Object of the invention is to provide an improved technique of suitably charging a battery assembly irrespective of the kind of the battery assembly. The representative battery charger 100 includes a power supply section 110, a detecting section 130 that detects voltage that represents a temperature index of the battery assembly 200, 300 a reference voltage that is compared with the detected temperature-representing voltage of the battery assembly. The battery charger 100 stops supplying the charging current when the temperature-representing voltage of the battery assembly reaches or exceeds the reference voltage. The reference voltage varies according to the charging voltage of the battery assembly and thus, the threshold temperature to complete the charge can be varied according to the charging voltage of the battery assembly 200, 300. As a result, an overload state for the battery assembly with high charging voltage and high temperature can effectively be prevented.