A battery module for use in a battery pack is disclosed. The battery module (12, 12′) comprises a plurality of battery cells (24, 25) and a sensing circuit (62) extending across the battery cells. A monitoring unit (46) is connected to the sensing circuit. The monitoring unit comprises a continuity detector (66) arranged to detect a break in continuity of the sensing circuit (62). This can allow a thermal event occurring in one of the plurality of cells to be detected.