Can a Ro-Ro Still Carry Grid Battery Cargo?

Yes — but on a much smaller part of the ship than before. Since 1 January 2026, IMDG Amendment 42-24 has moved UN 3536, lithium batteries installed in a cargo transport unit, to on-deck stowage only. On a container ship that reallocates a slot. On a ro-ro or PCTC, where nearly the whole cargo envelope is enclosed vehicle space, it reduces the acceptable stow to the weather deck.

What actually changed on 1 January 2026

The stowage category for containerised battery energy storage systems moved from A to D. Category A permitted stowage on deck or under deck; category D permits on deck only, with stowage codes SW1 — protected from sources of heat — and SW2, clear of living quarters. The change came in through IMDG Amendment 42-24, adopted by IMO Resolution MSC.556(108) and mandatory from 1 January 2026. The reasoning is straightforward: a BESS in thermal runaway vents flammable and toxic gas for a long time, and an enclosed hold turns that vented gas into an explosion hazard while denying access for firefighting. On deck, it vents to atmosphere and a team can reach it.

The regulatory direction was set earlier. In July 2023 a member state put a simulation of an on-board BESS explosion to the IMO, and recommended amendments covering improved stowage, increased hold ventilation capacity, and assessment of the vessel's stability for free surface effect where large volumes of firefighting water enter a hold. Two of those three concerns are answered simply by not putting the unit in a hold.

Why that lands harder on a ro-ro than a box ship

Because a ro-ro's defining feature is that its cargo space is enclosed. A PCTC is a stack of closed decks; a con-ro or ro-ro carries breakbulk and containers on roll trailers inside that same envelope. A rule written around a container ship's on-deck/under-deck distinction maps awkwardly onto a hull where the weather deck may be a fraction of total capacity and, on many designs, is not laid out for a 30-tonne box that must stay clear of accommodation and heat sources. The practical result is not that ro-ros are barred from the cargo — it is that the acceptable stow is scarce, and scarce stow is where commercial pressure to reinterpret a category comes from.

The energy mass is the reason for the caution

A utility BESS container is a different order of hazard from a car. IUMI's March 2025 assessment notes individual units usually weigh over 30 tonnes, and the fire load is not a single pack but a dense assembly of cells engineered for grid-scale discharge. Gard's guidance is blunt about the controlling variable: state of charge directly influences both the likelihood of a thermal runaway fire and the growth rate and peak heat release once it starts, so a minimal SoC in transit is the single most effective lever available to the shipper. Nothing in the ship's fixed firefighting arrangement changes that curve after the fact.

The regulatory floor is also thinner than it looks. IUMI's view is that IMDG requirements around these units remain relatively broad, leaving hazards unaddressed. EMSA's own guidance on battery energy storage on ships — published 14 November 2023, last updated 11 November 2025 — opens by stating there is no international regulatory instrument on the safety aspects of using batteries in ships, and is itself non-mandatory. A category change is a stowage rule, not a fire-protection standard.

What the fire plan has to cover that it probably doesn't

An on-deck BESS creates a detection and response problem the vessel's existing arrangement was not built for. Enclosed vehicle decks get fixed detection and drencher coverage because SOLAS requires it there; the weather deck typically gets neither, and open-deck detection is a known gap — wind, sun-loading and spray defeat the threshold logic that works below. So the ship is now required to place its highest-energy cargo in the zone where it is least likely to see an early off-gas or temperature signature, and where the first indication may be visible flame. That is a defensible trade against explosion risk in a hold. It is not a complete answer, and an owner should not read the category change as one.

What owners and underwriters should ask for

• A declared state of charge, in writing, before the unit is accepted — Gard treats minimal SoC in transit as the primary control, and it is a shipper input the ship cannot generate for itself.
• Battery type, in-built safety features, and the manufacturer's emergency response guidance for that specific unit, requested from the charterer or shipper at booking rather than after the alarm.
• A stow plan that actually satisfies SW1 and SW2 on that hull — heat sources and accommodation proximity are design-specific on a ro-ro, and 'on deck' is not automatically compliant.
• Open-deck detection coverage for the stow position, since the fixed arrangement that protects enclosed vehicle decks does not extend there.
• A stability check where firefighting water could enter an enclosed space — the free surface effect concern raised in the July 2023 IMO submission survives the stowage change wherever water is applied near a hold opening.
• For underwriters: ask whether the operator accepts UN 3536 at all, and on what written conditions. An unstated acceptance policy on a ro-ro is an unpriced accumulation.

Sources

• 1. IMDG Code Amendment 42-24, adopted by IMO Resolution MSC.556(108) and mandatory from 1 January 2026: UN 3536 (lithium batteries installed in a cargo transport unit, Class 9) stowage category revised from A to D — on deck only — with stowage codes SW1 (protected from sources of heat) and SW2 (clear of living quarters) — imo.org. [VERIFY: the A→D category change and the SW1/SW2 assignment are reported consistently across amendment summaries but were not read directly from the IMDG text; confirm against the in-force Amendment 42-24 entry for UN 3536 before publish.]
• 2. Gard — 'Shipping battery energy storage systems: high energy, high risks?': UN 3536 covers lithium batteries installed in cargo transport units; Special Provision 389 applies only to UN 3536 and covers securing of batteries within the unit; state of charge directly influences both the likelihood of thermal runaway fire and the growth and peak heat release rate, so minimal SoC in transport is recommended; owners should require special handling instructions, SoC, battery type, in-built safety systems and emergency response guidance from charterers and shippers; a July 2023 member-state submission to the IMO simulating an on-board BESS explosion recommended improved stowage, increased hold ventilation capacity, and assessment of stability for free surface effect from firefighting water — gard.no.
• 3. IUMI — 'Risks associated with transporting containerised Battery Energy Storage Systems (BESS) by sea' (12 March 2025): individual BESS units usually weigh over 30 tonnes; classified Class 9, UN 3536; thermal runaway triggered by collision, compression, overcharging, over-discharging, external short circuit or high temperature, or by internal defect; current IMDG requirements remain relatively broad, leaving some potential hazards unaddressed — iumi.com.
• 4. EMSA — 'Guidance on the Safety of Battery Energy Storage Systems (BESS) on board ships' (published 14 November 2023, last updated 11 November 2025): non-mandatory guidance developed with the European Commission, Member States and industry, stating that there is currently no regulatory instrument at international level on the safety aspects of using batteries in ships — emsa.europa.eu. [VERIFY: EMSA's guidance addresses batteries used for ship power; confirm the extent to which it speaks to BESS carried as cargo before relying on it for cargo stowage points.]
• 5. Carrier acceptance practice — leading carriers reported as requiring utility-scale BESS units to load at roughly 25–35% state of charge, with dangerous-goods pre-approval before booking is confirmed. [VERIFY: this range appears only in freight-forwarder guidance, not an approved primary; confirm against a carrier's published DG acceptance policy or drop the figure before publish.]
• 6. Companion RoRoSAFE analysis — 'CINS Li-Ion Container Guidelines Meet RoRo' (how container-derived guidance transfers to vehicle-carrier loading), 'Weather Deck vs Enclosed Detection' (why open-deck sensing is the harder problem), and 'Is There a Charter Clause for EV Cargo?' (the contractual route to the data the ship is not given).