A car power source apparatus is provided with a driving battery which drives an electric motor, heaters which heat the driving battery, a control circuit which controls power to the heaters, a temperature sensor which detects battery temperature and issues a detected temperature signal to the control circuit, and a surrounding temperature sensor which detects surrounding temperature and also issues a detected temperature signal to the control circuit. When the car is not being driven and the driving battery is heated via the heaters powered by the driving battery, the set temperature is changed corresponding to the detected surrounding temperature in the control circuit.