Most serious lithium-ion incidents are not really about one cell. A single cell entering thermal runaway is a contained, survivable event in a well-designed pack. The incidents that produce large losses are the ones where that first cell heated its neighbours past their own onset temperature, and the failure walked through the pack. Propagation, not initiation, is usually what separates a warranty claim from a catastrophic loss.

How propagation happens

A cell in runaway releases energy by several routes at once. It conducts heat through direct contact with adjacent cells and through the structure holding them. It vents hot gas and ejecta which can heat neighbouring cells convectively and, if the gas ignites, radiatively. Ejected material can create external short circuits between nearby cells or busbars. Each of these paths can push an adjacent cell past its onset temperature, and each adjacent cell that goes adds its own energy to the process.

Whether the chain continues depends on pack design decisions: cell spacing, the thermal conductivity of the materials between cells, the presence and performance of barriers or intumescent layers, venting paths that direct hot gas away from other cells rather than across them, and the thermal management system's ability to remove heat during the event rather than merely during normal operation.

This has become one of the more active areas in battery safety research. Recent work has developed methods for assessing uncertainty in propagation behaviour (Applied Thermal Engineering, 2024) and evaluated active suppression using mini-channel cooling structures (2023), while surrogate modelling approaches aim to make pack-level propagation prediction tractable without full physical testing of every configuration (IEEE RAMS 2026).

What the forensic question actually is

After an incident, two questions run in parallel, and they have different answers and often different responsible parties. The first is initiation: which cell went first, and why. That is a cell-level investigation — internal short, separator defect, manufacturing contamination, overcharge, mechanical damage, external heat — and it points toward the cell manufacturer or the conditions of use.

The second is propagation: given that one cell failed, should the pack have contained it. That is a system-level question about pack architecture, barrier materials, venting design, and the thermal management strategy. It points toward the pack integrator and the system designer. A defensible investigation has to separate them, because a cell defect and a propagation-control deficiency are distinct failures that can coexist in the same incident.

Reading a burned pack

Establishing the origin cell in a pack that has largely been consumed sounds impossible and generally is not. Damage severity typically grades away from the initiation site. Non-destructive imaging — CT and X-ray — locates internal damage and can identify cells whose internal structure indicates they failed from the inside rather than being cooked from outside. Melting and annealing patterns on busbars and structure record the thermal gradient. Where they survive, battery management system logs provide voltage, current, and temperature history that can identify which cell diverged first and under what conditions.

Standards work is relevant throughout: UL 1642 and UL 2054, IEC 62133, and UN 38.3 for transport all bear on what the cell and pack were required to withstand, and on whether the qualification testing performed matched the conditions the product actually met in service.

Preservation

A failed pack is both evidence and a hazard. It should not be discarded, charged, cleaned, or disassembled, and it should be stored safely and isolated from heat and moisture. Beyond the pack itself, the charging equipment, the BMS and any stored data, and the installation context are all part of the record. Altering the cell can destroy the proof of what happened and weaken the case that depends on it.

For the cell-level mechanisms and the full examination protocol, see our specialization area on lithium-ion battery failure analysis.