Coupled Influence of SOC and Safety Valve Orientation on Thermal Runaway Behavior of Lithium-Ion Batteries

The effect of safety valve orientation on the thermal runaway behavior of prismatic lithium-ion batteries may vary with state of charge (SOC), yet this combined influence remains insufficiently understood. In this study, 15 Ah prismatic ternary lithium-ion batteries at 0% SOC, 50% SOC, and 100% SOC were externally heated with the safety valve oriented upward or downward. External venting phenomena, surface temperature, temperature rise rate, voltage response, gas concentrations, mass loss, and venting temperature were systematically characterized. The results showed that the overall thermal response weakened with decreasing SOC, while the influence of safety valve orientation varied across SOC levels. At 100% SOC, the orientation difference was mainly reflected in rapid heating intensity, whereas at 0% SOC, sustained rapid heating and thermal runaway occurred only in the downward orientation. Two-way ANOVA showed that SOC and safety valve orientation significantly affected both maximum temperature and maximum temperature rise rate. Their interaction was significant for the maximum temperature rise rate but not for the maximum temperature, indicating a stronger combined influence on the dynamic heating process. These findings provide experimental evidence for evaluating the influence of cell orientation on thermal runaway risk and offer practical guidance for battery installation, safety valve arrangement, and vent path design in battery systems.