bank. The load was then directly connected to the AC terminal of the inverter.
Three brands of batteries were tested: EconoPower, ValuCraft, and Kirkland. These were
tested as individual batteries, and two of them wereconnected in parallel circuit to form a battery
bank. LABs can be connected in series, in parallel, or in combination to increase power and
capacity. The only multi-unit battery combination tested was a parallel connection between
EconoPower and Kirkland batteries. The reason for choosing this combination is that these two
batteries were the only onesthat had substantial and similar capacity remaining. The purpose of
testing a multi-unit battery bank is to measure the useful energy pool from these batteries, so it
made sense to test the two healthier (or more robust) batteries.
3.6. Equipment
3.6.1. Batteries. The original intent of the research required sourcing car batteries
between the voltage of 10.5 and 11.5. Sourcing used automotive batteries with voltage range
between 10.5 and 11.5 was practically very difficult. Automotive parts and battery retailers donot
39
stock used batteries. They stocked reconditioned batteries, which are basically remanufactured
batteries. Eventually, four batteries used for this research were obtained from a local junk yard.
A voltmeter was used to test the voltages of the used batteries available in the junk yard. Most of
the available batteries had voltages below 7, and some had healthy voltage readings above 11.
However, there were no batteries in the target range between 10.5 V and 11.5 V. Due to
difficulty sourcing the batteries with the desired voltage range, I modified the scope of this
experiment from utilizing “dead” car batteries to “used” car batteries as a residential energy
storage device. I purchased three SLI batteries and one dual purpose battery: EconoPower,
Kirkland, ValuCraft, and an EverStart dual purpose.
LABs are often defined by their Battery Council International (BCI) group number such
as group 24, type 27. The BCI group number categorizes batteries by the physical dimensions of
the exterior case. This group designation is important, because some application calls for specific
group sizes. In the US, battery manufacturers do not publish nor rate their batteries in amp-hour
units, but instead use reserve capacity (in minutes). Reserve capacity (minutes) is the duration
that a battery will last if a fully-charged battery is drained with 25 amp load. The amp-hour
capacity can be determined easily from reserve capacity by reverse calculation. If the reserve
capacity is 90 minutes, dividingby 60 minutes caculates that the battery will last 1.5 hours.
During the 1.5 hours, the load is 25 amp so 1.5 hours multiplied by 25 amp –representing 25 amp
load— is 37.5 amp-hours. Below is the equation to solve for the capacity in amp-hour from
capacity in minutes.
90 𝑚𝑖𝑛𝑢𝑡𝑒𝑠𝑋
1 ℎ𝑜𝑢𝑟
𝑋 25 𝑎𝑚𝑝 = 37.5 𝐴𝑚𝑝𝐻𝑜𝑢𝑟
60 𝑚𝑖𝑛𝑢𝑡𝑒𝑠
Each battery listed the manufacturer’s reserve capacity in minutes operating at 80° F.
The reserve capacity in amp-hours is available for only two types of batteries, Kirkland and
EverStart. Reserve capacity in amp-hours calculated by using the above equation from reserve
40
capacity minutes is lower than amp-hour capacity published by a third party. For example,
Kirkland battery’s amp-hour capacity is 50 amp-hours, but its capacity in minutes is only 105.
Using the above conversion equation, 105 minutes is only 43.8 amp-hours, which is 12.4% less
than 50 amp-hours. The EverStart battery is a dual-purpose battery that did not belong to a
specific BCI group and its capacity in amp-hours was printed on the top cover of the battery.
The EconoPower battery is physically the largest battery followed by ValuCraft and
Kirkland. However, the smallest Kirkland battery had the highest cranking amp rating at 1000
amps. Based on performance figures, there seems to be no direct relation between cranking amp,
size, and reserve capacity. The smallest battery, Kirkland, has the highest cranking amp but its
calculated capacity rate sits in the middle of the pack.
The manufacture date of three car batteries, Kirkland, EconoPower, and ValuCraft are
very close together, but each battery belongs to a different BCI size group. From the date of
manufacture, the three car batteries were either 19 or 20 months old at the time of experiment.
The EverStart battery was the oldest at 50 months. From the manufacture date, the performance
of the three regular car batteries can be estimated to be similar to each other. Estimating the
performance levels based solely on their age is difficult due to a lack of published data.
Table 5
BCI Group and Size Specifications
BCI
34
24F
24
L
260
273
260
Millimeters
W
173
173
173
H
200
229
225
Volume
(mm3)
8996000
10815441
10120500
41
L
10 1/4
10 3/4
10 1/4
Inches
W
6 13/16
6 13/16
6 13/16
H
7 7/8
9
8 7/8
Volume
(inch3)
549 8/9
659 1/9
619 5/7
Table 6
Battery BCI Group Size and Performance Specifications
Brand
BCI
Group
Size
Kirkland
Econo
Power
ValuCraft
34
24F
24
EverStart
N/A
Manuf.
Date
07-12
07-12
08-12
09-09
Model
Cranking
Amp
12877
GR24
F
24-VL
24DP4
1000
Performance
Reserve
Reserve
Capacity
Capacity
Minutes
Amp@ 80F (per
Hours
manuf)
105
43.8
Amp-Hour
Capacity
(per manuf)
50
750
650
110
90
45.8
37.5
N/A
N/A
675
140
58.3
105
3.6.2. Hydrometer and specific gravity. The water and sulfuric acid mixture is an
active ingredient in the electrochemical reaction of the LAB. The hydrometer measures the
specific gravity of the battery’s electrolyte and complements the voltage test in determining the
battery’s SOC. The hydrometer used in this experiment is OTC 4619 Professional Hydrometer.
3.6.3. Onset Hobo data loggers. The Hobo Data logger of Onset Computer Corporation
was used to track the voltages of batteries. The U-12 data logger is able to measure temperature
and humidity and, with additional accessories, the voltage. The continuous voltage data is used to
track behavior of batteries and to calculate amount of energy that the battery is able to dispense.
3.6.4. 12-V AC/DC trickle charger. A 120-V AC/DC trickle charger plugs into a
conventional 120-V AC wall outlet and has positive and negative leads that clamp to a pair of
terminals, a negative and a positive, of a 12-V automotive battery. It has charge rate settings of 2,
10, and 50-amps.
3.6.5. Xantrex C40 inverter.The inverter is a device that converts direct current (DC)
electricity to alternating current (AC). A Xantrex C40 inverter was used here, which was
42
designed for DC input of 10 to 15 volts, making it ideal for use with an automotive LAB. When a
12-V battery is used to convert to an AC power source, a low voltage of 12 volts needs to be
converted to a high voltage of approximately 120 volts. This conversion is a two-step process.
The inverter first amplifies the low DC voltage into 145 DC volts, and then converts it to 115
volts, 60 Hz AC. (Prowatt 800 manual)
The C40 inverter is only able to accept input from DC voltages between 10and 15-V.
Combining two batteries in a series would double the voltage of the battery bank to 24 volts, so
the only viable connection is a parallel connection which would keep the voltage at 12 volts. The
voltage output of the batteries remains as if it is 12 volts, but the battery capacity doubles.
The inverter also serves as a regulator to prevent the extreme discharge of batteries. The
operating voltage of the inverter is 10 volts or higher. The inverter self-monitors battery voltages
in the discharge process, and at 10-V and below the inverter shuts off to prevent permanent
damage to the battery.
43
4. Results and Analysis
4.1. Recharge and Voltage Test. Before the experiments began, the subject batteries
had been recharged with the trickle charger three weeks before the voltage test to revive and
maintain their capacity. If a LAB remains in a discharged state without being recharged, sulfation
can occur, leading to possible permanent damage. Sulfation is the irreversible buildup of lead
sulfate in the active mass of battery cells (2004, Ruetschi). The main precaution of this test is to
test the batteries without excessively overcharging the batteries, because this canresult in internal
grid corrosion (2004, Ruetschi). Hence the charging time was limited to one hour in each charge
rate, 2 and 10 amps. The main purpose of this test is to see whether the batteries will hold the
charge by checking the voltage levels before and after the recharge. The state-of-charge for each
battery was not determined and each had a distinct initial voltage before the recharge test.
The input stream of electrical current from the trickle charger had fluctuating voltages
between 13.8and 14.2 volts when the battery terminals were tested with the voltmeter. After one
hour of charge, the voltages of the batteries were immediately measured once the trickle charger
was disconnected. The biggest gain of voltage was 0.6 volts by the Kirkland and the smallest
gain of 0.18 by ValuCraft. These figures are not representative of true voltages of batteries, due
to the surface charges. To eliminate surface charges, the voltage of each battery was measured
six hours later (1998, Darden). Kirkland had the lowest initial voltage, the biggest gain during
the first voltage test after one hour recharge, but also had the biggest voltage drop during the six
hour period. The Kirkland eventually had the highest gain, 0.25 volt, among the batteries. The
EverStart that had the highest initial voltage but eventually gained the least voltage after the
recharge. The ValuCraft had the lowest range of voltage, the lowest gain during recharge and the
lowest loss during six hours of discharge. Table 7 shows the results of one-hour recharge atthe 2
amp recharge rate by the trickle charger. Due to the relatively high initial voltages of the batteries
and the short recharge time, the actual gain is minimal but all batteries had a net gain of voltages.
44
Table 7
Voltage
Test Result: 2 Amp Recharge
Battery
Kirk
EconoP
ValuC
EverS
Charge
rate
(A)
Charge
duration
(H:MM)
2
2
2
2
1:00
1:00
1:00
1:00
Initial
Voltage
(V)
Voltage
after
charge (V)
Voltag
e Gain
(V)
Voltage
after 6
hours rest
(V)
Rest
Voltag
e Drop
Actual
Gain
A
B
B–A
C
C–B
C–A
12.06
12.40
12.38
12.66
12.66
12.79
12.56
12.92
0.60
0.39
0.18
0.26
12.31
12.48
12.47
12.72
-0.35
-0.31
-0.09
-0.2
0.25
0.08
0.09
0.06
The recharge and voltage test was immediately followed by a larger charge rate of 10
amps. The initial voltage of each battery was the terminal voltage rate from the previous test.
Each battery was recharged for one hour with the trickle charger at the 10 amps setting. The
voltage was measured immediately following the recharge and after ten hours of discharge to
account for the surface charge. Like the previous results, Kirkland had the lowest initial voltage
and EverStart had the highest initial voltage. Following one hour of recharge, Kirkland gained
0.38 volt and lost 0.23 volt during the 10 hours discharge process. The actual gain for Kirkland
was 0.15 volt. The actual gain, accounting for surface charge removal, from the 10-amps
recharge is smaller than the previous 2-amps recharge. This is probably due to the fact that these
batteries are already adequately charged and they were past the bulk charging stage.
From conducting the recharge and voltage test, all four batteries were foundto be
operational and able to be recharged and hold recharges to display higher voltage readings when
tested with a voltmeter.
45
Table 8
Voltage Test Result: 10 Amp Recharge
Battery
Kirk
land
Econo
Power
Valu
Craft
Ever
Start
Charg
e rate
(A)
Charge
duration
(H:MM)
Initial
Voltage
(V)
V after
charge
(V)
A
B
Change
Voltage
after 10
hours rest
(V)
Rest
Voltag
e Drop
Actual
Gain
B–A
C
C–B
C–A
10
1
12.31
12.69
0.38
12.46
-0.23
0.15
10
1
12.48
12.71
0.23
12.51
-0.2
0.03
10
1
12.46
12.71
0.25
12.49
-0.22
0.03
10
1
12.7
12.94
0.24
12.74
-0.2
0.04
From the voltage test, the SOCwas determined to be very high for all batteries. The
EverStart battery has such a high voltage that it appears to be as good as a new battery. Three
other batteries were in good condition with each over 75% SOC. After the first test, all the
batteries appeared to be in near new or very healthy conditions. However, since the voltage test
alone does not correctly represent the SOC, specific gravity and initial load test were also done to
complement the voltage test.
Table 9.
Voltage and SOC of Four Batteries
Battery
Open Circuit Voltage
SOC
Kirkland
EconoPower
ValuCraft
EverStart
12.46
12.51
12.49
12.74
75%
80%
80%
100%
4.2. Initial Load Test. This preliminary load test measures the baseline capacity of the
batteries and determines the health of the batteries. Baseline capacity means attaining a
quantifiable value for the battery’s capacity, since these batteries are used batteries with no record
46
of previous performance. This test is also used to see whether any of the batteries are failing,
because a voltage test alone is not sufficient to determine operational order. A useful battery
accepts, stores, and dispenses electricity while a dead battery fails to perform such functions.
All batteries were recharged at10 amp recharge rate with varying charging time according
to their manufacturers’ specification and BCI group rating. The EverStart battery is rated at 105
amp-hours, so it was charged for 10 hours at 10 amps. Even with the highest open circuit voltage,
marking it as the healthiest batteries out of the group, EverStart battery failed to store any
electricity. This result was anticipated from the specific gravity test, because specific gravity of
an unhealthy battery is inconsistent. Specific gravity readings from ValuCraft, Kirkland, and
EconoPower batteries were consistent in all of the six cells, while EverStart had one or two cells
with abnormally low specific gravity that deviated from the mean. Specific gravity will be
discussed further in the next section.
From the preliminary load test, Kirkland and EconoPower batteries had the highest
capacity with 277 and 248 watt-hours, respectively. The ValuCraft battery was rated at less than
100 watt-hours after the preliminary capacity test. This test revealed that EverStart battery may
be a defunct battery and that the other three batteries are operational. The discharge duration
enabled estimates of the battery performance and showed that the ValuCraft battery has
significantly smaller capacity compared to the EconoPower and Kirkland batteries, which could
not be proven with the voltage test.
47
Table 10
A load test with trickle charger
Battery
Ever
Start
Valu
Craft
Econo
Power
Kirk
land
Input
Energy (Ah)
Load (A) Voltage (V)
A
B
Power (W)
C=AxB
Duration
(H:MM)
D
Capacity
(Watt-hr)
E=CxD
10h x 10amp
0.73
118.5
86.51
0:01
1.44
4h x 10amp
0.73
118.5
86.51
1:05
93.71
4h x 10amp
0.73
118.5
86.51
2:52
247.98
6h x 10amp
0.58
118.5
68.73
4:02
277.21
4.3. Specific Gravity. To determine the SOC from the specific gravity (SG)
measurements, all batteries were initially fully charged. Then the electrolytes from six cells
within each battery were measured with the hydrometer, and the SG values were averaged as a
mean. The mean SG was then compared to a SG to SOC conversion chart to extrapolate the SOC.
The EverStart battery appeared to be the most potent battery with an average SOC of 86%. The
EverStart battery had the highest average specific gravity, but this battery will be discussed later
forits inconsistent nature of SG among the six cells.
The ValuCraft, EconoPower, and Kirkland batteries each had consistent SG output
among all six cells. The ValuCraft and EconoPower batteries had SOC of 50% or more while
Kirkland battery was lowest with 42% SOC.
48
Table 11
Specific Gravity and State of Charge (SOC) of Fully Charged Batteries
Battery
Test 1
Mean S.G.
Test 2
SOC
Mean S.G.
Average
SOC
Mean S.G.
SOC
EverStart
1.257
83%
1.264
88%
1.261
86%
ValuCraft
1.217
67%
1.213
64%
1.215
66%
EconoPower
1.197
54%
1.183
46%
1.190
50%
Kirkland
1.171
39%
1.181
45%
1.176
42%
The graph below explains the relationship between specific gravity and the SOC. The
graph has two axes:left is SG and right is the SOC (%). Just based on the average specific gravity,
the EverStart is at near 80% SOC. The voltage yielded that the EverStart battery is 100% SOC,
and the specific gravity measurement also yields that it is a very high performing battery in spite
of being five years old. However, the EverStart battery was under scrutiny due to its abnormal
range of specific gravity across the six cells.
100%
1.280
1.260
1.240
1.220
1.200
1.180
1.160
1.140
1.120
80%
60%
40%
20%
0%
EverStart
ValuCraft
EconoPower
SG
Kirkland
SOC
Figure 21. Average specific gravity and SOC for fully charged battery.
49
State of Charge
Specific Gravity
Avg Battery Specific Gravity & State of
Charge
Specific gravity may be presented in a four digit integer (i.e. 1,000) or as a whole number
with three decimal digits (i.e. 1.000). The scale inside the hydrometer is in 1000 point scale
while specific gravity is measured on a 1.000 scale. The difference in specific gravity of each of
six cells in the EverStart battery is 40 points in the discharged state, and 100 in the fully
recharged state. When all the cells are in good order the specific gravity will render about the
same level within 25 points of each cell (Dyke, 1920). If the spread is persistently 25 points or
greater after the extended recharge, the battery should be checked again (Shumay Jr., 2005). The
specific gravity of cell 5 and 6 of EverStart were significantly lower than other cells, resulting in
a 100 point spread between the highest and lowest cells. From the voltage test the EverStart had
the highest voltage and yet rendered the most inconsistent specific gravity measurements. Cell 5
and 6 are possibly the bad cells and later it was found that EverStart battery is inoperable, most
likely due to malfunctions from cells 5 and 6.
Table 12
Specific Gravity of EverStart battery
EverStart
Cell
Discharged
state
Recharged
state1
S.G.
S.G.
1
1201
1234
2
1196
1294
3
1211
1304
4
1171
1299
5
1186
1204
6
1181
1204
Highest
1211
1304
Lowest
1171
1204
40
100
1191
1257
14
48
Difference
Average
Standard Deviation
50
4.4. Amp-Hour Capacity Calculation Algorithm. As discussed in the methodology
section, Mahmoud’s algorithm calculates the amp-hour capacity of LAB with the battery’s
voltage and specific gravity. For this test, two setsof SG were measured for each battery; twice at
the discharged state twice and twice at the fully recharged state. At each discharged and charged
state, Mahmoud’s algorithm was utilized to solve for amp-hour capacity. The EverStart battery
was omitted from the calculation, because no significant result could be assigned to the
malfunctioning battery.
Besides the calculated amp-hour capacity, the range, or the difference between the
maximum and minimum capacity, is discussed in this section. The range, not discussed in
Mahmoud’s paper, is significant in this test because there was a lack of equipment to measure SG
while the experimental battery was under the load. Each battery was fully discharged before they
were fully recharged and only the maximum and minimum SG was measured. These SGs were
then used to calculate the maximum and minimum capacity. In this test design, the range could
serve as a vital piece of data that could be translated as the capacity itself.
The amount of
energy that was used to recharge the battery from the fully discharged state equals the capacity.
Therefore the capacity at full minus the capacity at empty equals the available capacity. This
argument is also possible because there is a systematic over-calculation of capacities in both
empty and full states. All the batteries were calculated to be at least 30% full even though they
were discharged to near complete depletion during the load test. If the calculation itself is
inflating the results, then the range could be a better indicator of the capacity.
The capacity of a new ValuCraft battery is 37.5 amp-hours. Using Mahmoud’s modified
algorithm, this test solved for amp-hour capacity based on the battery’s VOC and specific gravity.
A fully discharged ValuCraft battery had a calculated capacity of 19.4 which is 52% of a new
fully-charged battery. A fully charged battery had 32 amp-hours of capacity, which is 84% of a
new, fully-charged battery. A fully discharged battery should normally have near zero capacity
51
remaining, but a fully discharged ValuCraft battery still had 52% of capacity remaining. This
seems abnormal, but it is also due to the fact that these voltages are uncompensated for the
surface charge. If the surface charge was given time to settle, the voltage would be lower and
would result in a smaller amp-hour capacities. The fully charged state measurement in
Mahmoud’s study is an open circuit voltage at no load, as is the test condition for fully charged
batteries for this experiment. The range of the first run is ignored, because the battery was not
fully discharged. On the second run, the range is 31.6 – 19.4 = 12.2 amp-hours. This information
will later be compared with results from TJ House test to determine its validity.
Table 13
Valucraft Amp-Hour Calculations
Charge State
Voltage
(V)
Average
Specific
Gravity
Amp-Hour
Calculation
(AH)
Manufacturer
Amp-Hour
(AH)
SOC %
Partially discharged
12.40
1.191
27.7
37.5
74%
Fully charged
12.49
1.217
31.9
37.5
85%
Fully discharged
11.64
1.201
19.4
37.5
52%
Fully charged
12.51
1.213
31.6
37.5
84%
The Kirkland battery had the largest capacity out of the three batteries at 50 amp-hours.
The fully discharged Kirkland battery had a calculated capacity of 17.1 and 17.7, approximately
34-35% of a new fully-charged battery. A fully charged battery had 37.3 and 38.4 amp-hours of
capacity, which is 75 - 77% of a new fully-charged battery. The voltage measurements at the
discharged states did not account for the surface charge so the amp-hour calculated is overcalculated in this case as well. The ranges, the difference between the amp-hour capacity of
discharged and charged state, are 38.4 – 17.7 = 20.7 and 37.3 – 17.1 = 20.2 amp hours.
52
Table 14
Kirkland Amp-Hour Calculations
Charge State
Voltage
(V)
Average
Amp-Hour
Manufacturer
Specific
Calculation
Amp-Hour
Gravity
(AH)
(AH)
SOC %
Fully discharged
11.87
1.126
17.7
50
35%
Fully charged
12.62
1.178
38.4
50
77%
Fully discharged
11.81
1.128
17.1
50
34%
Fully charged
12.52
1.181
37.3
50
75%
Based on the manufacturer’s data, the capacity of a new EconoPower battery is 45.8
amp-hours. Based on the algorithm, a fully discharged EconoPower test battery had a calculated
capacity of 13.8 which is 31% of a new fully-charged battery. This is an odd result like the other
two batteries from an unaccounted surface charge. A fully charged EconoPower test battery had
29.8 and 31.1 amp-hours of capacity, which is in the 66 - 69% range of a new, fully-charged
battery. Based on the algorithm calculation, the EconoPower battery has the lowest state of
charge at the fully charged state, at a level below 70%.
The range from recharge from the first
run is 15.1 amp-hours and 16 amp-hours from the second run.
Table 15
Econopower Amp-Hour Calculations
Charge State
Voltage
(V)
Average
Amp-Hour
Manufacturer
Specific
Calculation
Amp-Hour
Gravity
(AH)
(AH)
SOC %
Fully discharged
11.87
1.126
16.0
45
35%
Fully charged
12.62
1.197
31.1
45
69%
Fully discharged
11.72
1.146
13.8
45
31%
Fully charged
12.65
1.183
29.8
45
66%
53
This is asummary of the range of maximum and minimum capacities for the three
batteries. The range could potentially be used as a parameter in determining the battery’s storage
capacity in amp-hours. The maximum, minimum, and the range will be compared with the TJ
House charge controller data to verify their validity. The charge controller in the PV system has a
LCD that displays the amp-hour capacity of the battery, so it provides more accurate data on the
amp-hour battery capacity. The first test of ValuCraft battery is invalid data, because the
minimum data is not truly a minimum capacity.
Table 16
Range of Recharge for Three Batteries Based on the Algorithm’s Calculation
Battery
ValuCraft
Kirkland
EconoPower
Test
1
2
1
2
1
2
Maximum
capacity
(amp-hours)
Minimum
capacity
(amp-hours)
31.9
31.6
38.4
37.3
31.1
29.8
Range:
Max - Min
(amp-hours)
27.7
19.4
17.7
17.1
16
13.8
4.2
12.2
20.7
20.2
15.1
16.0
Inaggregatingthe specific gravity data ofthe three batteries, ValuCraft in fully charged
status is almost as good as new at 85%. Both Kirkland and EconoPower batteries are also in
decent condition at 76% and 68%, respectively. Each battery belongs to a different BCI group
and has a different maximum capacity and was calculated accordingly.
54
Avg Full State of Charge (%)
90%
80%
70%
85%
60%
76%
68%
50%
ValuCraft
Kirkland
EconoPower
Figure 22.Average full SOC for each battery according to the algorithm.
4.5. TJ Power System Test. A series of capacity tests were conducted to assess whether
used car batteries can be utilized as a home energy storage device. Repeated capacity tests were
the best method to determine whether these batteries have enough potential to continuously
provide power to electrical loads. The electrical loads in this experiment consisted of a fan and
light bulbs, varied number of light bulbs depending on the load configuration. The initial test
subject included a coffee maker and a small microwave oven, but testing these appliances could
not be continued due to a limitation in the inverter capability. The inverter is a high performance
12-V model rated to sustain the operation of 800 watts load. The 800 watts inverter is capable of
providing 6.7 amps current at 120 volts, but both the coffee maker and the small microwave oven
drew 8+ amps, so the inverter failed to operate. Because of these technical limitations, the loads
were changed to include a large fan and table lamps. An appliance such as a heat pad is a useful
load due to its load size, but its energy consumption is difficult to track because it automatically
switches on and off to provide constant heat. One of the purposes of the experiment is to measure
the energy capacity with a uniform energy draw from start to finish, so a fan and multiple lamps
were ideal test subjects.
55
The most dependable method of determining the potential amp-hour capacity each
battery is measuring the amp-hour input energy in the TJ House PV power system. Contrary to
the setting of previous tests, the batteries used here are 50% or below the full potential capacity of
a new battery of its type. The EverStart battery is below 1%, which proves that it is either a dead
or malfunctioning battery.
Table 17
Maximum Amp-Hour Capacity of Batteries
Manufacturer’s
Battery
Input Energy
Capacity (Amp-
(Amp-Hour)
Hour)
SOC Percentage
Kirkland
23.6
50
47.2%
EconoPower
20.3
45.8
44.3%
ValuCraft
9.0
37.5
24.0%
EverStart
0.2
105
0.19%
The previous section discussed Mahmoud’s algorithm for calculation of amp-hour
capacity of batteries, its systematic over-calculation of capacities, and the range. Below is achart
that compares the amp-hour capacity of three batteries. The capacity measured by the charge
controller is the standard, and the two other measurements of capacities are compared to the
standard.
Both the calculations from Mahmoud’s algorithm and the range do not precisely match
the capacity measured by the charge controller. Analyzing the percentage difference, the range is
a better tool of measuring capacity than direct application of Mahmoud’s algorithm in this case.
56
Table 18
Comparison of Amp-Hour Capacity between Charge Controller, Algorithm, and Range
Battery
Kirkland
EconoPower
ValuCraft
Charge
controller
capacity
(Amp-Hour)
23.6
20.3
9
Algorithm
calculation
capacity
(Amp-hours)
31.6
37.9
31.6
% Diff
34%
86%
251%
Range from
algorithm
capacity (Amphours)
% Diff
20.45
15.55
12.2
13%
23%
36%
The EverStart battery is excluded from the TJ house test, because it failed to pass the
preliminary capacity test. The EverStart battery was installed in the TJ house to recharge,
however it only charged 0.2 amp-hour over several hours and failed to power a load. Based on
this initial experiment, the Kirkland battery had the highest capacity of all test batteries at 23.6
amp-hours. A close second is the EconoPower battery at 20.3 amp-hours capacity and ValuCraft
battery was able to store 9 amp-hours.
Each battery was then connected to the inverter and a load to check how much watt-hours
of energy it was capable of dispensing. The amount of dispensed energy is the watt-hour capacity
of each battery. The input energy in amp-hour was directly correlated to the amount of watt-hour
energy.
57
Table 19
Amp-Hour Energy Input Vs Watt-Hour Energy Output of 3 Healthy Batteries
Load
AC
Power
Duration
Output Energy
(amps)
voltage
(Watt)
(H:MM)
(Watt-Hours)
(A)
(B)
(D)
(C*D)
Input
Battery
Energy
(C = A *
Amp-hours
B)
ValuCraft
9
0.76
118.5
90.06
1:00
90.1
EconoPower
20.3
0.62
118.5
73.47
3:00
220.4
Kirkland
23.6
0.62
118.5
73.47
3:30
257.1
4.5.1. The ValuCraft battery load test. The ValuCraft battery behaved in a very
unpredictable and erratic manner in the recharge and discharge tests. The three relevant factors in
this test is the input energy, load in amps, and the output energy in watt-hours. The input energy
here is the fully charged capacity in amp-hours and the output energy is equal to the energy
capacity in watt-hours. The current size in amp is dependent on the load, the combination of light
bulbs and a fan. From the literature review on battery discharge, it is seen that output current and
total energy capacity are inversely correlated. This was proven by the load test. When the
current draw from ValuCraft battery was 0.76 amps, watt-hour capacity was 90.1. When the
electrical load decreased to 0.58 amps,it resulted in an increase in capacity to 97.4 amp-hours. In
three separate recharge cycle, the charge controller indicated the full charge status in three
different numbers ranging from 7.2 to 9.3, thereby displaying inconsistency even though the
battery was fully recharged. The output energy is fairly consistent with all ranging above 90
watt-hours. Ignoring the inconsistent input energy in amp-hours and taking account of the power
and output energy, it was proven that higher current draw results in less capacity.
58
Table 20
ValuCraft Battery Input and Output Energy
Input
Energy
Load
AC
(amps)
voltage
(A)
(B)
Power (Watt)
Duration
Output Energy
(H:MM)
(Watt-Hours)
(D)
(C*D)
(Amphours)
(C ) = (A * B)
9
0.76
118.5
90.06
1:00
90.1
9.3
0.58
118.5
68.73
1:25
97.4
7.2
0.58
118.5
68.73
1:25
97.4
Output (Watt-hour)
ValuCraft: Output Current vs Capacity
98
97
96
95
94
93
92
91
90
89
0.5
0.6
0.7
0.8
0.9
1
Output Current (Amp)
Figure 23.ValuCraft Power vs capacity graph.
4.5.2. The battery bank (Kirkland & EconoPower) load test. The Kirkland and the
EconoPower batteries were connected in parallel to double the size of the capacity while
maintaining the voltage of a single 12-V battery. The battery bank was fully charged and
subsequently fully discharged to measure the amp-hour and watt-hour capacity of the battery
bank.
59
Table 21
The Econopower and Kirkland Battery Bank Input and Output Energy
Date
Input
Energy
Amp-hours
Load
(amps)
AC
Voltage
(V)
Power
(Watt)
Duration
(H:MM)
Output
Energy(WattHours)
(A)
(B)
(C = A * B)
(D)
(C*D)
3/17
37.0
0.92
118.5
109.02
4:45
517.8
3/18
43.5
0.92
118.5
109.02
4:30
490.6
3/19
41.7
0.92
118.5
109.02
4:20
472.4
3/25
49.2
0.67
118.5
79.395
6:10
489.6
3/26
44.0
0.67
118.5
79.395
5:53
467.1
3/28
30.0
0.67
118.5
79.395
5:45
456.5
3/31
26.4
0.67
118.5
79.395
4:00
317.6
The most conspicuous trend evidenced in the data is the apparent decline of capacities, both input
amp-hours and output watt-hours. Input amp-hour is inconsistent and erratic, but the trend line
shows that the input amp-hour is declining. The output energy shows a more clear and consistent
decline. Output capacity here means how much energy the battery bank was able release to
power the loads when the battery was fully charged by solar modules. Inthe two week testing
period, it was apparent that the battery bank was losing its maximum potential of storing
electricity. The rapid decline in storage capacity can be attributed to the fact that the battery bank
—made of car batteries, not deep discharge batteries— was graduallydischarging to a complete
discharge state. From the literature review, it was seen that a LAB completely discharged each
time only has a service life of 16-20% compared to a battery that was only discharged to 30%
DOD for each discharge cycle. If a battery bank of similar energy capacity is regularly used in a
home to provide nocturnal energy, the use should be limited to 1 hour to 1.5 hours to elongate the
service life of the battery bank.
60
55
550
50
500
45
450
40
400
35
350
30
300
25
250
20
16-Mar
18-Mar
20-Mar
22-Mar
24-Mar
26-Mar
28-Mar
30-Mar
Output Energy (Watt-hours)
Input Energy (Amp-hours)
Battery Bank Input vs Output Energy
200
1-Apr
Input Energy (Amp-hours)
Output Energy (Watt-Hours)
Linear (Input Energy (Amp-hours))
Linear (Output Energy (Watt-Hours))
Figure 24. Decline of input and output capacities
4.6. Efficiency: Input energy vs output energy
Siemens did not disclose an efficiency rate of the SM55 PV modules, but it can be
calculated using other known specifications. The SM55 PV modules are rated to produce a
maximum of 55 watts. The PV modules are rated under standard test condition (STC) of 1kW/m2.
That means solar insolation has 1000 watts of power in an area of 1m2. (El Chaar, L. ,lamont, L. ,
& El Zein, N. 2011). The module efficiency, ŋ, is given by:
ŋ=
FF ×V oc ×I sc
Pin
FF is fill factor that measures the junction quality and series resistance. It is given by:
FF =
The specification of SM55 module is:
Vmp × Imp
Voc × Isc
61
Table 21
SM55 Electrical Parameters
Maximum Power Rating (Pmax)
[W]
55
Rated Current (Impp) [A]
3.15
Rated Voltage Vmpp [V]
17.4
Short circuit current Isc [A]
3.45
Open circuit voltage Voc [V]
21.7
FF =
ŋ=
17.4 V ×3.15 A
21.7 V × 3.45 A
= 0.732
0.732 ×21.7 V ×3.45 A
1000 W
= 0.055
The efficiency of 0.055 is equal to 5.5%, meaning that per 1,000 watts/m2 of solar power,
PV cell is capable of producing 55 watts. This matches the manufacturer specifications of 55
watts of maximum power. The area of one SM55 module is 0.4m2 so the area for five module
array is 2m2. Since one module can produce 55 watts of power, a five module array can produce
275 watts of power. The solar irradiance incident on 2m2 in test condition is 2000watts per 2m2
(1000 watts/1m2).
Thus, the PV module efficiency of the system is:
PV module efficiency (ŋ) =
275 𝑤𝑎𝑡𝑡𝑠
2000 𝑤𝑎𝑡𝑡𝑠
= 13.75%
13.75% is the maximum efficiency of the PV modules of the system. The highest
observed power from the 275 watt array was 120 watts. 120/275 = 43.6%. This lower than
expected percentage may be attributable to the Siemens modules’ old age, because PV cells lose
efficiency over time. Also, the operating condition are not the same as the STC, and usually less
favorable. So the realistic efficiency of the modules will be 13.75% X 43.6% = 6%
62
To calculate the total efficiency, all the component efficiency must be multiplied in this matter:
Total system efficiency (ŋ) = Module ŋ X Wiring ŋ X Inverter ŋ X Battery ŋ
Total system ŋ = 6% X 90% X 90% X 85% = 4.1%
Wiring efficiency, also referred as derating in the electrical industry, is established as an
industry standard at 90%. When electricity travels through wires, it loses some electricity in the
from ofradiating heat, retaining approximately 90% of the originating energy. De-rating has a
bigger influence when the wires are larger and longer, but in this small setup, 90% is an adequate
figure. Inverter efficiency is from the manufacturer data, and battery efficiency is 85% which is
12V divided by 14V, because the battery recharges at 14V, but discharges at 12V. At the location
and season of the experiment, the realistic PV module efficiency of the system was 6%. The total
system efficiency is 4.1%. This means that for every 1000 watts of solar irradiance input, 41
watts are converted as useful power.
Insolation is the sum of all incident solar energy (watt-hours) for the specific recharging
period on the 2m2 PV module array. Hour-by-hour solar insolation data isavailable from the
California Irrigation Management Information System (CIMIS). Cal Poly Pomona has an oncampus weather station as part of CIMIS that provided accurate local insolation data for this
experiment. The complete data set can be found in the appendix. The output energy column is
the measurement of energy (watt-hours) from the battery. For this experiment, a load test was
conducted as soon as the recharging occurred. The calculated efficiency of the entire system is
4.1%. All experiments ended in a lower than calculated system efficiency except for one sample.
63
Table 22
System Efficiency Comparing Solar Input to Battery Output in Watt-Hour.
Battery
Eco+
Kirk
Eco+
Kirk
Econo
Power
Kirk
land
Valu
Craft
Date
Insolation
(watt-hr)
Solar
Input
(amp-hr)
Charge
(Hour)
Avg
amp
/hr
Output
energy
(watt-hr)
System
Efficienc
y (sun to
load)
Wh
/ Ah
3/18/14
17,650
43.5
6.5
6.7
491
0.028
11.3
3/21/14
6,999
24.2
4
6.1
425
0.061
17.6
4/3/14
7,605
15.2
2.3
6.5
124
0.016
8.2
4/4/14
4,601
12.1
3.75
3.2
146
0.032
12.0
3/27/14
5,643
5
2
2.5
93
0.016
18.5
ValuC
3/27/14
2,376
7.6
3.5
2.2
75
0.032
9.9
ValuC
3/31/14
7,178
5.4
2
2.7
97
0.014
18.0
64
5. Conclusion
The electrical energy storage known as the battery powers people’s lives in this modern
era. Smartphones, laptop computers, automobiles, watches, and manyother electrical devices
dependon reliable batteries to function properly. Batteries no longer just crank combustible
engines, as in the 19th century; they now allow cars to be driven for hundreds of miles without
using a single drop of petroleum based fuel. The next big frontier for electrical energy storage is
reliably powering even bigger effects, like homes and commercial buildings.
Off grid buildings have utilized generators, solar modules, and batteries for power
twithout a connection to the grid, one of the biggest infrastructure systems that has ever been built.
Grid connected homes and buildings used energy from that grid. Being connected to the grid,
with its centralized electricity generation, was easy and cheap, but also necessary because there
was no other ways to source electricity. Distributed generation is the local generation of energy
from multiple sources such as: photovoltaic modules, wind turbines, biogas, etc. With maturing
technology, the relative ease of installation, and competitive price, solar power has arisen as the
next generation of a renewable distributed energy source.
Energy storage devices are essential in building PV power systems. The time between
power generation and use of energy is bridged by energy storage devices. PV modules depend on
diurnal solar radiance for electricity generation, but energy use may be extended deep into the
night. Batteries as energy storage devices store unused electricity and dispense it at the time of
use. Depending on weather conditions, solar radiance can cause irregular and unstable power
productions so batteries can also act as power conditioner to smooth out electrical power surges.
From a diverse range of batteries, the lead-acid battery (LAB) is the most affordable,
technologically mature, and easy to access / purchase. The two main types of LABs are starting,
lighting, ignition (SLI) and deep-discharge. SLI batteries are used to crank engines in vehicles
65
and deep-discharge batteries power golf carts, boats, and buildings. Deep discharge LAB looks
similar to SLI battery but has thicker lead plates to withstand deep discharges. SLI batteries have
thinner plates and can pack more plates into a limited space, because greatersurface area is
needed for higher instantaneous cranking amps, i.e. to crank a car’s engine. Because of this
difference in design, SLI batteries have not been used for purposes other than automotive.
Due to the ubiquitous nature of cars, there are so many SLI batteries in circulation. Due
to the toxic nature of lead and sulfuric acid, the two main components of a LAB, almost all LABs
are recycled. Before they are recycled, it is worth questioning whether there are potential
alternative uses for used car batteries. In comparison to SLI batteries, deep discharge batteries
are expensive and not always accessible. Whether it is geographic or economic reasons, SLI
batteries are difficult to access in some regions. The PV power and battery industry recommend
deep-discharge batteries for residential and building energy storage. If only some used SLI
batteries are available, how will they perform as a building energy storage device? How would
someone check for the health of used SLI batteries? Leaving a dome light on overnight drains a
car battery so how much energy can a SLI battery truly provide?
To answer these questions, three SLI batteries and one dual purpose battery was
purchased from a local junkyard to test how much potential they have as residential energy
storage devices. They were subject to several tests to determine their electrical potential. The
tests conducted in chronological order are: voltage test, hydrometer test, capacity test, amp-hour
capacity algorithm calculation, and field capacity test by connecting the batteries to an off-grid
PV power system.
All four batteries successfully passed the voltages test, designed to check whether
recharging batteries with a trickle charger results in voltage rise. The hydrometer is a device that
measures acid concentration by testing the specific gravity of sulfuric acid of battery’s six cells.
66
The specific gravity of each cell should be similar to each other,soa large deviation from the
mean of any single cell means a potentially dead cell. EverStart battery had a couple of cells that
had large deviations from the average specific gravity reading, indicative of a bad battery. As
expected, the EverStart battery failed to hold charge when it was recharged and discharged under
a load. The other three batteries successfully completed a recharge and discharge cycle. From
the capacity test, the Kirkland and EconoPower batteries were able to store similar amount of
energy at above 200 watt-hours (Wh). The ValuCraft battery was able to store 90 Wh of energy.
This test also established a more accurate baseline capacity for each used battery. Mahmoud’s
algorithm for calculating amp-hour capacity by measuring battery voltage and cell’ specific
gravities was used to estimate amp-hour capacity of each battery. The ValuCraft, EconoPower,
and Kirkland batteries at fully recharged state all had around 70% state-of-charge compared to the
manufacturers’ specifications. The last part of the experiment was to install batteries in the TJ
House PV power system and check performance by recharging it with solar power and
discharging it with a load.
The last test was conducted to observe how much solar energy is converted and stored in
LABs. After recharging, load tests were conducted to measure how much energy was discharged.
The charge controller that charges and regulates the batteries also functioned as an indicator to
display amp-hours energy flowing into the storage batteries. This amp-hour energy intake
showed the true amp-hour capacities of these batteries. Before using the charge controller, there
was no reliable method to measure amp-hour capacities. This test revealed that the Kirkland
battery could take up to 23.6 amp-hours of electricity. The EconoPower was able to store 20.3
amp-hours while the ValuCraft battery only had a capacity of 9 amp-hours. This test highly
diminished the validity and reliability of Mahmoud’s amp-hour calculations. The main reason
behind the over amplification of Mahmoud’s algorithm in this case may be attributed to lack of
more professional equipment that will safely measure specific gravity under load.
67
The Kirkland and EconoPower batteries had approximately 50% life remaining and the
ValuCraft battery had about 25% capacity remaining. Unfortunately, batteries lack a black box
that tracks the history of the battery. It is completely unknown how these batteries have been
used in the past, but to mitigate this, a series of test was conducted to gauge their current
conditions. The battery bank formed by combining the Kirkland and EconoPower batteries in
parallel circuit lasted 4 to 5 hours with two lights and a fan operating full time. This is a quite
high state of performance, given that the source is two used batteries that had been discarded for
no future use. It was unfortunate that a more demanding load could not be tested, but this
experiment shows that batteries are a sufficient backup energy source to charge phones, laptop
computers, and a radio in the event of an electrical outage. If used in third world countries
lacking a reliable centralized electrical distribution system, these used car batteries can offer the
comfort of lighting for several hours into the night.
To balance the service life and daily usage hours, the user of LAB energy storage system
must have a good awareness of the system specification and functionality. The service life of
LAB decreases to 20% if they are fully discharged in each cycle. Discharging the battery to 30%
DOD yields about 5 times more service life compared to a 100% DOD. In order to maximize the
useful life and daily usage, the user must calculate the daily usage and design a system where the
daily usage is approximately equal to 30% of the battery’s energy capacity potential.
5.1. Limitations
The test was limited by electrical equipment used to conduct testing. The Pentametric
monitoring system tracks amperages and voltages of a circuit that can simultaneously track
multiple positions within a circuit. This equipment would have enabled monitoring and tracking
amperages easily in every recharging and discharging process. Without this equipment, there
68
were limitations in calculating the exact quantity of input and output energy that the experiment
attempted to attain.
The other major piece of equipment that limited the range of the experiment is the
inverter. High power inverters are expensive, so the inverter used in this testing was unable to
handle a large amperage load such as a water heater, electric pan, and microwave oven. Using a
large load like a water heater or microwave oven can increase the load significantly and thus
would be able to test for more diversified conditions. The 800-watt inverter lacked the capability
to handle such large loads and limited the span of this study.
In addition, a safe and more professional hydrometer that can accurately measure specific
gravity under load would add significance to Mahmoud’s algorithm testing. Opening the cover
and drawing sulfuric acid under in the process of discharging under load prevented following the
exact algorithm process of Mahmoud’s study. As result, the only meaningful data to measure
amp-hour capacity was in a fully charged and open circuit state (zero load condition).
The experiment would be more useful with enough charge and discharge cycles to
observe the drop in energy capacity of each battery. The study that focused on cycling tests of
various types of batteries was conducted by the International Energy Agency in 2002 (IEA, 2002).
The limited sample size is another limitation of the study. Lead-acid batteries are
uniform in design and operation, with variations in performance from amount of raw materials.
Sample size would be a greaterconcern in a qualitative study based on interviews, but the
batteries also are affected by other factors such as the operating and environment temperature,
age, prior usages, so a greater sample size would further the benefits of this study.
69
6. References
Averill, B., Eldredge, P., (2011), Commercial Galvanic Cells .In General Chemistry: Principles,
Patterns, and Applications (19.5). Retrieved from
http://catalog.flatworldknowledge.com/bookhub/4309?e=averill_1.0-ch19_s05.
Buchmann, I., &Cadex Electronics Inc. (2001). Batteries in a portable world: a handbook on
rechargeable batteries for non-engineers. Cadex Electronics.
Darden, B., (1998, November).Car battery frequently asked questions 2.2. Retrieved from
http://www.rbsp.info/rbs/PHY337/lab1/b05.html.
Delpierre, G, Sewell, B. (1992-2006). The Lead-Acid Accumulator. Retrieved from
www.psychem.co.za/Redox/cells.htm
Dyke, A. L. (1920). Text Book for Dyke's Home Study Course of Automobile Engineering.St.
Louis , MO; AL Dyke.
El Chaar, L., & El Zein, N. (2011). Review of photovoltaic technologies.Renewable and
Sustainable Energy Reviews, 15(5), 2165-2175.
Fasih, A. (2006). Modeling and fault diagnosis of automotive lead-acid batteries.Retrieved from
http://kb.osu.edu/dspace/handle/1811/6534.
Gustavsson, M., &Mtonga, D. (2005).Lead-acid battery capacity in solar home systems—Field
tests and experiences in Lundazi, Zambia. Solar Energy, 79(5), 551-558.
Hansen, R. (1999). Valve-Regulated Lead-Acid Batteries for Stationary Applications.Department
of the Air Force. Air Force Pamphlet 32-1186.Retrieved from
http://www.wbdg.org/ccb/AF/AFP/afpam32_1186.pdf
Herrmann, B., Karl, H., Kopf, E., Lehner, G., &Saupe, G. (1987, January).Dynamic behaviour of
lead-acid batteries with special respect to photovoltaic applications.In Seventh EC
Photovoltaic Solar Energy Conference (pp. 231-235).Springer Netherlands.
70
Hua, S., Zhou, Q., Kong, D., & Ma, J. (2006).Application of valve-regulated lead-acid batteries
for storage of solar electricity in stand-alone photovoltaic systems in the northwest
areas of China. Journal of power sources, 158(2), 1178-1185.
Huacuz, J. M., Flores, R., Agredano, J., &Munguia, G. (1995).Field performance of lead-acid
batteries in photovoltaic rural electrification kits. Solar energy, 55(4), 287-299.
International Energy Agency.(2002). Testing of batteries used in Stand Alone PV Power Supply
Systems. (Rev.:0.36).
Murphy, T., (2012, July 24). My modest solar setup.Retrieved from http://physics.ucsd.edu/dothe-math/2012/07/my-modest-solar-setup.
Murphy, T., (2012, September 18). Blow-by-blow PV system efficiency: A case study for storage.
Retrieved fromhttp://physics.ucsd.edu/do-the-math/2012/09/blow-by-blow-pv-systemefficiency/.
Panasonic Corporation. (n.d.)VRLA Handbook. Retrieved from
http://industrial.panasonic.com/eu/i/21291/VRLAHBInteractive/VRLAHBInteractive.pdf
Pascoe, P. E., &Anbuky, A. H. (2004). VRLA battery discharge reserve time estimation. Power
Electronics, IEEE Transactions on, 19(6), 1515-1522.
Picciano, N. (2007). Battery Aging and Characterization of Nickel Metal Hydride and Lead-Acid
Batteries (Master’s Thesis).Retrieved from
http://kb.osu.edu/dspace/handle/1811/25087.
Ruetschi, P. (2004). Aging mechanisms and service life of lead–acid batteries.Journal of power
sources, 127(1), 33-44.
Shumay Jr., W., (2005).Using the battery hydrometer. Retrieved from
arconequipment.com/article_0995.html .
Siemens Solar.(1998). Solar Module SM55 manual. Rev. C.
71
Zhou, W., Yang, H., & Fang, Z. (2008). Battery behavior prediction and battery working states
analysis of a hybrid solar–wind power generation system.Renewable Energy, 33(6),
1413-1423.
72