Embodiments of an implantable electrolytic pump include a drug reservoir, a cannula fluidly coupled to the reservoir, a pumping mechanism for forcing liquid from the reservoir through the cannula, control circuitry for operating the pumping mechanism, and a power source comprising primary and auxiliary batteries in a stacked configuration, only the main battery being operatively coupled to the control circuitry during normal operation, the control circuitry operatively coupling the back-up battery upon detection of an electrical fault. The pump may further include a hermetic enclosure containing the control circuitry and the power source.