NOVELTY - The device (10) has switches (13,17,21,23,27) that are controlled by control circuit (29) to charge capacitors (15,19), when starter (25) is not in driving state. The electric power of battery (11) is supplied to starter, when sum voltage is less than fixed voltage, during driving state of starter. The battery is disconnected from starter and the electric power is supplied to starter, if sum voltage is more than fixed voltage, when the vehicle is in driving state, so that capacitors are connected in series respectively.    USE - Power supply device for vehicles.    ADVANTAGE - Since electric current hardly flows into the starter from the battery during driving state of starter by the series circuit when the capacitors are in series, the stabilization of the battery voltage can be achieved. Since the burden of a battery is reduced, the lifetime of the power supply device is improved with an idle stopping function.    DETAILED DESCRIPTION - The switch (13) is connected to the positive electrode of the battery. The capacitor (15) is connected through positive electrode of the battery, and switch (13), and the negative electrode is connected to the ground. The switch (17) is connected to positive electrode of capacitor (19). The negative electrode of capacitor (19) is connected through positive electrode of capacitor (15), and the switch (17), and is connected by a switch (17) and connection point. The switch (21) is connected between the connection point to which the switch (17) and the negative electrode of the capacitor (19) are connected, and the ground. The switch (23) is connected to positive electrode of capacitor (19). The switch (27) is connected between positive electrode of battery, and starter. The semiconductor switching element is connected to the positive electrode of capacitor (19).    DESCRIPTION OF DRAWING(S) - The drawing shows a block circuit diagram of the power supply device for vehicles. (Drawing includes non-English language text)    Power supply device (10)    Battery (11)    Switches (13,17,21,23,27)    Capacitors (15,19)    Starter (25)    Control circuit (29)