a power supply for an implantable cardioverter - defibrillator for subcutaneous positioning between the third rib and the twelfth rib and using a lead system that does not directly contact a patient &# 39 ; s heart or reside in the intrathorasic blood vessels and for providing anti - bradycardia pacing energy to the heart , comprising a capacitor subsystem for storing the anti - bradycardia pacing energy for delivery to the patient &# 39 ; s heart ; and a battery subsystem electrically coupled to the capacitor subsystem for providing the anti - bradycardia pacing energy to the capacitor subsystem .