A power supply device comprising: a battery module, which includes three battery banks (11, 12, 13) connected in series, each of which comprises a predetermined number of batteries 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 each of the battery banks (11, 12, 13) are individually connectable through discharging switches (SW1, SW2, SW3); an output voltage detecting circuit configured to detect an output voltage between the positive electrode output terminal and the negative electrode output terminal; a charging current adjusting circuit, which includes two charging switches (SW4 and SW5) and a current-limiting resistor, configured to adjust a charging current of the battery module; a plurality of state-of-charge detecting circuits (14, 15, 16) configured to detect a state-of-charge of each of the battery banks (11, 12, 13) of the battery module by detecting a temperature of each of the battery banks (11, 12, 13); and a control device configured to control charge and discharge of the battery module, wherein the control device includes: means configured to control the discharging switches (SW1, SW2, SW3), so that the output voltage is within a specific range; and means configured to control the two charging switches (SW4, SW5) in the charging current adjusting circuit on the basis of the state-of-charge of each of the battery banks (11, 12, 13) of the battery module. a battery module, which includes three battery banks (11, 12, 13) connected in series, each of which comprises a predetermined number of batteries 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 each of the battery banks (11, 12, 13) are individually connectable through discharging switches (SW1, SW2, SW3); an output voltage detecting circuit configured to detect an output voltage between the positive electrode output terminal and the negative electrode output terminal; a charging current adjusting circuit, which includes two charging switches (SW4 and SW5) and a current-limiting resistor, configured to adjust a charging current of the battery module; a plurality of state-of-charge detecting circuits (14, 15, 16) configured to detect a state-of-charge of each of the battery banks (11, 12, 13) of the battery module by detecting a temperature of each of the battery banks (11, 12, 13); and a control device configured to control charge and discharge of the battery module, wherein the control device includes: means configured to control the discharging switches (SW1, SW2, SW3), so that the output voltage is within a specific range; and means configured to control the two charging switches (SW4, SW5) in the charging current adjusting circuit on the basis of the state-of-charge of each of the battery banks (11, 12, 13) of the battery module. means configured to control the discharging switches (SW1, SW2, SW3), so that the output voltage is within a specific range; and means configured to control the two charging switches (SW4, SW5) in the charging current adjusting circuit on the basis of the state-of-charge of each of the battery banks (11, 12, 13) of the battery module.