PURPOSE: To prevent swell or rupture of battery due to overcharge and to prevent heating or smoking of charging circuit due to overcurrent by indicating a state where a rated output voltage corresponding to the number of series connected secondary batteries can not be obtained and immediately opening the charging circuit.

CONSTITUTION: When a cord 1' is connected with an AC receptacle, an AC mode takes place and a motor 3 is driven with an AC power supply 1. At this time, a battery 5 is also charged through a charging circuit 4. When the cord l' is removed from the AC receptacle, a DC mode takes place and the motor 3 is driven with the battery 5 power supply. Terminal voltage of the battery 5 is continuously detected through a control circuit 2 and when thus detected terminal voltage exceeds a set voltage, an abnormality is indicated on a display section 6 comprising a liquid crystal display or the like. At the same time, the charging circuit 4 is opened thus blocking charging of the battery even if it is connected with the AC receptacle. When the terminal voltage drops below the lower limit of normal voltage, an abnormal display indicated on the display section 6 and the charging circuit 4 is opened.