Abstract: Thermal runaway of lithium-ion batteries is accompanied by signal variations such as temperature rise, voltage drop, and volume expansion which are of great significance for battery performance evaluation and safety warning. In this paper, lithium-ion batteries with a ternary/graphite system are taken as the research object. The variation characteristics of temperature, voltage and expansion force signals during thermal runaway are explored at different state of charge (0%, 25%, 50%, 75%, 100%), and a three-level early warning scheme based on multi-parameter coupling is proposed. The results show that with the increase of state of charge, the thermal runaway trigger time of lithium-ion batteries is shortened, the maximum temperature during thermal runaway is increased, and the expansion force changes more rapidly with a higher stress peak. The reason is that batteries at higher state of charge store more energy, leading to more intense internal chemical reactions in the thermal runaway process. Meanwhile, it is found that the variation of expansion force precedes temperature and voltage signals by approximately 300 s during thermal runaway. The three-level early warning method dominated by expansion force provides sufficient time for implementing control measures.

Key words: lithium-ion battery, thermal runaway, state of charge, early warning, expansion force