Is the Ship's Own Battery a Fire Risk?

Yes — and it is the one lithium installation aboard that is engineered properly. More than 800 battery-powered ships are in operation worldwide, and a hybrid or battery-electric vessel carries its own lithium energy storage in a dedicated room built to class rules that demand cell-level thermal-runaway isolation, dedicated off-gas ventilation and internal suppression. The vehicle decks on the same hull get none of that.

The ship's own batteries are a separate, growing installation

This is a different system from the cargo, on a different regulatory track, and it is scaling fast. More than 800 vessels now operate with batteries aboard for propulsion — pure electric or hybrid — with around 60% of them trading in Europe. On a hybrid ship the battery energy storage system exists for peak shaving, backup and reserve power, and load optimisation rather than primary propulsion, which means it is permanently installed, permanently energised and sized to the vessel's electrical demand. EMSA published its Guidance on the Safety of Battery Energy Storage Systems on board ships to promote consistent treatment of these installations, updating it in November 2025 to cover equipment design, battery technology, planning, operation and safety. That guidance is non-mandatory, and EMSA's own position is that there is no international regulatory instrument governing the safety aspects of using batteries in ships — so in practice the binding requirements come from class.

What class actually demands of a battery room

A great deal, and it escalates sharply with stored energy. DNV's class guidance DNV-CG-0660 limits a battery room to 5 MWh unless additional safety features are fitted. Where a single space needs between 5 and 25 MWh, extra barriers become mandatory: single-cell thermal-runaway isolation design, dedicated off-gas ventilation provision, thermal barriers between modules, increased spacing, a shift to safer chemistries such as LFP or LTO, and internal fire-extinguishing and cooling systems. The reasoning behind it is explicit — ventilation alone will not prevent an explosion if a large number of modules fail in the same compartment, so the design has to confine fire and gas emission to as small a part of the system as possible. That is a serious engineering standard: it treats aggregated lithium energy in a confined space as a hazard requiring containment at the cell level, not just detection at the compartment level.

The asymmetry — and where the comparison breaks down

The same hull applies cell-level engineering to a few megawatt-hours in the machinery space and compartment-level smoke detection to far more energy on the cargo decks. The arithmetic is easy to sketch and worth sketching carefully: a thousand battery-electric cars at roughly 60 kWh each is on the order of 60 MWh, and a near-fully-electric vehicle carrier with several thousand units runs into the hundreds — comfortably past the 25 MWh band where class stops permitting a simple battery room at all. Set against a 5 MWh threshold, a single vehicle deck is not marginally larger; it is a different order of magnitude.

But the two are not the same hazard, and the honest version of this argument has to say so. A battery room is a dense, electrically interconnected array held at working state of charge in a confined compartment — one cell failing sits millimetres from its neighbours in a common electrical and thermal path. Cargo packs are individually enclosed in vehicle structures, physically separated by the stow, not electrically interconnected, each with its own management system and casing, and ideally shipped at reduced charge. Propagation behaves differently, and it would be wrong to claim a car deck is simply 'ten times more dangerous than a battery room'. The narrower and more defensible point is this: the engineering attention follows where energy is *concentrated and interconnected*, and is largely absent where the same energy is merely *aggregated*. Aggregation is still accumulation. A fire does not need an electrical path between vehicles when it has a physical one.

What owners and underwriters should take from it

• Treat them as two exposures, not one. A hybrid car carrier has a class-governed BESS installation and an IMDG-exempt cargo deck full of lithium; a survey that covers one has not covered the other.
• Ask what governs the installation, because class does the binding work here. EMSA's guidance is non-mandatory and there is no international instrument, so the actual requirements sit in the class rules and the flag approval.
• Use the battery-room standard as the reference point for what good looks like. Off-gas detection and cell-level granularity are already accepted marine practice — on the cargo side they are treated as optional extras.
• For underwriters, note that the two risks correlate on the same hull. A casualty in either space threatens the other, and a hybrid PCTC concentrates both.
• Watch the trend line rather than today's fleet. Battery-hybrid tonnage is growing, and the vessels most likely to take it are the same ones carrying rising BEV cargo shares.

Sources

• EMSA — 'Guidance on the Safety of Battery Energy Storage Systems (BESS) on board ships': non-mandatory guidance drawn up with the European Commission, Member States and industry, aimed at uniform implementation of essential safety requirements for battery installations aboard ships; covers equipment design, battery technology, planning, operation and safety, including hybrid powering functions (peak shaving, backup/reserve, load optimisation); updated November 2025. EMSA states there is currently no regulatory instrument at international level on the safety aspects of using batteries in ships — emsa.europa.eu.
• Fleet scale: more than 800 vessels in operation worldwide using batteries aboard for propulsion in pure-electric or hybrid configurations, around 60% of them operating in Europe — EMSA / industry reporting via Safety4Sea and IIMS. [VERIFY: figure is from secondary coverage of the EMSA guidance rather than a fetched primary; confirm the count and date before publish.]
• DNV — class guidance DNV-CG-0660: battery room capacity limited to 5 MWh unless additional safety features are installed; between 5 and 25 MWh in a single space requires extra barriers comprising single-cell thermal-runaway isolation design, special off-gas ventilation provision, thermal barriers between modules, increased spacing, safer chemistries such as LFP or LTO, and internal fire-extinguishing and cooling systems; ventilation alone is insufficient to prevent explosion where many modules fail in one compartment — dnv.com. [VERIFY: these thresholds come from secondary summaries of DNV-CG-0660 and the DNV Maritime Battery Safety Joint Development Project technical reference; confirm the current edition's figures against the DNV document before publish, as class guidance is revised.]
• Illustrative arithmetic only: 1,000 battery-electric vehicles at roughly 60 kWh each ≈ 60 MWh aggregate. [VERIFY: this is a stated calculation from an assumed average pack size, not a sourced fleet figure — it is used to show order of magnitude against the 5 MWh / 25 MWh class thresholds, and should be presented as such or replaced with a manifest-derived figure.]
• NTSB — Genius Star XI (Christmas Day 2023, North Pacific): improperly secured lashings allowed 41 containerised BESS units to shift, causing internal damage that drove batteries in three units into thermal runaway — the cargo-side counterpart to the installation risk discussed here — ntsb.gov.
• Companion RoRoSAFE analysis — 'Can a Ro-Ro Still Carry Grid Battery Cargo?' (BESS as cargo under IMDG 42-24), 'Genius Star XI: Anatomy of a Contained Battery Fire' (what happens when cargo BESS fails), and 'Multi-Fuel PCTC Changes the Deck' (how the ship's own energy systems are changing).