The Four Stages of Li-Ion Thermal Runaway
Li-ion runaway moves from SEI breakdown near 80–120 °C through venting to fire. Useful detection works in the venting window, before smoke and flame.
A lithium-ion cell does not go from normal to fire in one step. It passes through a sequence: abuse or an internal fault starts heat-generating reactions, the cell vents gas, smoke appears, and then fire, which in a pack spreads cell to cell. Each stage is detectable by different means, and at very different distances. Detection that is meant to change the outcome has to work in the venting stage, because by the time there is smoke, runaway has usually begun.
Stage 1: what starts the reactions?
Heat from abuse or an internal fault, and a cascade of chemistry that feeds on itself. Feng and colleagues' widely cited 2018 review sets out the sequence. The solid electrolyte interphase on the anode begins to break down at roughly 80 to 120 °C; the exposed anode then reacts with the electrolyte at about 120 to 160 °C; the polyethylene separator melts at around 130 °C (polypropylene near 170 °C), allowing internal short circuits; and the cathode decomposes between about 150 and 350 °C, releasing oxygen that sustains combustion inside the sealed cell. From outside, nothing is yet visible.
Stage 2: when does the cell vent?
When internal pressure opens the cell's vent, before smoke. The first release is electrolyte solvent vapour, such as dimethyl carbonate, followed by gases produced by the reactions, including hydrogen, carbon monoxide, carbon dioxide and methane. This off-gas stage is what gas-detection vendors target, quoting between 2 and 30 minutes of warning before runaway. In a vehicle the cell sits inside a sealed pack that releases gas through a designed vent path, so what a sensor outside the car sees is what survives that path and then the deck's airflow.
Stage 3: when does smoke appear?
Once the reactions accelerate beyond the cell's ability to shed heat. Temperature climbs steeply, the pack surface warms enough for infrared sensing to register, and visible smoke is released. Detection vendors describe smoke and fire as occurring almost together at this point: by the time smoke is seen, runaway has usually already begun. A deckhead smoke detector responds here or later, after the smoke has travelled to it and survived the ventilation.
Stage 4: how does it become a deck fire?
By spreading cell to cell inside the pack, and then vehicle to vehicle. Heat from one cell drives its neighbours through the same sequence, the pack burns with jetting flame, and toxic gases including hydrogen fluoride are released. On a car deck, radiant heat reaches the next vehicle; full-scale tests put spread to an adjacent car at around 20 minutes. At this stage the task is containment and suppression of a fire, not prevention.
How RoRoSAFE helps
RoRoSAFE is designed to act in Stage 2, when a cell vents electrolyte but has not yet ignited. Per-vehicle vent-gas and thermal sensing flags that window before visible smoke, so the crew responds while the event is still recoverable, not after runaway has started.
Pilot: one deck · installed alongside the berth · no drydock · 6 months of dashboard access
Sources
- 1. Feng, X., Ouyang, M. et al. — 'Thermal runaway mechanism of lithium ion battery for electric vehicles: A review', Energy Storage Materials 10 (2018) 246–267: reaction sequence and approximate onset temperatures (SEI decomposition ~80–120 °C; anode–electrolyte reaction ~120–160 °C; separator melting ~130 °C PE, ~170 °C PP; cathode decomposition with oxygen release ~150–350 °C).
- 2. Honeywell Li-ion Tamer — the four stages of battery failure (abuse, off-gas, smoke, fire); 2–30 minutes' typical warning from off-gas detection; smoke and fire near-simultaneous. Vendor material.
- 3. Full-scale vehicle fire tests — Energies (MDPI) 2019, 12(8):1465: spread to the adjacent car ~20 minutes.
- 4. Companion analyses — 'Can Electrolyte Vapour Warn Before Fire?', 'H₂, CO, CO₂: The Thermal-Runaway Signature' and 'How Long Is the Off-Gas Window, Really?'.
Questions, answered
What are the stages of lithium-ion thermal runaway?+
Abuse or an internal fault starts heat-generating reactions inside the cell; the cell vents electrolyte vapour and gas; smoke appears as runaway accelerates; and fire follows, spreading cell to cell and, on a car deck, vehicle to vehicle. Each stage is detectable by different means.
At what temperature does thermal runaway start?+
The first exothermic step, breakdown of the solid electrolyte interphase, begins at roughly 80 to 120 °C, according to Feng and colleagues' 2018 review. Separator melting follows at about 130 °C for polyethylene, and cathode decomposition releases oxygen between about 150 and 350 °C. Exact values depend on chemistry and state of charge.
How much warning does off-gas detection give?+
Detection vendors quote between 2 and 30 minutes before thermal runaway for off-gas detection in stationary battery storage. In a vehicle the gas must first escape the sealed pack's vent path and survive deck ventilation, so the usable window on a ship depends on where the sensor is.
Why is smoke detection too late for battery fires?+
Because smoke and fire arrive almost together once runaway accelerates. By the time smoke is seen, runaway has usually begun, and a deckhead detector only responds after the smoke has travelled to it. The stage where the outcome can still be changed is the venting stage, before smoke.
Continue the thread
H₂, CO, CO₂: The Thermal-Runaway Signature
The order matters more than the shortlist. Solvent vapour beats H₂ by about fifteen minutes — and LFP and NMC invert which gas dominates.
How Are Thermal Hotspots Detected?
Measuring temperature on a cargo deck is easy; deciding which rise is real is not. Relative thresholds and neighbour checks separate faults from noise.
Baselining a Three-Chemistry Car Deck
The manifest never says which chemistry is aboard. So a mixed deck cannot be baselined per chemistry — only per position, against the weakest signal.