A hybrid battery power source for implantable medical use provides relatively stable resistance during discharge and avoids the voltage delays that develop as a result of variable resistance increase in Li/SVO cells. The hybrid battery power source utilizes two batteries or cells, one being a primary battery of relatively high energy density and the other being a rechargeable secondary battery of low relatively stable internal resistance. The primary and secondary batteries are connected in a parallel arrangement, preferably via an intermediate voltage boost circuit having an inductor and a pulse generating control circuit therein. The energy storage capacitors of the medical device in which the hybrid battery power source is situated are driven in whole or substantial part by the secondary battery. The primary battery is used to as an energy source for recharging the secondary battery. By arranging the two batteries in parallel, with one serving as a primary battery and the other as a rechargeable secondary battery, all the benefits of the defibrillatory impulse will be obtained and the deficiencies arising from variable voltage delay found in prior art implantable power sources will not be present.