PURPOSE: To prevent a damage by an overcharge and an overdischarge of a battery by adding and reducing the charging current and the discharging current to a secondary battery accumulatively at every specific time, and displaying the resultant residual capacity, in a battery pack with built-in secondary battery.

CONSTITUTION: An A/D converter 3 outputs a binary number with mark, and the output is input to one side input terminal of a computing element 4, and the other side terminal of the computing element 4 is connected to a register 5 to store the operation result. The converting timing of the converter 3 and the storing timing to the register 5 are synchronized by a timer 6. And the value proportional to the charged power amount in the charging time is accumulated, while the value proportional to the value subtracting the discharge amount from the accumulated amount remains, in the register 5. The output of the register 5 is input to a display 7, where it is displayed as the battery residual capacity.