Many types of batteries can be used in photovoltaic systems. Lead acid type is the most common, but lithium-ion batteries are becoming increasingly popular. Table 1 compares these two most common battery types. Power batteries (traction batteries) are lead-acid batteries designed for deep discharge applications such as electric vehicles. Traction batteries are used in independent photovoltaic systems, unlike deep discharge batteries, as they use heavier, thicker plates and sturdy battery connections to withstand the mechanical stress generated by deep discharge.

Table 1: Two most common types of batteries used for photovoltaic system storage

A fully charged battery has a liquid electrolyte solution. Ventilated lead-acid batteries release hydrogen and oxygen. Therefore, sufficient ventilation must be provided for the ventilated and sealed battery system. It is usually recommended to provide sufficient ventilation requirements similar to combustion water heaters. There are no special ventilation requirements. Ventilated batteries should be equipped with flame arresters to prevent external ignition sources from exploding, and sealed batteries must have pressure relief holes.

Valve regulated lead-acid (VRLA) batteries have gel based electrolytes and have no same maintenance requirements as exhaust lead-acid batteries. However, they have lower tolerance to overcharging and higher temperatures, and are more expensive.