A power control system is described for use in an implantable cardioverter-defibrillator (ICD) having a low-power cell and a high-power cell. The power control system includes a fuzzy logic controller for gradually varying the relative amounts of energy drawn from the low- and high-power cells based upon a selected function to be performed by the ICD and upon various operational parameters of the ICD. Example functions include cardioversion therapy, cardiac defibrillation, cardiac pacing, cardiac monitoring and capacitor reformation. Example operational parameters include the remaining capacities of the low- and high-power cells, the amount of time since implant of the ICD, the number of defibrillation shocks already delivered, and the amount of pacing energy previously utilized. In each case, the fuzzy logic controller applies a set of fuzzy logic rules to determine the relative amounts of energy to be drawn from the two power cells to optimize energy source allocation and to extend the overall longevity of the ICD.