An early warning detection system for detecting battery thermal runaway in a battery pack or module, said system comprising a coating applied to the outside of one or more battery pack cells, said coating selected such that it decomposes at a temperature range useful for said detection and so as to emit a detectable volatile compound. Also provided is a method for preparing battery cells for use in a detection system, systems and batteries prepared according to the invention and their use in the detection of battery thermal runaway.