The power supply device comprises first wiring that connects an alternator that is driven by the engine and a constant voltage electrical load that is driven at 12V, a heat accumulator that accumulates cooling heat and warming heat, a thermoelectric converter which generates power by using the heat that has accumulated in the heat accumulator and which supplies the generated power to the battery, second wiring that connects the battery and a variable voltage electrical load that is driven at 5V to 16V, for example, a relay which is provided on third wiring that connects the first wiring and second wiring and which interrupts the first wiring and second wiring, and a charge and discharge ECU which controls the thermoelectric converter on the basis of travel route information and so forth and which controls the relay on the basis of the operating state of the engine, the heat accumulation state of the heat accumulator, and the state of charge of the battery.