A battery powered cardioverter or defibrillator employing output capacitors for delivery of cardioversion or defibrillation pulses. The capacitors are charged by means of a step-up transformer, coupled and uncoupled from the battery by means of a voltage controlled oscillator which varies its off and on times as a function of the voltage on the battery and the voltage stored on the output capacitor to allow for rapid charging of the capacitor even under conditions of battery depletion. The voltage controlled oscillator is constructed to provide a highly accurate timing signal with a low current drain.