A power supply device includes: a low-voltage direct current-direct current converter (LDC) to convert a voltage of a high-voltage battery to supply power; a first low-voltage battery to be charged by receiving power from the LDC and supply power to a general load; a bidirectional converter to convert a voltage supplied from the LDC and control a charge amount of a second low-voltage battery; the second low-voltage battery to be charged by receiving power from the bidirectional converter and supply power to a dual power source load; a first relay disposed between the LDC and the first low-voltage battery; a second relay disposed between the bidirectional converter and the second low-voltage battery; a second battery sensor to measure a voltage of the second low-voltage battery; and a control unit to control the bidirectional converter.