To eliminate a charging detection circuit or a EEPROM originally included in a battery compartment to reduce the cost and provide a battery charger having an energy level gauge.

This invention relates to a battery charger having an energy level gauge. This battery charger is built in a portable data processor and connected to the rechargeable battery pack to charge it, and includes a charger to charge the battery pack by connecting it to the power supply and an energy level gauge built in the charger to detect the remaining capacity of the rechargeable battery pack. The remaining capacity of the rechargeable battery pack is inputted to the portable data processor and a current detector circuit. The energy level gauge connects the power supply and the energy level detector circuit to charge the rechargeable battery pack. Accordingly, a battery charge level monitor function is obtained only by adding an extra circuit when manufacturing the charging circuit for the main system.