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Thermal batteries use an electrolyte that is solid and inactive at ambient temperatures. They can be stored indefinitely (over 50 years) yet provide full power in an instant when required. Once activated, they provide a burst of high power for a short period (a few tens of seconds to 60 minutes or more), with output ranging from watts to kilowatts. The high power is due to the high ionic conductivity of the molten salt, which is three orders of magnitude (or more) greater than that of the sulfuric acid in a lead–acid car battery.

One design uses a fuze strip (containing barium chromate and powdered zirconium metal in a ceramic paper) along the edge of the heat pellets to initiate burning. The fuze strip is typically fired by an electrical igniter or squib by application of electric current.

Another design uses a central hole in the middle of the battery stack, into which the high-energy electrical igniter fires a mixture of hot gases and incandescent particles. This allows much faster activation times (tens of milliseconds) vs. hundreds of milliseconds for the edge-strip design. Battery activation can be accomplished by a percussion primer, similar to a shotgun shell. The heat source should be gasless. The standard heat source typically consist of mixtures of iron powder and potassium perchlorate in weight ratios of 88/12, 86/14, or 84/16.[citation needed] The higher the potassium perchlorate level, the higher the heat output (nominally 200, 259, and 297 cal/g respectively). This property of unactivated storage has the double benefit of avoiding deterioration of the active materials during storage and eliminating capacity loss due to self-discharge until the battery is activated.

In the 1980s lithium-alloy anodes replaced calcium or magnesium anodes, with cathodes of calcium chromate, vanadium or tungsten oxides. Lithium–silicon alloys are favored over the earlier lithium–aluminium alloys. The corresponding cathode for use with the lithium-alloy anodes is mainly iron disulfide (pyrite) replaced by cobalt disulfide for high-power applications. The electrolyte is normally a eutectic mixture of lithium chloride and potassium chloride.

More recently, other lower-melting, eutectic electrolytes based on lithium bromide, potassium bromide, and lithium chloride or lithium fluoride have also been used to provide longer operational lifetimes; they are also better conductors. The so-called "all-lithium" electrolyte based on lithium chloride, lithium bromide, and lithium fluoride (no potassium salts) is also used for high-power applications, because of its high ionic conductivity. A radioisotope thermal generator, such as in the form of pellets of 90SrTiO4, can be used for long-term delivery of heat for the battery after activation, keeping it in a molten state.[4] Uses Thermal batteries are used almost exclusively for military applications, notably for guided missiles.[5][6] They are the primary power source for many missiles such as the AIM-9 Sidewinder, MIM-104 Patriot, BGM-71 TOW, BGM-109 Tomahawk and others. In these batteries the electrolyte is immobilized when molten by a special grade of magnesium oxide that holds it in place by capillary action. This powdered mixture is pressed into pellets to form a separator between the anode and cathode of each cell in the battery stack. As long as the electrolyte (salt) is solid, the battery is inert and remains inactive. Each cell also contains a pyrotechnic heat source, which is used to heat the cell to the typical operating temperature of 400–550 °C.