To increase flexibility of an available charger, by widening an allowable range of voltage/current setting accuracy thereof.

The battery pack comprises charge/discharge control elements SW1, SW2, connected in series to one or a plurality of secondary batteries 1 and protection function control means 2 for controlling the charging/ discharging control elements, by monitoring a terminal voltage of the secondary battery 1. A charge current control means 6 for controlling a charge current to a reset value is inserted in a serial circuit of the secondary battery 1 and charging/discharging control elements SW1, SW2. The protective function control means 2 includes a over-charge detecting function. The charge current control means 6 includes a bypass circuit, to bypass discharge current during the discharging operation. Accordingly, even when the charger for controlling a voltage and a current higher than the recommended range of the secondary battery 1 is connected, the voltage and current are adjusted with the over-charge detecting function and current control function, to widen the allowable range of the voltage/current setting accuracy of the available charger and increase a the flexibility thereof.