A battery pack detachably connectable to an equipment body to supply power to the equipment body, the battery pack including a battery cell, a microcomputer for communicating with the equipment body, a connection terminal connected to the microcomputer, a positive power supply input terminal connected to a cathode of the battery cell; and a negative power supply input terminal connected to an anode of the battery cell. After the power of the equipment body is turned on, the microcomputer alternately sends to the equipment body by serial communication via the connection terminal information to be used in authentication processing executed by the equipment body and information to be used in a battery residual quantity count executed by the equipment body. After the authentication processing is complete, the microcomputer sends to the equipment body information to be used in updating the battery residual quantity count executed by the equipment body.