Thermal batteries are primary reserve batteries that are solid state at normal temperature. For many years, they have been the first choice of power supply for guided missiles and nuclear weapons. They use an electrolyte, such as a mixture of lithium chloride and potassium chloride, which is solid and inert at normal temperatures, but which is molten at the operating temperature. Older batteries used calcium or magnesium anodes, but lithium anodes are now common. The lithium may be present as an alloy with silicon or aluminum, or as the pure metal held in a matrix of another metal such as iron.
In the latter case, the lithium is in the molten state at the operating temperature of the battery, but it is held in the iron matrix by surface tension; otherwise liquid lithium would run and short out to the cathode. The cathodes usually consist of chromates or sulfides (e.g. K2Cr2O7, FeS2 and CoS2), that are reduced to other complex chromates and sulfides containing lithium. A typical battery contains a stack of cells, as in the figure here, each with its own anode, cathode, electrolyte and heat pellet. The working temperature of between 500 and 700oC is achieved by burning the electrically fired pellets of gas-less thermite. Activation takes between 0.2 second and a few seconds, depending on the size of the stack, and is initiated by an "electric match", or by a percussive primer, and a fuse train connected to all of the heat pellets.
The cell stack is thermally insulated from the case, partly to maintain a high working temperature, but also to minimize effects on nearby components. Once activated, the internal temperature depends on the balance between the heat loss through the insulation and the internal power losses. Active life ceases due to either exhaustion of the active materials or because the temperature falls too low. Thermal batteries are available in a range of sizes, giving output from a few watts for a few seconds to several kW for tens of minutes or even several hours. They are also being used increasingly for emergency aircraft supplies, electronic fuses, and for a variety of underwater devices. Unlike lithium primary batteries, they are particularly good at coping with pulsed loads.
Thermal battery demands are significantly lower than in the past due to cancellation of developmental weapons programs and other defense cutbacks with no foreseeable growth in the near future. However, thermal batteries are being assessed for additional military applications in sonobuoys and BAT smart missiles and for a commercial application as emergency power on More Electric Aircraft (MEA). A new series of sonobuoys may replace lithium sulfur di batteries with a thermal battery system. This application is in the development phase but if successful, could significantly increase military demand for thermal batteries. The development of the MEA will establish a commercial demand for thermal batteries if the emergency backup Systems using these batteries prove successful.