Effect of immersion height on thermal runaway triggered by overcharge in LiFePO4 batteries

Immersion cooling (IC) is an effective strategy for suppressing thermal runaway (TR) in lithium-ion batteries. However, the influence of immersion height on TR behavior has received limited attention, especially under partial immersion conditions, and the combined effects of immersion height and overcharge rate remain insufficiently understood. Therefore, experiments were conducted on 32 Ah LiFePO 4 batteries at different immersion heights and overcharge rates to clarify their effects on TR characteristics and quantify the heat transfer performance of IC during TR. Results show that immersion height significantly affects TR characteristics. Under the 3C overcharge condition, the full immersion (FI) mode reduces the maximum temperature to 139.6 °C and the maximum temperature rise rate to 5.65 °C/s, and completely suppresses open flame, transforming TR into a low hazard heat release process. The FI mode also delays safety valve opening and internal short circuit onset, with the total TR-triggering energy factor ( E ⁎ ) reaching 1.81 at 1C, indicating a significantly elevated energy threshold for TR. During TR, localized subcooled boiling occurs under IC conditions, enabling efficient heat transfer; the equivalent heat transfer coefficient ( h e ) of the FI mode reaches 5207.86 W/(m 2 ·K) at 3C. Moreover, IC effectiveness weakens under high overcharge rates, with TR suppression gradually shifting from delaying onset to mitigating hazards. These findings provide experimental evidence for optimizing liquid level design and assessing safety limits in energy storage systems employing immersion cooling.