Knowledge / Ship systems

Marine-battery thermal runaway: understanding propagation from cell to module

Separate electrical isolation from ending an internal chemical event, and examine propagation tests, off-gas, cooling and the limits of energy-capacity figures.

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Thermal runaway in a lithium-ion cell occurs when internal heat generation exceeds heat removal and becomes a self-accelerating temperature rise. Propagation from that cell to neighbouring cells and modules is a separate risk. Marine-battery safety depends on preventing initiating faults, detecting abnormal behaviour, containing propagation and managing the heat and gas released.

Cell, module, pack and battery space

A cell is the electrochemical unit. Cells form modules, and modules may form packs or racks; terminology varies by manufacturer, so the system boundary must be stated. The battery management system, or BMS, performs monitoring and protective functions. Energy management determines when and how much power the battery exchanges with the ship’s other sources. Treating these as the same function obscures failure responsibilities.

MCA MGN 550 Amendment 1 addresses both propagation from a single-cell defect and overheating risks in an electrically isolated battery. Opening a contactor may interrupt energy exchange with the external circuit; it does not establish that internal chemical reactions already under way have stopped.

Preventing initiation differs from containing propagation

Detecting excessive voltage, temperature or current can help prevent some initiating events. In another case, an internal cell defect may develop before the system can observe its first indication. A safety explanation limited to monitoring normal operating limits is therefore incomplete.

Heat transfer between cells, module enclosure design, the direction of vented gas and cooling availability during a fault influence propagation. Successful electrical protection and a safe battery space are different outcomes. Even if the initiating event remains within one cell, its gases can accumulate elsewhere or hot particles can affect surrounding materials.

What does a non-propagation result actually cover?

EMSA’s BESS guidance relates propagation testing to cell/module arrangement, operating conditions and protective auxiliaries; it also connects the number of cells represented in gas analysis to the propagation outcome. Saying only that a test passed is insufficient. The ways in which the tested arrangement represents the installation must be understood.

A reader should look for cell chemistry, module geometry, state of charge, the scope of ageing evidence and the representation of neighbouring modules. If an auxiliary operating during the test would be unavailable during the shipboard fault, direct transfer of the result becomes weaker. A replacement module with identical external dimensions but different cells or internal construction can require the existing evidence to be reconsidered.

kWh and kW answer different questions

Assume a nominal capacity of 1 000 kWh. An illustrative operating window from 80% to 20% state of charge provides 600 kWh. At a constant demand of 400 kW, neglecting losses and additional reserves, duration is 600/400 = 1.5 h. That state-of-charge window is a teaching assumption, not a recommended limit for actual batteries.

The electrical energy 600 kWh equals 2.16 GJ. It should not simply be treated as the heat released in a battery fire: chemical contributions from cells and surrounding materials, reactions and the extent of the event must also be considered. Electrical capacity alone does not determine fire load or gas volume.

In a separate power example, 600 kW at 1 000 V DC requires an ideal current of 600 A. Delivering the same power at 500 V requires 1 200 A. Assuming a fixed path resistance of 0.010 Ω, I²R loss rises from 3.6 kW to 14.4 kW. This shows the relationship between voltage, current and local heating; it is not a thermal-runaway model.

Gas management is more than an air-change figure

DNV’s 2020 joint-research summary reports that ventilation alone may be inadequate to mitigate gas accumulation when a substantial part of a battery system becomes involved. Limiting propagation can also limit the gas source. That dated finding does not supply a universal fan capacity or detector threshold.

Gas composition, release rate, temperature and destination matter. Total fan flow does not establish local dilution next to a venting cell. Potential flammable and toxic atmospheres, ignition sources and discharge towards occupied areas must be considered together. A detector’s type, location and response time bound what event it can actually observe.

What a simple dilution model cannot establish

For mathematics only, assume a perfectly mixed 40 m³ volume, a constant 0.40 m³/s exchange with clean air and a gas release that has completely stopped. A mass balance gives c(t)/c(0) = exp(−Qt/V). The time constant V/Q is 100 s. After 300 s the ratio is exp(−3) ≈ 0.0498, leaving about 4.98% of the initial concentration.

This is not a safe re-entry time. Initial concentration and hazard limits have not been specified; renewed release, poorly mixed pockets, temperature effects and fan failure are excluded. If gas continues to evolve, the no-source equation already has the wrong boundary condition. Actual space assessment depends on measurements, appropriate gas information and the vessel’s emergency arrangements.

The consequence of a small battery also depends on location

NTSB’s S-Trust finding identifies thermal runaway in a handheld-radio battery cell as the cause of a bridge fire on 13 November 2022, with loss of navigation, communication and alarm equipment. This was not a propulsion-battery test. It illustrates how proximity to essential equipment can magnify the consequences of a small energy source.

For a large energy storage system, consequences likewise extend beyond damage to the battery rack. Nearby distribution equipment, cooling, cables and escape routes can be affected by the same event. Two redundant electrical sources dependent on one fire or gas space do not become physically independent merely because there are two of them.

Read the documents, installation and operating state together

EMSA’s BESS page identifies its guidance as non-mandatory. Applicable flag and classification requirements and manufacturer limits must be established separately. A product certificate or the title of a guidance document does not establish acceptance of the complete shipboard integration.

A useful technical file connects cell/module identity, verified operating limits, the scope of propagation evidence, gas and heat assumptions, alarm and isolation functions, and auxiliary-system failure behaviour. The central question is which functions remain after a specified cell fault. Energy density and nominal battery capacity alone cannot answer it.

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