An implantable cardiac stimulation device with a hybrid battery network having first and second batteries with different battery chemistries connected in parallel with each other. The first dense cell battery has a higher resistance and greater energy density than the second fast cell battery, and the fast cell battery has greater current carrying capability than the dense cell battery. The dense cell battery has a higher voltage than the fast cell battery. In one embodiment the dense cell is a Lithium Carbon Monofluoride cell and the fast cell is a Lithium Silver Vanadium Oxide cell. During high current demand of high voltage capacitor charging, the fast cell provides the vast majority of the current due to its lower resistance. Following capacitor charging, the fast cell is recharged from the dense cell at a rate limited by the voltage difference between the cells.