PURPOSE: To provide a car battery device of a two-power supply configuration which can make the lifetimes uniform without causing increase in the capacity of battery and its weight.

CONSTITUTION: A car battery concerned comprises a starting battery SB in connection with a starting ignition device IG and a starter motor S, a load battery LB in connection with a car on-board load L and an alternator G, and a relay contact RY connected between the starting battery and load battery, wherein the relay contact is closed when the alternator voltage has exceed the specific value which is higher than the circuit opening voltage of the load battery, while the contact is closed for a certain time when the discharge voltage of the load battery has sunk to the specified value. Because the a heavy load generated in a short duration other than the steady load is shared by the starting battery, the capacity of the load battery can be set small to a great extent. Thereby unbalance of lifetimes can be eliminated without increasing the weight of battery by selecting the capacity balance with the starting battery properly.