A power supply device includes a battery module in which batteries are connected in series; a negative electrode output terminal to which a negative electrode terminal of the battery module is connected; a positive electrode output terminal to which a positive electrode terminal of the battery module and a connection point between predetermined batteries are individually connected through discharging switches; an output voltage detecting circuit for detecting an output voltage; a charging current adjusting circuit for adjusting the charging current of the battery module; thermistors for detecting the state-of-charge of the battery module; and a control device for controlling the charge and discharge of the battery module, wherein the control device includes means for controlling the discharging switches, so that the output voltage is within a specific range, and means for controlling the charging current adjusting circuit on the basis of the state-of-charge of each battery of the battery module.