Provided is a power supply device integrally combined with an electrical device body including a motor, comprising a rechargeable battery for supplying power to the motor, a microcomputer for detecting a residual capacity and a battery voltage of the battery, and a switching element provided between the battery and the microcomputer. The microcomputer stops charging the battery when the detected residual capacity has become 100%, and when the detected value of the battery voltage has become lower than a peak value after the value of the battery voltage passes the peak value, making it possible to prevent overcharging of the battery. The microcomputer also controls to turn off the switching element to stop the power supply from the battery to the microcomputer when the residual capacity becomes less than a predetermined threshold value, making it possible to reduce power consumption.