A battery pack (1) is connected to an external equipment (9) and to a power source circuit (10) designed to supply the power to the external equipment (9). The communication of the information is made between a micro-computer (11) in the battery pack (1) and the external equipment (9) via a terminal (8). At least the information indicating the present residual capacity of a battery main unit (4) is transmitted by such communication from the micro-computer (11) to the external equipment (9). The residual capacity is found by the calculations performed by the micro-computer (11) on the current, voltage, internal resistance and the temperature of the battery main unit (4). In this manner, the residual capacity may be accurately found on the side of the external equipment (9).