Thermal event identification

A thermal event identification device to identify thermal events in a battery pack comprising at least one battery cell, the thermal identification device comprising:

• a heat sensitive component comprising a substrate and a capsule containing an electrolyte, wherein:
• the substrate is electrically conductive;
• below a first temperature, a resistance of the substrate has a first resistance value;
• and
• in an event that the heat sensitive component is heated to above the first temperature the electrolyte is released from the capsule such that the substrate is soaked in the electrolyte and the resistance of the substrate changes to a second resistance value;
• a first electrical connection and a second electrical connection, wherein the heat sensitive component is electrically connected to the first and second electrical connections such that a resistance of the substrate of the heat sensitive component can be measured;
• and
• a controller configured to determine whether a thermal event has occurred based on the resistance of the heat sensitive component, wherein in use, the heat sensitive component is configured to be adjacent to a battery cell.