67 E-Mobility Engineering | September/October 2025 Runaway mitigation strategies As battery packs grow in terms of both size and energy density, preventing thermal runaway propagation has become a paramount safety concern. Multiple protection strategies have emerged, each with distinct mechanisms and implementation challenges. Immersion cooling has demonstrated particular effectiveness in testing scenarios, where dielectric fluids have successfully contained thermal events originating from deliberately punctured cells. The fluid’s ability to rapidly absorb and distribute heat, combined with proper venting system design, can prevent cascade failures even when multiple cells are driven into thermal runaway simultaneously. Material science plays an equally critical role in runaway prevention. Phase -change materials (PCMs) between cells can absorb large amounts of heat via latent heat storage, while specialised thermal barriers using aerogels or mica sheets provide insulation against radiant and conductive heat transfer. Some systems leverage the properties of water–glycol mixtures themselves – when exposed to extreme heat, the water component vaporises, absorbing significant thermal energy through phase change. Venting system design represents another crucial consideration. Effective thermal runaway mitigation requires careful routing of vent gases away from unaffected cells, often through dedicated manifolds or channels. This becomes particularly challenging in immersion systems where particular attention must be taken to ensure vent paths avoid displacing large volumes of dielectric fluid near the cells. Modern designs incorporate pressure relief valves and burst discs calibrated to specific failure modes, with geometries optimised to maintain fluid contact with cells even during gas expulsion events. Fail safe design Robust thermal management systems incorporate multiple layers of protection to maintain safety when primary cooling functions are compromised. At the most basic level, comprehensive sensor networks monitor temperatures throughout the pack, fluid flow rates and pump performance. These feed into battery management systems (BMSs) capable of detecting incipient failures through subtle changes in system parameters. When anomalies are detected, the BMS can initiate graduated responses ranging from power derating to complete system shutdown, depending on severity. The thermal mass of battery systems themselves provides inherent short-term protection. Even with failed cooling, most packs can absorb several minutes of heat generation before reaching critical temperatures. This buffer period allows time for protective measures to engage, although the duration varies significantly with pack design and operating conditions. Some implementations augment this inherent capacity with passive thermal buffers – PCMs or heat sinks that extend the window for safe shutdown or limited ‘limp home’ operation. Redundancy approaches differ substantially by application. Automotive systems typically prioritise cost and weight savings over full redundancy, instead relying on robust singlechannel designs proven to meet safety standards even with complete cooling failure. In contrast, aerospace applications such as eVTOL aircraft often incorporate fully redundant cooling circuits and pumps, reflecting the catastrophic consequences of total system failure in flight. Across all implementations, the trend toward digital twin technology enables more sophisticated failure prediction, with real-time system models comparing actual performance against expected parameters to identify degradation before it causes a failure. Regulation and simulation Battery developers face increasingly stringent global safety standards including UN ECE R100, ISO 6469, and China’s forthcoming GB380312025 rules. These standards mandate rigorous thermal runaway mitigation, vibration resistance and electrical safety protocols that fundamentally influence cooling system design. Compliance verification now requires extensive simulation and physical testing, adding approximately 10–15% to development timelines because engineers must demonstrate system resilience under worst-case scenarios. Advanced simulation tools have become indispensable for navigating these requirements. Engineers employ multi-physics modelling to replicate thermal shock tests, mechanical abuse conditions and propagation scenarios Miba employs a U-flow configuration and parallel routing of individual Flexcooler ribbons to each thermal interface to maintain stable temperatures even among cells in high C-rate charging (Image courtesy of Miba)
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