A power supply apparatus is provided that has a plurality of secondary batteries connected in series, outputs a composite voltage of all of the secondary batteries, and outputs an output of a part of the secondary batteries as a partial voltage. The power supply apparatus includes a detecting unit that detects states of the secondary batteries, a changing unit that changes an order of series connection of the plurality of secondary batteries based on a detection result of the detecting unit in such a manner that the partial voltage is outputted from a secondary battery that is in a relatively good state.