With the wide application of lithium-ion batteries in underground mines, the safety issue of mining batteries has become increasingly prominent. In this paper, overcharge tests were performed on 200Ah LiFePO4/C battery cells and battery modules at different rates (0.5C, 1C, 1.5C) to study the thermal runaway characteristics of large-capacity batteries used in mining. The results show that the thermal runaway behavior of the lithium-ion batteries is divided into three stages: battery expansion, slow flue gas injection, violent flue gas injection-subsequent natural cooling, etc. As C-rate increases, the overcharged capacity required in each stage gradually decreases. The temperature after thermal runaway of the battery can reach up to more than 400℃, and the maximum temperature in the battery module experiment is significantly higher than that in the battery cell experiment. High temperature will pose a severe challenge to the safety of underground mines, and corresponding cooling and protective measures need to be taken. The thermal effect of overcharged batteries in the battery module does not cause thermal runaway reactions of adjacent batteries. The critical conditions for the thermal runaway chain reaction of mining batteries still need further study.