Abstract: A method and apparatus is provided for determining when a battery, or one or more batteries within a battery pack, undergoes an undesired thermal event such as thermal runaway. The system uses a conductive member mounted in close proximity to, or in contact with, an external surface of the battery or batteries to be monitored. A resistance measuring system such as a continuity-tester or an ohmmeter is coupled to the conductive member, the resistance measuring system outputting a first signal when the temperature corresponding to the battery or batteries is within a prescribed temperature range and a second signal when the temperature exceeds a predetermined temperature that falls outside of the prescribed temperature range.