Photovoltaic systems increasingly use rechargeable batteries to store energy to be later used at night. Batteries used for storage also stabilize the electrical grid by levelling out peak loads, and play an important role in a smart grid, as they can charge during periods of low demand and feed their stored energy into the grid when demand is high.
Common battery technologies used in today’s PV systems include the valve regulated lead-acid battery– a modified version of the conventional lead–acid battery, nickel–cadmium and lithium-ion batteries. Compared to the other types, lead-acid batteries have a shorter lifetime and lower energy density. However, due to their high reliability, low self-discharge as well as low investment and maintenance costs, they are currently the predominant technology used in small-scale, residential PV systems, as lithium-ion batteries are still being developed and about 3.5 times as expensive as lead-acid batteries. Furthermore, as storage devices for PV systems are stationary, the lower energy and power density and therefore higher weight of lead-acid batteries are not as critical as, for example, in electric transportation. Other rechargeable batteries that are considered for distributed PV systems include sodium–sulphur and vanadium redox batteries, two prominent types of a molten salt and a flow battery, respectively.
Photovoltaic systems with an integrated battery solution need also a charge controller, as the current and varying voltage from the solar array requires constant adjustment to prevent damage from overcharging. Basic charge controllers may simply turn the PV panels on and off, or may meter out pulses of energy as needed.