Abstract: A charging system for a vehicle battery comprises a voltage detecting circuit having at least one element, such as a Zener diode, having an impedance variable in accordance with a voltage from an auxiliary rectifier connected to the armature coils of the alternator of the system. When the generating voltage is low at the initial operating period, the impedance of the voltage detecting circuit is kept high, and thus the generated current from the auxiliary rectifier is all supplied to the field coil of the alternator without being consumed by a driving circuit of a charge-indicating lamp. As the voltage from the auxiliary rectifier exceeds a predetermined voltage, the impedance of the detecting circuit lowers to cause the driving circuit to energize the lamp. The voltage detecting circuit may be constructed of one or more than one Zener diode, or of a plurality of diodes connected in the forward direction.