Does Transport Mode Blind an EV's Own BMS?

Largely, yes. A stowed EV sits in what UN GTR No. 20 calls usual parking mode: main software off, the battery's power line disconnected, the cooling system not operating and the battery management system not energised. Transport modes switch off more on top of that. NHTSA's FMVSS No. 305a proposal of 15 April 2024 found that most real-world single-cell thermal runaway fires started in exactly this state.

That matters for anyone designing detection for a vehicle deck, because it removes the one sensor that sits inside the battery. Loading guidance tells crews to isolate each vehicle at stow, for good reasons. The consequence is that the car's own monitoring is at its weakest during the voyage, which is when the ship most needs it, and the vehicle offers no signal a deck system can lean on.

What state is a stowed EV actually in?

Switched off, isolated and, by definition, unattended. The ICS, International Group of P&I Clubs and TT Club Common Guidance on the Presentation and Loading of Vehicles asks for the ignition off and the key stored in the cabin. It also asks for an ignition-off confirmation for EVs and hybrids, and for transport isolation mode or a battery kill switch to be engaged where fitted. That is done in consultation with the manufacturer, because some transport modes also release the handbrake.

Vehicle regulation describes the same state in its own terms. NHTSA's proposal distinguishes three operational modes: active driving possible, external charging and parking. The usual parking mode it quotes is the vehicle with main software off, gear in park, energy supply and battery power line disconnected and cooling not operational. The controls that manage safe operation of the battery, the BMS included, are not energised, and occupants are typically not present. A car lashed to a deck fits every part of that definition.

What does transport mode switch off?

Whatever the manufacturer decides, and it is designed for the 12V battery, not for fire safety. Transport or shipping mode is a low-power state set at the factory before a new car leaves the plant, to stop dozens of control units draining the 12V battery during weeks of transit and storage. Ford's version, for example, disables the audio system, interior lights and accessory delay, and on some models switches itself off once the car exceeds a set speed. Dealers clear the mode during pre-delivery inspection.

What none of the published descriptions say is what happens to battery supervision. The functions a low-power mode is built to suppress are the ones that keep electronics awake: telematics, remote alerts and periodic wake-ups. Whether a given model keeps any BMS monitoring alive in transport mode is model-specific and largely undocumented outside the manufacturer. For a ship loading thousands of vehicles from dozens of brands, that means the vehicle's self-monitoring cannot be assumed on a single car.

Does a sleeping EV still check its own battery?

Some do, on their own schedule, and the schedule is not built around a fire. Kia's owner's manual describes Aux. Battery Saver+. With the car off and all doors, hood and tailgate closed, the car periodically uses the high-voltage battery to recharge the 12V battery, with 360 V present in the vehicle while it runs. Each activation lasts at most 20 minutes, and the function stops after ten consecutive activations, on the assumption the 12V battery is faulty. BMS designs in the patent literature range from fixed periodic wake-ups to low-power monitors that wake the controller when a voltage or temperature threshold is crossed. No standard sets an interval.

Even a car that wakes and sees a problem has no one to tell. UN GTR No. 20's thermal-propagation warning, and the 5-minute thermal-event alarm in China's GB 38031-2025, are aimed at occupants. On a car deck the cabin is empty and locked, and there is no data path from the vehicle's BMS to the ship.

Why NHTSA dropped the occupant warning

Because the fires it reviewed did not happen while anyone was driving. GTR No. 20's warning requirement exists to give occupants time to get out and is framed around active driving possible mode. In the FMVSS No. 305a proposal, NHTSA declined to adopt it. Most real-world single-cell runaway fires, it said, occurred with the vehicle parked, propulsion off and no occupants, in home garages, parking garages or on the street, with state of charge generally in the upper range. It cited South Korean records of 35 EV fires since 2018, of which 20 began in the battery of a parked vehicle at more than 90% state of charge.

NHTSA proposed something else instead: documentation of each manufacturer's risk assessment and mitigation for single-cell runaway across all three operational modes, parking included. Compliance would follow two years after a final rule. Its own wording is that a warning is a secondary mitigation that "would not prevent the thermal propagation from occurring in the first place." In regulatory language, that is an acknowledgement that the parked, de-energised car is the case in-vehicle warnings do not cover.

What this means for deck detection design

Treat every stowed EV as an unmonitored battery and detect from outside it, continuously. First, a detection layer should not depend on any vehicle being awake: coverage that only works when the car reports would fail in usual parking mode. Second, detection has to be continuous rather than sampled on an interval, because a vehicle's own wake cycles are measured in hours or days while a venting cell develops in minutes. Third, the baseline should expect legitimate events such as a car briefly energising its high-voltage system to top up its 12V battery, so thresholds key on the pattern of a venting cell rather than on any warm spot. Finally, state of charge still matters. The loading guidance's 30–40% target sits well below the >90% band in the Korean parked-fire data, and holding it is one of the few battery-side controls the ship keeps.

Sources

• 1. NHTSA — Notice of proposed rulemaking, FMVSS No. 305a, Electric-Powered Vehicles: Electric Powertrain Integrity; Global Technical Regulation No. 20, Incorporation by Reference. 89 FR 26704, 15 April 2024, Docket NHTSA-2024-0012 (read from the GovInfo full text): three operational modes and their definitions (fn. 89–91); usual parking mode definition including BMS not energised and cooling not operational (fn. 96); most single-cell runaway fires occurred with the vehicle parked, propulsion off, no occupants (fn. 86); "NHTSA's Decision Not To Propose a Warning Requirement"; warning as a secondary mitigation strategy; documentation across all three modes; proposed compliance two years after the final rule.
• 2. South Korean EV fire records cited by NHTSA in the same notice (fn. 97: UNECE IWG EVS 23rd session, document EVS23-E1TP-0200, "EV Fire Records of Korea"): 35 EV fires since 2018, 20 originating in the REESS while parked at SOC above 90%.
• 3. UN Global Technical Regulation No. 20, Electric Vehicle Safety (UNECE WP.29): thermal-propagation warning to allow occupant egress, or 5 minutes before a hazardous situation in the passenger compartment.
• 4. ICS / International Group of P&I Clubs / TT Club — Common Guidance on the Presentation and Loading of Vehicles (Version 1.4): ignition off and key in cabin at stow; ignition-off confirmation for EVs and hybrids; transport isolation mode or battery kill switch per OEM advice; some transport modes release the handbrake; 30–40% SOC. Covered with its source in the linked post on the vehicle loading guidance.
• 5. Kia — owner's manual, "12V Aux. Battery Saver+" (ownersmanual.kia.com): cycle mode with the vehicle off and doors, hood and tailgate closed; high-voltage battery recharges the 12V battery; 360 V present during operation; maximum 20 minutes per activation; stops after ten consecutive activations.
• 6. Ford Transport Mode, as described by a Ford dealer (robsightford.com): disables audio, interior lights and accessory delay to reduce 12V drain; on some models exits automatically above a speed threshold; cleared before delivery.
• 7. US patent literature on BMS sleep and wake strategies (e.g. US 11,171,569, wake-up method and system for a battery management system; US 12,341,165, method for monitoring a battery system using threshold and predicted-value wake-ups): illustrates the range of designs; no common interval or standard.
• 8. GB 38031-2025 thermal-event alarm (5 minutes) is covered with its source in the linked article Does GB 38031 Change Marine Detection?.