A battery bank is necessary for off grid PV systems. It stores excess energy from the PV modules and supply it back to the connected loads when the sun is not available.

It is recommended to use deep cycle batteries for off grid solar PV systems applications. Deep cycle batteries have the capacity of being charged and discharged hundreds of times before they wear out.

Lead-acid batteries are the most used. Car batteries don't work with solar PV. Their thin plates don't allow them withstanding deep discharges.

Lead acid batteries are made of separate 2-volt compartments known as cells.

The cells are filled with sulfuric acid, serving as electrolyte, and, inside of each cell is a series of thick, parallel lead plates. Insulators prevent short circuit between the plates and partition walls separate each cell preventing sulfuric acid to flow from one cell to the next.

To build up the voltage, the cells are wired in series. Electricity can flow out or in the battery through the negative and positive terminal.

All the cells are encased in a heavy-duty plastic case.

Lead acid batteries can be of two type: flooded or sealed. They work in two way; converting electrical energy into chemical energy at the charging and reconverting chemical energy into electrical energy at the discharge. The discharge, off course corresponds to when batteries supplies power to the load.

The thickness of plates allows multiple deep discharges of a deep cycle battery. Charging and discharging being the cycle.

Unlike flooded batteries which require to be filled before installing them, sealed batteries are factory filled; they are delivered ready for use.

Sealed lead acid batteries are also known as maintenance free batteries because fluid level never need to be checked and they never need to be filled with water.

Two types of sealed lead acid batteries are available: AGM standing for absorbed glass mat and gel cell batteries.

The advantage of sealed batteries over flooded batteries is that they charge faster, they do not release explosive gases, they are more tolerant to low temperatures and they have lower self discharge rate.

Flooded lead acid batteries on the other hand are less expensive, they store more electricity, they have longer lifespan and can be rejuvenated after they have been left deep discharged for a long period of time.

Most of large PV systems use flooded tubular plate lead-acid batteries. This type of batteries can easily exceed 5 to 7 years if they are properly maintained and used in a PV system with a suitable charge controller. Under lower depth of discharge, longer operational life could be achieved.

The system might require more than what a single battery can supply. This might be the voltage or the amp-hour storage capacity.

In such a case, installers have to wire the batteries in series or parallel and build up the voltage or the amp-hours.