Application of rechargeable battery charging voltage is determined by a signal applied to a battery-connect terminal (94) from external electrical equipment. In the case of a charging voltage application, the current detection section is judged as malfunctioning when the maximum battery cell voltage detected as a time series rises from a voltage below 3.8V (the second voltage) to a voltage above 4.1V (the first voltage) even though rechargeable battery charging and discharging current detected as a time series is greater than −5 mA and less than 20 mA.