A full-wave rectifier connected to an AC source supplies a pulsating DC voltage. A thyristor is interconnected between the output of the rectifier and the battery. A voltage divider including a capacitor and the emitter-collector circuit of a transistor is connected across the output of the rectifier. The voltage divider has tap connected to the gate of the thyristor. A voltage proportional to the battery voltage is applied to the base of the transistor. Variations in the emitter-collector resistance of the transistor, which correspond to changes in the battery output voltage determine the cut-in time of the thyristor. When the battery voltage is sufficiently high the thyristor blocks completely.