PURPOSE: To eliminate the need for setting residue when a user exchanges a battery pack by writing the residue of a battery and the number of charge and discharge cycles to an E2PROM in the battery pack.

CONSTITUTION: The inside of an electronic device has a charging circuit 3 capable of charging a battery pack 1 on the usage of an external power supply and a microcomputer 7 for monitoring the battery pack 1 monitoring the time of the charge and discharge of the battery pack 1 and the number of cycles, the inside of the battery pack 1 has an E2PROM9, and a means writing the residue of battery and the number of charge and discharge cycles to the E2 PROM9 by the microcomputer 7 is provided. The micro-computer 7 counts a charging time, calculates the residue of the battery at every fixed time and writes the data to the E2PROM9. The residue of the battery is written similarly to the E2PROM9 during discharge. Accordingly, the residue of the battery is read from the E2PROM9, and the residue may be computed on the basis of the residue of the battery, thus eliminating the need for setting by a user.