Battery failure compensation for a power supply used in an implantable defibrillator

A fully implantable power supply for use with or in a fully implantable defibrillator, which has a fibrillation detector in circuit with the power supply and a power inverter switchably in circuit with the power supply. The power supply comprises an energy source which includes a plurality of batteries arranged in series, each of the batteries having a pair of output terminals, each of the batteries producing a multilevel voltage across its pair of output terminals, the voltage being at a first level when the battery is fully charged and dropping to a second level at some point during the discharge of the battery. Circuitry in the form of a plurality of unidirectional conducting devices is provided for creating a first conductive path between the serially-connected batteries and the fibrillation detector, and a second conductive path between the inverter and the batteries that are producing the first level of voltage.