POWER SUPPLY DEVICE

In a power supply device, a battery switching unit is a circuit capable of switching a series-connected relay, a parallel-connected relay, a parallel-connected relay, and a parallel-connected relay to switch to a series circuit connecting a first battery and a second battery in series or a parallel circuit connecting the first battery and the second battery in parallel. When charging the first battery and the second battery, a controller controls the battery switching unit to switch to a parallel circuit, controls the parallel-connected relay based on voltages applied to a first cathode terminal and a first anode terminal, and further controls the parallel-connected relay based on voltages applied to a second cathode terminal and a second anode terminal.