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`Training: Basic
Course : Electricity
Module: Solar Energy
Item: Solar Photovoltaic Technology
Detail: Batteries
Code: 4.3-6.2
Prepared by: Mr. Kajjaku W
BATTERIES
A solar module produces electricity during daytime when the sun is shinning. the modulecannot store
energy. when the electricity should be used during the night, it must be stored for example in
batteries. Other forms of storing energy are for example compressed air,fuel cells or pumped water.
Battery principle
Fully charged batteries have negative plates consisting of sponge lead (Pb) and positive plates
consisting of lead Dioxide( PbO2). These plates are submerged in an electrolyte solution of sulphuric
Acid (H2SO4).
When discharging a battery ,the sulpur leaves the solution and moves over to the plates. when it gets
there, it gives up an electron and moving electrons are electricity. when recharging a battery, the
sulphur is driven back into the solution.
charging
Pb + PbO2 + 2 H2SO4
2 PbSO4 + 2 H2O
Discharging
TYPES OF BATTERIES
a)
b)
c)
d)
NICaD ( Nickel Cadmium)
Lead – Acid batteries
Gel batteries
Absorbed Glass Mat( AGM)
The most common types of second ary battery systems on the world market today are ( a) and (b)
above. Nicad batteries (sometimes incorrectly alkaline ) are more expensive and less efficient per unit
of storage than lead –acid batteries,for this reason most installations choose lead acid batteries.
Lead –acid cells
It is generally agreed that lead-acid batteries are the most readily solution available to the problem of
storing PV charge. There are a number of types of lead acid batteries,which fall in two general
categories namely:✓ shallow discharge
✓ deep discharge
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Shallow discharge
The most common type of a shallow discharge is a starter battery. They are meant to get combustion
engines going. They have many thin lead plates which allow them to discharge a lot of energy very
quickly for short amount of time. However, they do not tolerate being discharged deeply, as the thin lead
plates needed for starter currents degrade and re-charging cycles. They are not the best choice for energy
storage for solar electric systems
Deep discharge
Deep cycle batteries have thicker lead plates that make them tolerate deep discharges better. They cannot
dispense charge as quickly as a starter battery but can also be used to start combustion engines. The
thicker the lead plates, the longer the life span, all things being equal. Battery weight is a simple indicator
for the thickness of the lead plates used in the battery. The heavier the battery for a given group size, the
thicker the plates, and the better the battery will tolerate deep discharges.
Batteries for Solar PV
The deep cycle batteries are most appropriate to use in solar electric systems. However, since deep cycle
batteries are more expensive than shallow cycle batteries .Many small solar systems use common car
batteries for energy storage.
Storage capacity
The capacity of a battery to store charge is expressed in ampere hours (Ah). If a battery can provide one
ampere (1A) of current flow for one hour, it has a real –world capacity of 1 Ah. If it can provide 1A for
100 hours, its capacity is 100 Ah. Likewise, 20 A for 2 hours equals 40 Ah capacity.
✓ Manufacturer’s rate their batteries based on the tests performed over 20 hours with a discharge
rate of 5% of the expected capacity an hour.
✓ The efficiency of the battery is different at different discharge rates. If the current is drawn from
the battery at a higher rate, the capacity is reduced.
✓ The capacity of a lead acid battery is also reduced with decreasing temperatures. At 0 degrees
centigrade, the charge is reduced by 20%.
State of charge
The state of charge is a measure of the energy remaining in the battery. The state of charge of a lead-acid
battery can be measured with:• a Hydrometer
•
a Voltmeter
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A hydrometer measures the density of the sulphuric acid electrolyte in each cell (also referred to as
specific gravity), which is directly related to the state of charge of the battery’s lead-acid batteries are
discharged, the sulfuric acid within each cell is converted to water ,which has a lower density than
sulphuric acid. Thus the specific gravity of the cell decreases. a hydrometer is being used as follows:
•
•
•
•
Draw sulphuric acid into the hydrometer
The scale floats at a level according to the density of the acid
Read the specific gravity of the cell from the scale floating in the acid
Translate the specific gravity into a state of charge using the table below
State of Charge
100%
75%
50%
25%
0%
Specific Gravity
( grams/liter 250 C)
1265-1275
1225-1235
1190-1200
1155-1165
1120- 1130
As the state of charge of a lead –acid battery decreases the voltage of the battery also decreases and can be
measured with a voltmeter. To measure the state of charge with the voltmeter :
• Disconnect the load and the solar charge from the battery
• Wait for 20 minutes to allow the cell voltage to stabilize, immediate measurement will be
inaccurate.
• Connect the voltmeter’s leads to positive and negative terminals of the battery.
• Read the voltage and translate into a state of charge using the graph below
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Cycle
A battery in a solar electric lighting system is charged each day by the solar cell module and then
discharged by the load each night. Each charge period together with the following discharge period is
called a cycle. for example, in one cycle a 100 Ah battery might be charged up to 95% state of charge
(12.68 V) during the day, and then discharged by lights and television to 75% state of charge (12.42 V)
that evening.
The rated cycle life of a battery is the number of cycles a battery is expected to last before its capacity
drops to 80%.
Charge and Discharge
The charge current of battery of should not be higher than 10% of the rated capacity, thus a 50 Ah battery
should not be charged at a current of more than 5A. When batteries are charged with a current exceeding
10% of their capacity, the electrolyte level drops because of gassing and the cells can be damaged. Low
currents are more efficient than higher currents for charging batteries.
Depth of discharge
The depth of discharge indicates how much a battery is discharged in a cycle before it is charged again.
Shallow cycle batteries should not be discharged below 80% state of charge. Deep cycle batteries should
not regularly discharged below 40% state of charge.
Example: If the state of charge of a 100 Ah battery (12 V) is rated at 75%, the depth of discharge is 25%.
Self discharge
Is the tendency of batteries to lose charge when they are left without being charged. The rate of
discharged depends on the age, condition and type of battery.
Overcharging
Overcharging of batteries on a regular basis causes water to be ‘boiled’ out of the electrolyte and will
eventually warp the plates from over heating and corrode away the positive plates.
Gassing begins when the batteries have reached a state of charge where they can only accept a small
amount of amperage. Trying to push more amperes into the battery than the battery can accept, these extra
amperes split water molecules into hydrogen and oxygen gas.
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The main requirements to be met by a storage battery for storage battery for solar power system are:
• Ability to withstand several charge /discharge cycle
• A low self- discharge rate
• Little or no need for maintenance
Maintaining batteries
Batteries will last between 2 and 10 years or even longer if properly maintained and managed.
maintaining
batteries involves the following tasks:
• Checking the state of charge regularly
• Checking the electrolyte level in each cell and replacing the lost water only with deionized water, if de-ionized water is not available, rain water can will do but never use tap
water.
• Cleaning the top of the battery prevents a higher rate of discharge
• Cleaning the terminal and contacts ensures a good electrical contact
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