POWER SUPPLY DEVICE

In a power supply device (1), a battery switching unit (50) is a circuit capable of switching a series-connected relay (56), a parallel-connected relay (51), a parallel-connected relay (52), and a parallel-connected relay (53) to switch to a series circuit connecting a first battery (11) and a second battery (12) in series or a parallel circuit connecting the first battery (11) and the second battery (12) in parallel. When charging the first battery (11) and the second battery (12), a controller (60) controls the battery switching unit (50) to switch to a parallel circuit (Q1), controls the parallel-connected relay (51) based on voltages applied to a first cathode terminal (cd1) and a first anode terminal (ad1), and further controls the parallel-connected relay (52) based on voltages applied to a second cathode terminal (cd2) and a second anode terminal (ad2).