BACKUP POWER SUPPLY APPARATUS AND BACKUP POWER SUPPLY APPARATUS CONTROLLING METHOD

The backup power supply apparatus is an apparatus that supplies power to a power-supply target apparatus operating by power supply from an external power source when a power supply state of the external power source is emergency. The backup power supply apparatus includes: a power storage section; a series regulator that is located on a line connecting the power storage section and the power-supply target apparatus; and control circuitry that changes an output voltage of the series regulator in multiple stages in accordance with a change in the power supply state of the external power source and a change in a voltage of the power storage section.