A safety switch is available that protects against the thermal runaway condition that can occur during use, misuse or abuse of high-energy-density lithium ion cells. The Therm-O-Disc device offers low impedance, small envelopes, and a range of operating temperatures. The compact unit offers cell-level protection for batteries down to AA size.The device uses shape-memory alloy as a temperature/current sensor. The sensor returns to its "remembered" high-temperature shape upon transformation, resulting in an open circuit before reaching thermal runaway temperatures. Other features include shock resistance, limited resettability to minimize nuisance trips from momentary shorts, and a wide range for setting trip temperature and trip current.