Abstract: In a method and a device for determining and indicating a residual capacity of a battery, a charging amount is measured in a charging mode in which a charger having a sensor for detecting a fully charged state is connected to the battery. If the charging amount in the charging mode is more than a trickle charging amount, the measured charging amount is added to a present charging amount to obtain the present charging amount and a ratio of the present charging amount to a fully charged state is indicated on a display unit. If the charging amount in the charging mode is less than a trickle charging amount, it is judged that the present charging amount is the fully charged amount and this amount is the fully charged state. The device further includes a current-to-voltage converting circuit, an analog-to-digital converter, and an operation control unit such as a MPU connected to the display unit and executing and controlling the operation of the device.