Study of the temperature and flame characteristics of two capacity LiFePO₄ batteries in thermal runaway

The thermal runaway of 20 Ah and 24 Ah LiFePO₄ batteries under different stage of charge conditions is tested using the combustion chamber. Surface temperature, mass loss rate, flame temperature and flame height are analyzed in detail during thermal runaway. The temperature at the center of the battery surface and the negative electrode is relatively high. Thermal runaway can be divided into two phases when the battery is heated continuously. The temperature increases sharply in the second phase of thermal runaway, which presents a high risk. The rising rate of temperature and the maximum surface temperature increase with increasing state of charge and battery capacity. The mass loss rate is the largest for the batteries with high capacity and high stage of charge. The flame temperature at 50mm above the top of the battery is the highest, which can be up to 950°C. The maximum flame temperature is mainly affected by the battery capacity, regardless of the state of charge. The maximum flame height and maximum liftoff height of the jet flame can reach up to 90 cm and 14 cm, respectively. The battery capacity and state of charge have little effect on the flame height and the liftoff height. The state of charge is found to play an important role in the formation of jet flames and five cycles of jet flame can occur in the 100% state of charge. The research of temperature and flame characteristics can provide a theoretical basis for improving the safety of LiFePO₄ batteries.