Pressure-balanced primary and secondary batteries suited for supplying power to downhole tools and for powering a downhole telemetry network. The battery comprises an electrochemical generator disposed within at least a partially compliant enclosure or housing. The compliant enclosure permits pressurization of the electrochemical generator, thereby compensating for the increased temperature downhole. By permitting the electrochemical generator to be pressurized, the thermal range of the battery is extended to beyond 120.degree. C. The electrochemical generator comprises flexible components comprising an ionic conductor intermediate anode and cathode materials, a current collector, and a terminal. The battery may be installed within a recess in the wall of the downhole tool or within an annulus formed by the inside wall of the tool and a liner. In another embodiment of the invention, the battery may be located within an operative portion of a selected downhole tool, itself.