Abstract: A battery life extender for use in an implantable tissue stimulator. The tissue stimulator comprises a battery which powers a volatile memory, control circuits and a pulse output circuit all of which are connected in parallel across the battery. The invention provides a means for controlling current through the output circuit as a function of a difference voltage between the battery output voltage and a reference voltage. The reference voltage and battery voltage comprise inputs to a differential amplifier which in turn provides an output control voltage which controls a field-effect transistor (FET) connected in series with the output circuit and battery. As this control voltage rises, current through the field-effect transistor is reduced, thereby reducing the amplitude of the pulses from the output circuit and the current drain from the battery.

Abstract: A battery monitoring means for an implantable living tissue stimulator system in which various battery voltages are telemetered to an external receiving means, these voltages being related to the internal impedance of the implanted battery. More specifically, a battery loading circuit is provided which incorporates a switch means for loading the battery in accordance with a predetermined sequence. In a specific embodiment, first and second resistors are sequentially connected across the battery. The battery output voltage is telemetered to an external receiving means during this sequential connection. By knowing the values of the two resistors, the internal impedance of the battery can be calculated, this impedance being related to the remaining life of the implanted battery.

Abstract: A telemetry system for use in a living tissue stimulator system in which an externally located oscillator is controlled by impedance changes in an impedance reflecting circuit located in an implantable tissue stimulator. In a first embodiment the impedance reflecting circuit is an LC circuit thereby frequency modulating the externally located oscillator and in a second embodiment it is an LR circuit thereby amplitude modulating the externally located oscillator. More specifically, the externally located oscillator drives an LC circuit in which the inductor is positioned in magnetically coupled relationship to an inductor in the impedance reflecting circuit. The externally located oscillator is chosen so that its frequency and amplitude is partially determined by the impedance of the LC circuit and the magnetically coupled impedance reflecting circuit.