The battery pack (1) is connected to an external device (9) and to a power supply circuit (10) for the external device (9), and it energizes the external device (9) or is charged by the power supply circuit (10). Communication is made between a microcomputer (11) inside the battery pack (1) and the external device (9) through terminals (8) and at least information representing a present residual charge of a battery (4) is transmitted from the microcomputer (11) to the external device (9). This residual charge is calculated by the microcomputer (11) using the current, voltage, internal resistance and temperature of the battery (4). In this way, the residual charge can be obtained with a high level of accuracy on the side of the external device (9).