What Causes EV Fires in RoRo Ships?

When a vehicle carrier makes the news for the wrong reason, the explanation is usually one phrase: "EV fire." It is not wrong, but it is not useful either. EV battery cells fail at very low rates in the field — orders of magnitude lower than ICE engine fires per vehicle-mile. The reason RoRo casualties dominate the loss tables is not battery failure rate. It is what happens to a battery fault inside a 6,000-vehicle enclosed steel box.

The five compounding factors

1. Mechanical damage during loading

Vehicles are driven on and off ramps at sharp angles, secured with lashing chains pulled tight against the chassis, and parked within centimetres of each other. Battery packs in the underbody are exposed to crush, puncture, and impact loads that the OEM crash structure was not designed to absorb at low speed.

2. Pre-existing internal cell defects

Cell-level manufacturing defects — separator contamination, dendrite growth, contaminant particles — usually surface within the first 100 charge cycles. New EVs roll onto carriers with very low cycle counts, meaning latent defects are statistically more likely to be present and undiscovered.

3. State of charge at loading

There is no industry-wide standard for SoC at loading. Some operators specify 30%; many manufacturers ship at 50–60% or higher to support delivery test drives. A higher SoC means more energy available to release in a thermal event and a higher peak temperature in runaway.

4. Environmental stress

Salt mist accelerates corrosion at any exposed cell terminal or BMS connector. Vessel motion induces low-frequency vibration on lashed vehicles for days or weeks. Engine-bay temperatures on hot-route sailings cycle higher than the OEM static-test envelope.

5. The enclosed deck itself

The four factors above raise the probability of an event. The fifth turns a manageable event into a casualty. On an enclosed cargo deck, ventilation dilutes early thermal and VOC signatures so they do not reach ceiling-mounted sensors at threshold. Legacy suppression systems were designed for hydrocarbon fires, not lithium-ion thermal runaway. Surrounding vehicles add fuel load. Crew access is restricted by ramp geometry and lashing.

Why this matters for detection

The takeaway from the multi-factor view: the battery fault is not the failure mode. The failure mode is the detection lag between fault and intervention. A system that observes per-vehicle thermal state continuously, on every deck, at sensor density that survives the dilution problem — that is the layer that changes the outcome.

Sources

• EMSA — "Guidance on the Carriage of Alternative Fuel Vehicles in RoRo Spaces" (2023, updated 2024).
• Larsson, F. et al. — "Toxic fluoride gas emissions from lithium-ion battery fires," Scientific Reports 7, 10018 (2017).
• IUMI — "Risk mitigation for the safe ocean and short-sea carriage of electric vehicles" (Sept 2025 revision).
• Allianz Global Corporate & Specialty — "Safety and Shipping Review 2024," RoRo casualty commentary.
• US NHTSA — "Risks Associated with Lithium-Ion Batteries in Electric Vehicles" (2024 update), for comparative EV vs ICE fire-rate context.
• [VERIFY: "first 100 charge cycles" cell-defect emergence window — widely cited in battery-engineering literature; primary citation pending.]