Battery chargers are essentially a full bridge rectifier circuit that converts alternating current (AC) to direct current (DC). There are three types of charging characteristics, namely constant current, step charging and constant voltage with current limiting.
The latter regime is used on standby applications to ensure the battery is in a fully charged state with minimal water loss. The voltage is set above the nominal voltage to compensate for open circuit losses caused by local action within the cell. This is done by replenishing active material on the positive plate and the migration of acid from the pores of the plate to the bulk electrolyte consequently increasing the density of the electrolyte.
Battery charger technologies available in the market are the robust thyristor controlled chargers and the compact switch mode rectifiers.