A battery pack system is provided that can be replaced easily without modifying the side of an appliance using this battery pack system, on which charging is controlled by a voltage control, and includes a secondary battery with high performance that undergoes charging by a current control. A battery pack system 10 is provided with a battery 13 including at least one unit cell serving as a secondary battery with a positive electrode containing a nickel oxide as an active material, a detecting portion 14 for detecting a battery state including a charging state in the battery, and a current adjusting portion 15, that is connected in series with the battery, for adjusting a charging current value according to the charging state detected by the detecting portion.