POWER SUPPLY DEVICE

In a power supply device (1), switching section (40) switch a connection between batteries (10A and 10B) to a series connection or a parallel connection. In a case where the connection between the batteries (10A and 10B) is switched from the series connection to the parallel connection to charge the batteries (10A and 10B) by an external charger (CG), a controller (50) does not switch the connection to the parallel connection, does not charge one battery having a larger voltage out of the batteries (10A and 10B), and separately charges the other battery when a potential difference between a voltage (VA) of the battery (10A) and a voltage (VB) of the battery (10B) is a predetermined threshold value or higher; and the controller (50) switches the connection to the parallel connection and charges the batteries (10A and 10B) when the potential difference is lower than the threshold value.