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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). 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