A defibrillator battery pack comprises a housing having a first set of battery cells having an upper set and a lower set of cells and a second set of battery cells connected in parallel with one of the upper or the lower sets of the first set of battery cells. The first set of battery cells is used for charging a capacitor bank of a defibrillator. The second set of battery cells cannot be used for charging and is only used for developing a nominal 5 volts to drive a microprocessor and other circuitry components of an electrical control system of the defibrillator. This arrangement increases the life of the battery pack in the lower voltage range, which is advantageous for operating the microprocessor. This arrangement also maintains the intelligence of the electrical control system because the battery cells supplying power to the microprocessor will always fail after, and not before, failure of the battery cells supplying power for charging the capacitor bank.