An implantable medical device includes circuitry for generating and delivering electrical stimulation therapy. A power source is included in the implantable medical device for storage of the energy for the stimulation therapy. Techniques and circuits are provided for minimizing energy losses associated with the storage of the stimulation therapy energy. The implantable medical device circuitry includes charging circuitry that is operated in at least a first mode and a second mode, such that the charging circuit is operable in one of the at least first and second modes based on whether an intrinsic cardiac event is detected. The charging circuit is operable to charge the output capacitor to a first threshold capacity that is less than a pacing capacity of the output capacitor until a given cardiac cycle elapses without a sensed intrinsic cardiac event during operation in the first mode.
