Abstract: A battery charging circuit having an electromagnetic transformer with primary and secondary coils, a rectifier connected between the secondary coil and the battery, a first transistor connected in series with the primary coil and a resistor to form a circuit in parallel with a power source, the base of the first transistor being connected to the secondary coil, and a second transistor electrically connected to the base of the first transistor. The first transistor has a high switching frequency, and the battery is charged with a constant reverse current during its blocking phase, regardless of the power source voltage. The first transistor is protected from reverse kick-back voltage pulses induced during its blocking phase by the provision of a diode and Zener diode connected in series across the primary coil and a capacitor interposed between the secondary coil and the base of the first transistor.