A battery is a device that converts chemical energy contained in its active materials directly into electrical energy by means of an electrochemical reaction.
Why do we need batteries for Solar photovoltaic system?
Batteries accumulate excess energy generated by the PV system and store it to be used at night or when there is no other energy input. Batteries can discharge rapidly and yield more current that the charging source can produce by itself, so pumps or motors can be run intermittently.
The battery’s capacity for holding energy is rated in amp-hours: 1 amp delivered for 1 hour = 1-amp hour
Battery capacity is listed in amp hours at a given voltage, e.g. 220 amp-hours at 6 volts. Manufacturer’s typically rate storage batteries at a 20-hour rate:
220 amp-hour battery will deliver 11 amps for 20 hrs
This rating is designed only as a means to compare different batteries to the same standard and is not to be taken as a performance guarantee. Batteries are electrochemical devices sensitive to climate, charge/discharge cycle history, temperature, and age. The performance of your battery depends on climate, location and usage patterns.
For every 1.0 amp-hour you remove from your battery, you will need to pump about 1.25 amp-hours back in to return the battery to the same charge state of charge. This figure also varies with temperature, battery type and age.
The batteries used for Photo voltaic system must be rechargeable, allow deep discharge, and should have long life span, easily serviced and have high capacities and low self-discharge.
The most widely used batteries fro solar photovoltaic system are Lead-acid batteries. Lead acid batteries are rechargeable, typically, they have capacities in the range of 1 to 12000 Ah, they have 500 to 800 discharge cycles and life time is about 2 to 3 years. The lead acid batteries perform very well for a wide range of temperatures from -15 degree centigrade to 60 degree centigrade.
The replacement and maintenance of lead acid batteries are very simple. All these factors make lead acid batteries a good choice for PV applications. There are varieties of lead acid batteries available.In general, the lead acid batteries are classified into two categories:
- Liquid vented
- sealed or VRLA ( Valve egulated Lead Acid ).It is two types:
- Absorbed glass mat (AGM ) Battery
- Gel battery (gel cell)
Liquid Vented Batteries:
Liquid vented batteries are generally used in automobiles. When a battery is charged and discharged, the hydrogen and oxygen gas is produced to chemical reactions. The gas produced during charging-discharging cycle must be vented out of the battery. In the process of venting out the gas, some water from battery is lost.
This loss of water must be supplied to the battery again, otherwise the battery will become dry and will not function properly. Therefore, for such bateries, some maintenance like regular water refilling is required.
Sealed or VRLA batteries
Sealed or Valve regulated lead acid (VRLA) batteries do not have any caps from where the gases produced in the charge-discharge cycle can exit as in case of liquid vented batteries.
The VRLA batteries are designed in a manner that hydrogen and oxygen produced in the batteries are recombined again to produce water. The valve is provided in the battery to regulate the pressure inside the battery.
Since during charge-discharge cycle no water is lost, no refilling of water is required for sealed or VRLA batteries.
They are maintenance free. Sealed batteries have the elctrolyte or acid either gelled or put into a sponge like glass mat. They have advantage/ disadvantage of being completely liquid tight. They can be operated in any position, even sideways or upside down, acid will not leak.
Absorbed Glass Mat (AGM)
In this type of batteries, the fibrous silica glass mat is used to suspend electrolyte. It forms semi solid (Gel) electrolyte along with empty pockets which helps in recombination of gas like hydrogen generated during charging process. This decreases the risk of hydrogen explosion.
The gel battery is similar to absorbed Glass mat batteries. The only difference here is silica gel is used instead of fibrous silica glass.