Experimental Characterisation of Jet Fires from NMC Battery Thermal Runaway: Passive Fire Protection perspective

Overcharging of lithium-ion batteries can lead to thermal runaway events that produce high-velocity gas and flame ejection, commonly described as jet fires. However, limited data exist on the thermal and physical characteristics of such fires. This study presents an experimental characterisation of jet fires generated from an overcharged 40 Ah NMC (Nickel Manganese Cobalt) pouch cell confined within a steel test chamber equipped with pressure transducers and thermocouples. The chamber featured a 12.7 mm (½ inch) vent, through which a pressure rise exceeding 1 bar and the subsequent release of a gas plume were observed at the onset of thermal runaway. Ignition of the vented gases by an ignition source produced a transient jet flame approximately 0.75 m in length. Heat fluxes were recorded using two instrumented steel plates positioned 0.5 m and 1.0 m from the vent, with a peak incident heat flux of 65 kWm⁻² at the nearer plate. The gas momentum was sufficient to displace the nearest 10 mm-thick steel plate, demonstrating significant velocity despite moderate thermal output. These results suggest that NMC battery jet fires differ substantially from conventional hydrocarbon or cellulosic fires, exhibiting strong momentum-driven behaviour along with expulsion of hot solid particulates, but relatively low incident heat transfer. Tests were conducted with varied vent sizes and multi-cell configurations with the aim to further refine understanding and to test the resilience of Passive Fire Protection (PFP) coating systems against this distinct fire phenomenon. Current battery fire testing and installation standards, such as UL 9540A and NFPA 855, evaluate the performance of energy storage systems under prescribed thermal runaway fire scenarios that primarily emphasise radiant heat exposure to adjacent units and nearby structures. While these methods are effective for assessing general fire propagation and spacing requirements, they do not fully capture the unique hazards associated with lithium-ion battery thermal runaway. In particular, thermal runaway events are characterised by transient, high-velocity jet flames driven by rapid internal gas venting, rather than by sustained, radiation-dominated combustion. UL 2596 is one of the few standards to date that incorporates a test methodology representative of this jet-flame behaviour. Consequently, where passive fire protection is required to mitigate fire spread between enclosures, existing standards may underestimate the directional and momentum driven effects of battery fires, leading to potentially inaccurate evaluations of fire spread, suppression requirements, and separation distances. Moreover, passive fire protection measures such as fire-resistant barriers or enclosures are not explicitly integrated into current safety evaluations, leaving a gap in guidance for mitigating the combined mechanical and thermal hazards posed by battery jet fires. This study addresses this gap by experimentally characterising the jet flame behaviour associated with thermal runaway in NMC lithium-ion batteries.