POWER SUPPLY SYSTEM AND METHOD FOR CONTROLLING POWER SUPPLY SYSTEM

A power supply system is installed in a vehicle. The power supply system includes a first load circuit configured to operate using electric power from a main battery, the first load circuit being connected to a first load; a second load circuit configured to operate using electric power from the main battery or an additional battery, the second load circuit being connected to a second load; a first relay provided on a power supply line for electrically connecting the first load and the second load, the first relay that electrically connects or disconnects the first load circuit to or from the second load circuit; a second relay that electrically connects or disconnects the second load to or from the additional battery; and a controller that determines a state of a start switch for starting the vehicle. The first relay is in an on state when the start switch is off. The second relay is in an off state when the start switch is off. When determining that the start switch has been switched from off to on, the controller is configured to turn on the second relay and then turn off the first relay. A failure diagnosis for the first relay is performed with the first relay being off and the second relay being on.