The Four Stages of Li-Ion Thermal Runaway
Stage 1 is invisible. Stage 4 is unrecoverable. Detection technology that wants to matter operates between Stages 1 and 2 — not between Stages 3 and 4.
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.
Questions, answered
What are the four stages of lithium-ion thermal runaway?+
Stage 1, internal pressure rise (invisible to any deck sensor); Stage 2, electrolyte off-gas vent — the pre-fire window with carbonate VOCs and trace hydrogen; Stage 3, runaway initiation with CO/CO₂, rising surface temperature, and visible smoke; and Stage 4, propagation with HF release dangerous to crew, where suppression effectiveness drops sharply.
Which stage should detection technology target?+
Stages 1–2 — the pre-fire window. Products that claim to catch fires without naming the stage are usually catching Stage 3 at best. Detection at Stage 4 is salvage logistics, at Stage 3 is suppression logistics, and at Stage 2 is where the outcome is genuinely changeable.
How fast do the stages progress?+
Roughly 5–30 minutes from Stage 2 to Stage 3 at the cell (ambient-dependent), about 2–5 minutes Stage 3 to Stage 4 at the pack, and under a minute from Stage 4 to uncontrollable deck-level propagation. The narrowing windows are why earlier detection disproportionately changes the outcome.
Continue the thread
H₂, CO, CO₂: The Thermal-Runaway Signature
The gas-phase signature of a stressed Li-ion cell has a specific order and a specific tempo. Knowing it tells you what to sense, where, and when.
How Are Thermal Hotspots Detected?
The detection problem on a cargo deck is not measurement — IR sensors are commodity. It is deciding which delta in which cell at which time is real.