BATTERY SYSTEM

An ECU outputs, when it determines that there is a battery cell which needs charging, address information to a battery monitor AFE 6. The battery monitor AFE 6 turns ON a transistor specified by the address information and outputs an active ON/OFF signal to a balancing circuit 4. To the balancing circuit 4, a DC converter 30 to which each of battery cells 2a are connected is provided, and selectively charges the battery cells by operating the DC converter 30. When the ON/OFF signal is active, the DC converter 30 is formed of a magnetic-amplifier type forward converter and selectively charges the battery cells having a low cell voltage when the voltage of the battery cells is about lower than 4.2 V, and eventually maintains the cell voltage at about 4.2 V to prevent overcharging to the battery cells 2a.