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Thermal batteries are primary reserve batteries that employ inorganic salt electrolytes. These electrolytes are relatively non-conductive solids at ambient temperatures. Integral to the thermal battery are pyrotechnic materials scaled to supply sufficient thermal energy to melt the electrolyte. The molten electrolyte is highly conductive, and high currents may then be drawn from the cell.
The activated life of a thermal battery depends on several factors involving cell chemistry and construction. Once activated, and as long as the electrolyte remains molten, thermal batteries may supply current, discharging the active materials to the point of functional exhaustion. On the other hand, even with excess active materials present, the batteries will eventually cease functioning due to loss of internal heat and subsequent re-solidification of the electrolyte. Hence two of the primary factors behind thermal battery active life are: 1) Compositions and massed of the active cell stack materials (i.e. anodes and cathodes), and 2) other construction details, including the overall battery shape and types and amounts of thermal insulation. Depending on the battery design, which is ultimately determined by the specific requirements of the application, the activated thermal battery may supply electric power for only a few seconds, or may function for over an hour.
Initiation of a thermal battery is normally provided by an energy impulse from an external source to a built-in initiator. The initiator, typically an electric match, an electro-explosive device (squib), or a percussion primer, ignites the cell stack pyrotechnics. Rise time, the time interval between the initiation impulse and that time at which the battery can sustain a current at voltage, varies as a function of battery size, design, and chemistry. Rise times of several hundred milliseconds are not uncommon for large units. Small batteries have been designed to reliably achieve operating conditions withing 10 to 20 milliseconds.
The shelf life of an unactivated thermal battery is typically 10-25 years, depending upon design. Once activated and discharged, though, they are not reusable or rechargeable.
Current developments in extending the activated life capabilities of thermal batteries have widened their suitability and application potential in new military as well as industrial/civilian systems.