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6. Batteries and Controllers
Herb Wade
Consultant
Solar PV Design Implementation O& M
March 31- April 11, 2008
Marshall Islands
Marshall Islands March 31-April 11, 2008
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6. Batteries and Controllers
• Contents
6-1. Batteries for Solar Systems
6-2. Controllers
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6-1. Batteries for Solar Systems
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Types of Batteries
• Lead-Acid
– Cheapest, mature technology, readily available in a wide range
of types
– Easily damaged by improper discharge control, some types
require periodic maintenance
• Nickel-Cadmium
– Expensive, mature technology, not readily available
– Not sensitive to overcharging and high discharge levels
– Long life, minimal maintenance
• Other
– Under development for electric and hybrid cars. Main
advantage light weight and high energy density, no
maintenance, not touchy regarding charging and discharging.
Expensive.
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Lead Acid Battery Construction
• Basic requirement: To allow the reversible chemical
reaction that absorbs then releases electricity to function
efficiently:
Pb + 2H2SO4 + PbO2
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Lead
-
Sulfuric Acid
Lead Oxide
+
PbSO4 + 2H2O + PbSO4
Lead Sulfate
-
Water
Lead Sulfate
+
Lead is a metal. Lead oxide is a hard, black gritty solid.
Lead Sulfate is a softer whitish solid. Sulfuric Acid is
a liquid as is water.
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Cycle life
• A basic measure of the life of a battery for any
application is its rated cycle life, a characteristic
provided by the manufacturer based on actual tests by
cycling batteries
– The number of full charge/discharge cycles that a
battery can provide before losing 20% of its rated
capacity
 A battery slowly loses capacity as it ages but
once about 20% of capacity is lost the ageing
process accelerates and the battery quickly fails
completely. So a battery is considered to be at
the end of its useful life when its capacity has
decreased to 80% of its rated capacity
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Cycle life (2)
 Partial cycles add up to make full cycles. So five
20% discharge cycles = 1 full cycle. Ten 10%
cycles = 1 full cycle, etc. So in theory a battery
that is always discharged 20% each day and has
a cycle life of 1000 cycles will in theory last
1000/0.20 = 5000 days (over 13.5 years). But
cycle life usually represents the maximum life
not average life so actual life usually is less than
indicated by cycle life. Still cycle life is a good
indicator of the comparative life of batteries. A
100 cycle battery probably will only last half as
long as a 200 cycle battery.
 Cycle life changes with rate of discharge. Cycle
life at C10 is much less than at C100 for example.
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Cycle life relative to DoD
• For a high quality solar battery, if the average DoD is
80% the cycle life is rated at 600 cycles (about 2 years)
• For an average DoD of 40%, the cycle life is rated at
1450 cycles (about 10 years)
• For an average DOD of 20%, the cycle life is rated at
2000 cycles or more than 25 years.
Unless maintenance is excellent and the system very
well designed, it is possible for a very good battery to
last longer than 15 years in solar service and more than
20 years in stationary backup service. So cycle life
should be specified to allow it to survive at least that
long at the average depth of discharge for the solar
system.
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Internal construction
• Determined by the type of use and cost
– Type of use mainly the speed that the battery must
deliver and/or accept power and depth of discharge
 1. Starting battery: Very high current (speed of
energy delivery) for a short time. Never
discharged more than 1% or 2%
 2. Traction battery (like for electric car or boat
motor): Medium current delivery for medium
time. Often discharged 50%-80%
 3. Solar battery: Slow current delivery for long
time. Sometimes discharged 80%, mostly 20%30%
 4. Backup battery: Most of the time kept at full
charge then must reliably deliver energy to
operate equipment (telecom, UPS, etc.) to deep
discharge
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Postive plate construction
• Flat plate: Large surface area in contact with acid
allows fast chemical reaction and high current delivery
– Need lots of surface in contact with electrolyte to
allow chemical reaction to work. Tiny grains of lead
oxide (like sand) packed (pasted) into a lead grid.
Huge surface area but grains deeper into the pack
are harder for the electrolyte to get to quickly.
– The more external surface area the more current so
for starting batteries that need high current, many
thin plates are used. But fragile and high surface
area also means high probability of loss of grains of
lead oxide from the plate surface
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Positive Plate Construction (2)
• Tubular plate
– Lead oxide grains packed in a tubular shape around
a central electrode inside a porous tube. Minimal
surface area to lose grains of lead oxide and held in
place by tube so the battery has a long life but
because electrolyte is slower to penetrate the thick
layer of active material high current cannot be
maintained. Best for lower current but long times of
discharge. Excellent for deep discharge applications
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Internal changes with charging
• Lead sulfate is a larger molecule than lead oxide so
when discharging and lead oxide is converted to lead
sulfate the material swells. The difference in bulk
between charged and discharged plates is around
10%.
– Deep discharge of flat pasted plates causes
swelling that can push grains of active material off
the surface of the plates and they then fall to the
bottom of the battery and are no longer available to
be charged or discharged. Causes loss of Ah
capacity
 Can be reduced by putting porous sheets over
the plate surface but that slows down rate of
chemical reaction and reduces the maximum
current the battery can produce
 Starting batteries are therefore quickly damaged
by deep discharge
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Starting battery
• Starting battery: Large number of thin plates to maximize
the rate of the chemical process and therefore to instantly
produce high current for engine starting. Only produces
current for a few seconds so the total depth of discharge
(DoD) is normally 1% or less. So swelling of the plates is
minimal and few problems with loss of surface grains.
– If used in solar with DoD of 20%-30% there is
substantial swelling and the large surface area of the
many thin plates allows rapid loss of grains from the
surfaces and a short life (6 mo to 2 years according to
the quality of construction of the battery)
– Type of use effectively C0.2 I.e. very high rate of
discharge though for a very short time usually
– Cycle life only 5-50 according to quality of construction
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Traction battery for solar use
• Traction batteries are used for electric vehicles like golf
carts, small boats (trolling), industrial fork lifts, etc. Fairly
high current but much less than starting motors. Need to
deliver energy as long as possible between charges so DoD
of 80% is common.
– Flat plates to provide fairly high surface area and high
enough current but better batteries can use porous sheet
to reduce surface grain losses
– Much thicker plates than starting battery since lower
current needed. The longer time it takes for electrolyte to
get to internal grains in the plate (and for the water that
is produced to get out) the lower the current production.
But thicker plates have less surface area for loss of
active material grains so longer life
– Typical type of use is C5 to C10 so the motor can run all
day before the battery is discharged.
– Typical cycle life 200-500 according to quality of
construction and rate of discharge
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Tubular positive plate batteries
• Relatively low maximum current capability because
getting electrolyte to interior grains in the tube is fairly
slow
• Least possible surface area for losing grains of material
so very long life
• Physically bigger than flat plate batteries because
tubular construction takes more internal space for the
same amount of active material
• Best for C20-C100 applications. Modest current delivered
over a long period - e.g. most solar systems such as
SHS and remote telecom power
• Cycle life 500-3000+ according to quality of
construction and rate of discharge
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Back up batteries
• Sometimes called “stationary” batteries. Used as an
emergency power source if the main source fails.
Typically kept at full charge for long periods but then
may be deeply discharged
– Telecom back up batteries, UPS batteries for
computer power backup, electronic control
equipment backup, etc.
– Service life based on frequency of power outages.
Typically very long service life relative to other
applications. Cycle life not very relevant since
batteries are not normally cycled between charge
and discharge. Life largely determined by resistance
to sulfation, water loss rate and resistance to
internal corrosion.
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General Battery Characteristics
• Nominal voltage (number of 2V cells)
• Capacity in Ampere hours
• Open cell or sealed
• Liquid or Gel electrolyte
• Cycle life
• Acceptable repeated depth of discharge
• “Starting Amps” “Number of Plates” or “Starting
Minutes” not useful for solar specifications
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The effect of discharge rates
• The more hours taken to discharge a battery, the more
energy can be transferred because with a slow
discharge the chemical process that produces
electricity is more efficient. So a battery delivers more
Ah at C100 than at C10 by a quite significant amount.
Note that C100 means that the battery takes 100 hours
to discharge fully while C10 means it only takes 10
hours to discharge fully. A 100Ah battery at C100 may
become a 65 Ah battery at C10 discharge rate.
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Battery Ah ratings
• A 100 Ah battery at C100 may be a 65 Ah battery at C10. So to
compare batteries, the battery Ah rating must include the Cx rate
for the stated capacity and you must compare at the same Cx rate.
• Manufacturers of solar batteries, particularly those of questionable
quality, often give Ah ratings using a C100 discharge rate. That
gives a substantially inflated Ah value but that capacity is never
reached in practice.
• Always compare battery capacities at the same discharge rate,
preferably C10 or C20 (C20 represents the typical solar discharge
rate for SHS and is best though C10 comparisons are commonly
done and are ok. Just be sure all comparisons are at the same Cx
rate.)
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Physical strength of construction
•
For longest service life, pure lead is the best material for making
negative plates and for other components internal to the battery.
Pure lead batteries have the lowest water loss rate of any leadacid battery and the fewest problems with internal corrosion.
– Unfortunately pure lead is very soft and not physically strong.
So for batteries that will be in vehicles or that will have to
survive rough transport, pure lead batteries are too weak
internally and vibration and physical shocks cause internal
damage.
– Most batteries have either antimony or calcium added to lead
to make it stronger
 Antimony added to the lead provides long cycle life but
tends to cause a battery to lose water faster than may be
desirable
 Calcium added to the lead causes a shorter cycle life but
very low water loss
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Battery uses vs lead additive
• For stationary (backup) batteries that have to have very
low water loss and very long service life, pure lead is
usually used but they are sensitive to shock and
vibration so have to be handled carefully, They have
the longest cycle life.
• For sealed batteries that have to have very low water
loss and need to be able to withstand shocks and
vibration, calcium is often added to the lead structure.
The disadvantage is that the cycle life is reduced.
• For open cell batteries where water can be added to
cells when electrolyte levels fall, antimony is often
added to the lead structure. The disadvantage is that
the water loss is higher than either pure lead or calcium
added to lead. The cycle life is about as long as pure
lead however.
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Measuring the level of charge
• Battery voltage. About 10.5V represents full discharge.
Full charge voltage for a battery with no current flow in
or out will be around 12.6V. When charging, full charge
is reached at about 14.2V
• Electrolyte specific gravity. This is an indication of
acid concentration and is measured using a
hydrometer. The higher the value the greater the
charge (1.26 to 1.28 is full charge, 1.0 to 1.1 is about
fully discharged).
Neither battery voltage nor specific gravity is an
accurate measure of charge, especially in old batteries.
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Accuracy of charge estimation
• The use of voltage to determine level of charge is not
very accurate because the rate of charge or discharge
affects the voltage too. As the battery gets older the
accuracy of voltage readings as an indicator of state of
charge during charging or discharging gets less and
less because the battery’s internal resistance goes up.
• The use of specific gravity is accurate when a battery is
new but as the battery ages, the hydrometer tends to
show a lower charge than is actually present due to
increasing sulfation
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Voltage changes during charging
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Hydrometer measurement
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Procedure for Hydrometer
Measurements
• Clean the top of the battery with a rag and water
• Flush out the hydrometer with clean battery water
• Open one cell at a time placing the cap upside down on the top of
the battery. Never open all caps at once.
• Draw the cell electrolyte into the hydrometer so the float does not
touch the bottom of the hydrometer tube and read the value
• Return the electrolyte to the cell. Add water if needed.
• Replace the cell cap and open the next cell.
• Draw the liquid into the hydrometer and read. Repeat for all cells.
• After completing, flush out the hydrometer with battery water and
replace in shock proof carrier.
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Interpreting Hydrometer Readings
• The reading indicates the relative level of charge for the
cell. All cells should be about the same (within about
0.03 of each other)
• If any cells are significantly different from the others, it
is a bad sign. An equalizing charge should be made if
possible.
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Equalizing Charge in the Field
• Connect the battery directly to the panels (bypass the
controller)
• Disconnect all loads and ask the user not to reconnect
at night during the charging process
• Bring the battery to full charge
• Continue charging the battery for at least one bright sun
day after full charge is reached.
• Keep a close watch on water level and keep the cells
filled but do not over fill.
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Causes of Battery Failures
• Sulfation – Most common problem. Sulfation is where
part of the cell becomes resistant to charge. Caused by
the battery remaining at partial charge for long periods.
May be offset by equalizing charges when cells are
seen to have unequal specific gravity.
• Internal corrosion – results in high internal resistance
and open circuits. Caused by cheap design, adding
acid instead of water and stratification of the acid in
some types of batteries
• Internal shorts – results in one or more cells not
producing voltage. May be due to cheap construction,
overheating or mechanical damage
• Loss of active material from plates – caused by
excessive depth of discharge and age. Mostly a
problem with cheaper batteries.
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What is Sulfation?
• When a battery discharges, Lead Sulfate is created.
When the battery is recharged, the Lead Sulfate is
supposed to dissolve. But if the Lead Sulfate is not
dissolved after a week or so because the battery is not
fully charged, it tends to form a mass that is very
difficult to dissolve when charging does take place.
Over time the amount of Lead Sulfate increases and
the battery loses its ability to be charged fully. The
effect is a loss of capacity. A 100Ah battery may
become a 50Ah battery after serious sulfation has
occurred.
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What are the signs of sulfation?
• When battery voltage indicates a full charge but the
hydrometer reading indicates a partial charge, that is a
strong indicator that serious sulfation has occurred in
the battery.
• Lead Sulfate is white in color. When looking at the
plates in a battery, if the battery has been fully charged
and the plates look light in color or have white sections,
that is an indication of sulfation
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Failure modes of batteries
• Total loss of power. Zero volts, cannot charge. Caused
by an internal open circuit. This may be because of
corrosion eating through a cell connector or mechanical
damage
• Gradually decreasing capacity. The time to charge and
discharge gets shorter and shorter. Caused by sulfation
or loss of active material from the plates or both.
Accelerated by deep discharge conditions and
operation at partial charge levels for weeks at a time.
• Reduced voltage at full charge. Cannot get the battery
to charge to more than about 10V. Caused by a short in
a cell making one cell inoperative. Excessive discharge
and mechanical damage are typically the reasons for
this mode of failure.
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Main Causes of Early Battery Failure
(Open Cell Batteries)
• Use of wrong kind of battery
• Panel capacity inadequate so the battery does not
come to full charge regularly
• System design not based on lowest solar month
causing batteries to stay at partial charge condition
during low months
• Controlers not working properly
• Over use of electricity keeping the battery in a constant
state of partial discharge
• Inadequate or incorrect maintenance
• Addition of acid to cells instead of water
• Addition of impure water to cells
• High temperature of operation (35º or more)
• Too deep a discharge for the type of battery being used
• Mechanical damage caused by hammering on the
posts, lifting the battery by the terminals, jolting the
battery too much in transport.
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Main Causes of Early Battery Failure
(Sealed Batteries)
• Repeated overcharging (wrong controller setting)
• Failure to maintain a high average charge level thereby
encouraging sulfation
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• High operating temperature (over 30°C)
• Repeated deep discharge
• Mechanical damage caused by hammering on the
terminals, lifting the battery by the terminals, jolting the
battery too much during transport
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How are batteries damaged by excess
discharge?
• When a battery discharges the plates swell. The deeper
the discharge the more the plates swell. Batteries that
are not designed for deep discharge have flat plates
that have the active material pressed into pockets in the
plate. When the plates swell greatly due to deep
discharge, some of the active material is pushed out of
the pockets and falls to the bottom of the battery
causing loss of capacity and possible shorting of cells.
Batteries that are designed for deep discharge have
plates that have the active material wrapped in porous
membranes to prevent the swelling from causing the
active material to fall off. This adds considerably to the
cost but increased battery life.
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Time between charging for idle
batteries
Ambient Temperature during
storage
Maximum time before
recharge
20°C
30°C
40°C
6 months
4 months
2 months
If a battery is being stored fully charged, a high
temperature of storage means the battery must
be recharged more frequently than if a cooler
storage temperature can be maintained. Delaying
charging beyond these limits allows sulfation to
occur and a permanent loss of some capacity.
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Self-discharge
• The effect of storage temperature on self-discharge
percentage of high quality tubular cell batteries
The number in the table is the percent of charge lost at the given
time period and temperature. For example, a charged battery
stored at 40C for 4 months will lose 36% of its charge
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Storing dry charged batteries
• High quality dry charged batteries can usually be safely
stored for up to 2 years without significant damage
provided:
– Storage is at 25°C or cooler
– Cell seals remain in place (usually a special cell cap
or a tape seal over the cap vent hole)
– Storage is in original packing and batteries remain
upright in the normal operating position
• Shorter storage life will result if these conditions are not
observed. In particular breaking the cell seal or storage
at 35°C or higher will result in rapid deterioration of
batteries. NEVER store wet or dry charged batteries in
unshaded, unventilated containers.
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Filling dry charged batteries
• Fill slowly. It is advisable to take 30 seconds or more
per liter of electrolyte put in a battery. Small batteries
with more than one cell should have the liter split about
equally among all the cells.
• If the cells do not get hot to the touch, let the battery sit
for about two hours and then charge to full charge,
effectively an equalizing charge. It is important that
there be NO LOAD applied during the time of the first
charge. The service life of the battery may be seriously
reduced if a load it turned on before the first charge is
fully completed.
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Damaged dry charged batteries,
commissioning in the field
• If the cell gets noticeably hot to the touch, that is an
indication that the cell has been damaged by poor
storage conditions or excess time in storage. The
battery will have a drastically shortened life if it is not
properly charged after filling. Do NOT commence
charging sooner than 12 hours after filling
• For proper charging, connect directly to the solar
panels without a controller in the circuit and allow to
charge WITH NO LOAD APPLIED for at least two
weeks checking the battery water daily and replacing
any that has been lost.
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Damaged dry charged batteries,
commissioning with a charger
• If a mains or generator powered charger is available, a
shorter time can be taken for charging. Method one is
to charge at constant voltage of 2.40V per cell for 96
hours continuously or until the SG of all cells is the full
charge rated value.
Or method 2:
• Charge continuously at a constant current of between
0.02 C10 and 0.05 C10 for 10-12 hours or until the SG
of all cells is the full charge rated value.
Constant current charging can restore seriously
degraded dry charged batteries. NO LOAD SHOULD
BE APPLIED DURING THE CHARGING PROCESS.
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Choices for Solar Use
• Open cell batteries, either flat plate or tubular cell types,
provide the best value and longest life but electrolyte
levels have to be checked and water added when
needed
• Tubular cell, deep discharge batteries provide the
longest life and should be used where access for
replacement is expensive or very difficult
• Valve regulated, sealed batteries are only
recommended where there is no one to properly
maintain an open cell battery
• Worst choice is an automotive type “maintenance free”
battery.
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Increasing battery voltage
• Add cells or batteries in series. Increments may be 2V
(single cells for large batteries), 6V (three cells in one
case – medium sized batteries), 12V (six cells in one
case, smaller batteries). No problems usually develop
because of series connections though a battery with a
shorted cell can create overcharging conditions for the
rest of the cells because the lower battery voltage that
results from a shorted cell makes the controller think
that the battery still needs charging so the charging
current is not shut off when the good cells do come to
full charge. The result is excessive water loss from the
battery, a definite symptom of a shorted cell in an SHS
battery along with voltage that is lower than expected.
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Increasing Ah capacity
• Put in a larger battery. It is always preferred to use a
single large battery than to connect batteries in parallel
• It is possible to parallel identical batteries just as it is
possible to parallel panels. However never should more
than two batteries be placed in parallel and even then
do not expect as long a life as a single larger battery. If
one cell of either battery loses capacity, both batteries
may rapidly develop sulfation problems. Note that many
battery manufacturers void battery warranties if more
than two are paralleled.
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Large battery Bank
48V battery bank (Cook Islands)
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Battery label
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Battery characteristics from label
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Battery Safety
•
Most injuries relating to batteries are the result of dropping them or being
hurt somehow by their weight. Do not carry batteries by the connections,
always support the battery from the bottom or sides of the case. Preferably
use a special carry strap made for the purpose. For large batteries share
the load with another person. Many smaller batteries have built in handles.
Use them
•
For open cell batteries, note that the electrolyte is dilute sulfuric acid and
can cause mild chemical burns on the skin and is toxic if swallowed. If the
acid gets into your eyes, immediate flushing with water is vital to avoid eye
damage. For that reason, keep a full bucket of water nearby when working
with batteries and battery acid.
•
Be sure cell caps have clear ventilation holes. A plugged ventilation hole will
cause pressure build up in the cell and will cause the battery case to swell
and may cause damage to the battery.
•
Never lay tools on top of the battery. A short circuit could occur and may
damage the battery, cause an explosion or cause burns.
•
Do not smoke around batteries that are charging. Explosive gas is present
in the cells.
•
Do not take the caps off battery cells when charging.
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Don’t lay tools on batteries!
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When should you add more acid?
• Unless electrolyte is actually spilled out of the battery,
you should never add acid, only pure water. It is not the
sulfuric acid that evaporates, it is water only. Adding
more acid gradually increases the strength of the acid
and increases the rate of internal corrosion but in no
way increases the charge in the battery or makes it
easier to charge.
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