Abstract: An implantable defibrillator having a charging circuit for charging a high-voltage energy storage capacitor from a low voltage battery. Current from the battery is switched on and off through the primary winding of a voltage step-up transformer to induce a fly-back current in the secondary winding. The fly-back current is rectified and applied across the energy storage capacitor. The frequency of switching of the primary current is varied in relationship to the voltage of the battery to maintain a substantially constant average charging current as battery voltage decreases. Current through the primary is monitored on a cycle-by-cycle basis and switched off if it exceeds a preset limit.