A battery pack has a secondary battery connected to a plus side terminal. A protective circuit protects the secondary battery from overcharge and over-discharge. A calculation circuit operates with a minus side terminal as a reference for calculating a remaining capacity of the secondary battery. An N-channel MOS transistor is connected between the secondary battery and the minus terminal for controlling charge and discharge of the secondary battery in accordance with a signal from the protective circuit. A level shifter circuit is connected between the calculation circuit and a communication terminal.