One cell fails in seconds. Whether one cell becomes a pack, a rack, or a building was decided by design choices made years earlier — and that is usually where the liability sits.
Start a conversation with our AI Research Concierge, already scoped to thermal runaway. Pick a starting point, or describe your situation directly.
Thermal runaway is the self-accelerating chain of reactions inside a lithium-ion cell once heat is generated faster than it can escape. The sequence is well characterized: the protective layer on the anode begins to decompose at roughly 90 to 120 °C, the separator softens and melts at around 130 to 180 °C and lets the electrodes touch, and above roughly 150 to 200 °C the cathode releases oxygen into a flammable electrolyte — at which point the cell vents a mixture of hydrogen, carbon monoxide, and hydrocarbons and may ignite or explode. The forensic work divides in two. Initiation asks what pushed the first cell over the threshold: an internal defect, overcharge, mechanical damage, or external heat. Propagation asks why the neighbors followed, and that is an engineering question about cell spacing, thermal barriers, venting, battery management, and suppression — the questions UL 9540A testing and NFPA 855 exist to settle before a system is ever installed.
Five triggers account for nearly every initiation; the sixth mechanism is why a single cell so often becomes a much larger event.
Manufacturing contamination, separator damage, electrode misalignment, or lithium plating from cold or high-rate charging growing into a dendrite — the trigger that needs no external abuse.
A missing, defeated, or failed battery management system, or a mismatched charger, driving cells past their voltage limits into plating, gas generation, and heat.
Crush, puncture, or drop damage deforming the electrode stack — frequently latent, with the runaway arriving well after the impact.
An adjacent fire, an overheating connector or busbar, poor thermal management, or a sun-loaded enclosure taking a cell to its onset temperature from outside.
Capacity fade, separator degradation, gas generation, and swelling lowering the onset temperature in a pack that has outlived its safe life.
Heat conducted through casings and busbars, and vented gas igniting, carrying the failure from one cell to a module and from a module to a rack.
Thermal runaway work pairs the cell-level metallurgy with the electronic record, because the hours before the event are often documented in a battery management system that survived it.
A thermal runaway event rarely stays a technical question for long:
A pack that has partially run away still holds stranded energy and can reignite days later. Do not put it on a charger to see what happens, do not drain it, and do not open it. Isolate it, monitor it, and preserve the battery management system and the charger with it — the data inside them is often the only record of the hours before the failure.
A sequence of exothermic reactions, each one heating the cell into the next. The solid electrolyte interphase on the anode starts to break down at roughly 90 to 120 °C, exposing the anode to the electrolyte and generating flammable gas. Around 130 to 180 °C the separator softens and melts, the electrodes touch, and the internal short releases the stored energy as heat. Above roughly 150 to 200 °C the cathode decomposes and releases oxygen, so the cell now supplies its own oxidizer, and the electrolyte breaks down into hydrogen, carbon monoxide, carbon dioxide, methane, ethylene, and hydrogen fluoride. The cell vents, and the vented gas either burns as a jet or accumulates and deflagrates. Smothering does not stop it, which is why gas management and suppression strategy dominate the standards.
Often, within limits. The initiating cell is identified first, from CT and from the pattern of damage across the pack, and its internal condition is compared against its neighbors and against exemplars. Overcharge leaves lithium plating and a characteristic anode condition; mechanical damage leaves deformation that predates the thermal damage; an internal short from contamination can leave the particle itself, or the localized melt where it sat. Complete consumption of the cell can foreclose a definitive answer, which is why the battery management record, the charger, and the failure history of the same cell model matter as much as the metallurgy.
That is usually the central liability question, and the answer is design-specific. Propagation is governed by how much heat and vented gas one failing cell delivers to its neighbors and how much the design absorbs: cell spacing, thermal barriers, orientation, venting paths, busbar conduction, and whether the management system could isolate a module. For stationary systems, UL 9540A testing was created to measure exactly this at cell, module, unit, and installation level, and NFPA 855 sets what the installation must demonstrate. A pack that propagated under conditions its own test data said it should contain is a very different case from one that was never tested.
More than most parties realize. A battery management system records cell voltages, temperatures, current, fault flags, and balancing activity; an EV adds crash and diagnostic data; a stationary system adds site monitoring, gas detection, and suppression logs; a consumer charger may hold a charge-cycle history. Some of it lives in volatile memory that dies with the pack, some in cloud telemetry with a retention window, and some is overwritten within days. A preservation letter for the electronic record should go out at the same time as the one for the physical evidence.
Cells and packs: UL 1642 and UL 2054, IEC 62133, and UN 38.3 for transport. Micromobility: UL 2849, UL 2271, and UL 2272, which New York City requires by law and which the CPSC proposed to make mandatory nationwide in June 2026. Automotive: UL 2580 and the SAE J2464 and SAE J2929 abuse tests. Stationary storage: UL 1973 for the battery, UL 9540 for the system, UL 9540A for propagation testing (fifth edition, March 2025), and NFPA 855 for installation, whose 2026 edition requires large-scale fire testing and treats thermal runaway propagation prevention as an active protection method. The edition in force when the product was designed and sold, not the current one, is the one a case is measured against.
Technical briefings from our work in this area.
all electrical, electronics & batteries insightsTell us what happened. We will triage it and connect you with the right expert — usually within one business day.