In this paper a new charging algorithm is proposed to charge lead-acid batteries in photovoltaic (PV) systems. This algorithm can return discharged lead-acid batteries to their 100% state of charge (SOC) quickly and at the same time can avoid the associated problems of the excessive gassing phenomenon at overcharge. The proposed algorithm can be applied in the PV systems by using a DC-DC converter, which differs from the traditional on/off regulators in that it cannot only be used to charge the battery and protects it from overcharging, but it can also be used to quickly and safely charge the battery to 100% SOC through better exploitation of the available PV energy. The simulation results verify that, using the proposed algorithm, the discharged battery can always restore its 100% SOC compared with the conventional charging algorithms. Copyright (C) 2010 John Wiley & Sons, Ltd.