The Four Stages of Li-Ion Thermal Runaway — and What Each Implies for Detection

A lithium-ion cell going into thermal runaway passes through four observable stages before failure. They are not equally detectable — and detection products that claim to "catch fires" without naming the stage they catch are usually catching Stage 3 at best. The trade calls Stage 1–2 detection "pre-fire detection" for a reason.

Stage 1 — internal pressure rise (invisible)

The earliest stage. SEI layer begins to break down at elevated temperature. Internal pressure rises within the sealed cell. Outside the cell, nothing is observable to any deck-level sensor. Detection here requires per-cell instrumentation built into the vehicle (BMS) — the marine layer cannot see it.

Stage 2 — electrolyte off-gas vent (the pre-fire window)

Cell vents electrolyte vapor: carbonate VOCs (DMC, EMC, EC), trace hydrogen. Mass is small; rate is detectable with sufficiently sensitive gas sensing. This is the window the off-gas detection industry operates in — typically advertised as a 5–30 minute lead over visible smoke.

Stage 3 — thermal runaway initiates

Internal temperature climbs steeply. CO and CO₂ appear as electrolyte decomposes. Surface temperature on the pack begins to rise — detectable by IR sensors at distance. Smoke becomes visible.

Stage 4 — propagation and HF release

Adjacent cells ignite; pack-level runaway propagates. HF appears at concentrations dangerous to crew. Suppression effectiveness drops sharply once propagation begins.