Thermal batteries are primary reserve energy sources consisting of an alkali-metal and a transition metal chalcogenide electrochemical couple in a fused salt electrolyte. The electrolyte is solid and non-conducting at ambient temperatures and melts after integral pyrotechnic heat sources are ignited by a percussion primer or electro-explosive device. From their inception thermal batteries have proven superior for applications which require long shelf life, high reliability, imperviousness to dynamic environmental conditions and extremely high power. Primary applications include power for guidance and control, warhead fusing and telemetry for missiles. Recent improvements in specific energies and energy densities up to 70 Wh/kg and 155 Wh/l have opened new applications for aircraft emergency power and torpedo propulsion.