Thermal runaway in a lithium-ion cell is an internal chemical reaction that releases heat. It can keep going without oxygen and without any visible flame, so it is not the same thing as an ordinary fire. A cell first becomes unstable and gives off gas. Its voltage then drops to zero as an internal short dumps all of its stored energy, and the temperature can briefly reach 300 to 600 °C. Any electrical, thermal or mechanical abuse can start it, and so can an internal defect. At the McMicken BESS, lithium plating and dendrite growth inside a cell caused the internal short. When cells are tightly packed with no thermal barrier, heat passes from cell to cell and module to module. Leftover heat can restart cascading minutes after the fire appears to be out. Runaway without flame gives off toxic and flammable gases such as H2, CO, HCl, HCN and HF. These gases can collect in a closed enclosure and explode. Clean-agent suppression such as Novec 1230 does not stop cascading. Heat must be removed, for example by cooling with water, or the fuel must be dispersed. Design controls include thermal barriers between cells, liquid cooling, air gaps, smaller compartmentalized battery groups, and ventilation to open air. Crews need training and emergency response procedures that cover cascading runaway, flammable gases, and safe entry into a system after a failure.
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controlled_byc1 drafted · lead Cell-to-cell thermal barriers proposed
Thermal or physical barriers between cells and modules, such as aluminum plates, heat-resistant plastics, intumescent or phase change materials, slow or stop cascading from cell to cell. Missing barriers contributed to cascading at McMicken.
sec=6.3 h=9f4b4a9bddcdsec=4.3 h=fdfe2f919c6esec=6 h=b218da0ba88dc2 drafted · lead Liquid battery cooling proposed
Cooling plates in contact with a water or glycol loop pull heat away from neighboring cells. The report rates this better than air cooling or plain plates that have no heat sink.
sec=6.3 h=9f4b4a9bddcdc3 verified · fact Water cooling extinguishment proposed
Cutting off oxygen has little effect on a reaction that feeds itself. Heat has to be removed, for example with water, or the fuel has to be dispersed.
sec=3.2 h=336fb71f0d6ctbl=14 h=b927736a3445verified by: review-workflowverified on: 2026-10-24
c4 drafted · lead Battery compartmentalization proposed
Smaller cells and isolated sub-groups that vent on their own limit the fuel mass, the heat and the volume of explosive gas.
sec=6.3 h=9f4b4a9bddcdc5 verified · fact Cell air gap spacing proposed
A designed air gap between prismatic cells cuts heat transfer, but an air gap alone may not prevent cascading.
sec=6.3 h=9f4b4a9bddcdverified by: review-workflowverified on: 2026-10-24
c6 drafted · lead Energy storage ventilation proposed
Venting to open atmosphere keeps flammable off-gas from collecting inside enclosures.
sec=6.3 h=9f4b4a9bddcdsec=6 h=b218da0ba88dc7 verified · fact Thermal runaway protection device proposed
Conditions: VRLA, lithium-ion and lithium metal polymer battery systems
shall be provided with a listed device or other approved method to preclude, detect, and control thermal runaway
sec=h61 h=702216dd0fffverified by: review-workflowverified on: 2026-10-24
c8 drafted · lead BESS emergency response proposed
Training and emergency response procedures should cover cascading thermal runaway, flammable gases and how to enter a system after a failure.
sec=6 h=b218da0ba88d