A backup power supply system supplies power to one or more loads in a situation where a power supply has caused a failure. The backup power supply system includes a plurality of power storage devices and a switching unit. The plurality of power storage devices are charged by the power supply. The switching unit switches electrical connection between the plurality of power storage devices to either a first state where the plurality of power storage devices are connected to the power supply in parallel or a second state where the plurality of power storage devices are connected to each other in series. The switching unit switches the electrical connection to the first state while the plurality of power storage devices are being charged and switches the electrical connection to the second state when the power supply has caused the failure.