Abstract: A battery charger 2 equipped with an AC/DC converter charges a secondary battery module 1 constituted with a plurality of unit cells 10. The unit cells 10 are each connected in parallel with a constant voltage circuit 33 constituted with a Zener diode 332 and a resistor 331 connected in series, and the unit cell 10 achieving a state of full charge is detected based upon the difference between the potentials at the two sides of the resistor 331. As a battery controller 32 lowers the charging power used by the battery charger 2 in response to the full charge state being achieved, it is ensured that the secondary battery module 1 is charged thoroughly while preventing individual unit cells 10 from becoming overcharged.

Abstract: A battery charger 2 equipped with an AC/DC converter charges a secondary battery module 1 constituted with a plurality of unit cells 10. The unit cells 10 are each connected in parallel with a constant voltage circuit 33 constituted with a Zener diode 332 and a resistor 331 connected in series, and the unit cell 10 achieving a state of full charge is detected based upon the difference between the potentials at the two sides of the resistor 331. As a battery controller 32 lowers the charging power used by the battery charger 2 in response to the full charge state being achieved, it is ensured that the secondary battery module 1 is charged thoroughly while preventing individual unit cells 10 from becoming overcharged.