Battery thermal event detection system using an electrical conductor with a thermally interruptible insulator

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 an insulated conductive member mounted in close proximity to, or in contact with, an external surface of the battery or batteries to be monitored. A voltage measuring system is coupled to the conductive core of the insulated conductive member, the voltage 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.