By ATN Editorial Team
Battery immersion cooling submerges lithium-ion cells directly in a non-conductive dielectric fluid rather than cooling them through a cold plate, and in 2026 it is moving from hypercars and motorsport towards series production programmes. Every battery thermal management decision is a compromise between how much heat the cells generate and how quickly the cooling system can remove it. The dominant production solution, a water-glycol cold plate bonded to the base or side of the cells, cools each cell through a stack of thermal interface materials, housing walls and adhesive layers. Immersion cooling largely removes that conductive path between the cell and an external cold plate. The cells sit in direct contact with the coolant itself. This guide covers how the technology works, what the published data shows, who is running it in 2026 and what still stands between it and volume production. It builds on ATN’s earlier feature on how immersion cooling will impact EV battery design.
What Is Battery Immersion Cooling?
Battery immersion cooling places cells in direct contact with a dielectric fluid, a liquid with near-zero electrical conductivity that can touch live busbars and cell terminals without creating a short circuit. The fluid wets most or all of each cell’s surface, so heat can leave through much more of the cell surface rather than through one face. Because the fluid contacts far more of the cell than a cold plate or cooling channel, temperature gradients across and between cells shrink, which matters because cell ageing and usable power both track the hottest point in the pack, a constraint familiar from high power density motor design.
In a typical automotive implementation the pack is not simply flooded like a tank. A modest volume of fluid, roughly 4.5 to 16 litres in TotalEnergies’ demonstrator packs (larger packs will hold more), is pumped through a closed internal loop, guided along the cell surfaces by spacers, then passed through a fluid-to-coolant heat exchanger. Removing the cold plate also removes thermal interface materials, and in some architectures the dedicated battery chiller as well.
Single-Phase vs Two-Phase: The Two Architectures
Single-phase systems keep the fluid liquid throughout the cycle. Heat is absorbed by the fluid’s sensible heat capacity and rejected in an external heat exchanger. Nearly every automotive application to date, from the McLaren Speedtail to XING Mobility’s commercial packs, is single-phase, because the system is a conventional pumped loop that packaging and service engineers already understand.
Two-phase systems use a fluid with a boiling point tuned so it evaporates on the hot cell surface, absorbing its latent heat of vaporisation, then condenses elsewhere in the enclosure. The phase change absorbs far more energy per unit of fluid and needs less pumping, but it demands pressure management, condensing hardware and fluids that have historically been fluorinated chemistries. Specialists such as Carrar are developing two-phase automotive systems, but deployment maturity is well behind single-phase.
Dielectric Fluids: The Chemistry Question
The fluid is the system. Candidate families are synthetic esters and hydrocarbon oils, silicone oils and hydrofluoroethers (HFEs). The trade-offs are physical: HFEs combine low viscosity, low flammability and well-behaved boiling characteristics with densities above 1.5 g/cm3, a direct weight penalty, while esters and hydrocarbon blends sit below 1 g/cm3. All of them carry markedly lower thermal conductivity and specific heat than water-glycol, so immersion outperforms despite the fluid, not because of it: equivalent heat transfer comes from the far larger wetted area, direct convection at the cell surface and the removal of interface resistances rather than from fluid properties alone. Fluid viscosity and material compatibility still drive pump, hose and seal specification.
Regulation is reshaping the fluid menu. 3M announced it would exit PFAS manufacturing by the end of 2025, withdrawing its Novec fluid line that served early immersion and electronics cooling applications, and per- and polyfluoroalkyl substance scrutiny in Europe is encouraging development of ester and hydrocarbon alternatives. The lubricant majors have moved in: TotalEnergies markets a dedicated dielectric fluid range and Castrol formulated its ON EV Thermal Fluid for immersion cooling, alongside suppliers such as FUCHS, Lubrizol, M&I Materials and Engineered Fluids.
Immersion vs Cold Plates: What The Data Shows
The most cited like-for-like test is a 2021 study in the journal Energies that ran the same 21700 cell module with a cold plate and with single-phase immersion. Immersion produced 2.5 to 3 times higher thermal conductance, lower peak cell temperature and smaller internal temperature gradients at high discharge rates, and a 15 to 25 times lower pressure drop across the flow rates tested. Those figures apply to the specific module, geometry and coolant in that study rather than to immersion in general, and the same study is honest about the limits: at a gentle 0.5C discharge the peak temperature difference between the two approaches was about 2 degrees Celsius. Immersion earns its complexity at high C-rates, not in steady cruising.
Demonstrator programmes point the same way. TotalEnergies reports that converting a Volvo XC90 PHEV pack to immersion delivered around 7 times higher cooling efficiency normalised by cell surface area, with an 8 per cent cost and 6 per cent weight reduction at pack level, and that its immersed Renault Megane E-Tech demonstrator halved 0 to 80 per cent charging time while cutting cooling energy consumption by a factor of 8. These are the company’s own figures from its own demonstrators, but they are consistent with the peer-reviewed comparisons.
Safety: Thermal Runaway Suppression
Immersion’s strongest card may be safety. In overcharge testing published in Applied Energy, fully immersed lithium iron phosphate cells did not enter thermal runaway at a reduced charging rate, with peak temperature held to around 110 degrees Celsius, and the fully immersed configuration scored a fraction of the hazard rating of the same cell in air. In UL 9540A-style testing reported by immersion system suppliers, circulating dielectric fluid prevented cell-to-cell propagation in the published tests, and in one publicised 20-cell test runaway triggered in a heated cell did not spread to its neighbours.
The mechanism is thermal rather than chemical. Thermal runaway does not depend on atmospheric oxygen, since the decomposing electrolyte supplies its own oxidising species, so the fluid does not stop the initiating failure. What it does is absorb heat around every cell, dilute and quench vented gases and keep neighbouring cells below the temperatures at which they would fail in turn. It is not a substitute for cell quality, battery management or venting design, but it changes the propagation maths that pack engineers have to demonstrate for regulatory approval.
Who Runs Battery Immersion Cooling in 2026
McLaren. The Speedtail, delivered from 2020, was the first production road car with cells permanently immersed in a dielectric oil. McLaren credited its Formula E experience and quoted a pack power density of 5.2 kW/kg, which it claimed as the highest of any road car battery at the time.
Mercedes-AMG. The AMG ONE’s 400V high performance battery cools each of its 560 cells individually with around 14 litres of electrically non-conductive fluid circulating past every cell, holding the pack at a constant working temperature of around 45 degrees Celsius regardless of load. The company brands this direct cell cooling rather than immersion: each cell is washed individually by the circulating fluid rather than sitting in a shared bath. Mercedes-AMG describes direct cell cooling as the most efficient way to remove heat from the cells, while acknowledging the approach is too complex for mass production in its current form.
XING Mobility. The Taiwanese company has built immersion packs for construction machinery and speciality vehicles for a decade and now supplies its IMMERSIO cell-to-pack technology for Caterham’s Project V electric sports car, whose production prototype debuted at Tokyo Auto Salon in January 2026. XING is also carrying the same architecture into stationary storage and data centre backup batteries.
The pattern. Analyst firm IDTechEx characterises adoption as concentrated where packs are power-dense rather than energy-dense: high-performance hybrids, motorsport, luxury electric vehicles and construction equipment. Volume passenger car programmes remain committed to cold plates, which are proven, cheap and already demonstrate high charging power.
The Barriers: Weight, Cost and Serviceability
Three things keep immersion out of mainstream packs. First, the fluid itself adds mass and cost, only partly offset by deleting cold plates and interface materials, and industry analyses still put immersion’s total system cost well above a cold-plate equivalent. Second, standard battery modules are not designed to hold liquid: enclosures must be redesigned for sealing, corrosion and long-term material compatibility, and every wetted component must be qualified against the fluid. Third, serviceability. A pack full of oil changes workshop procedures, end-of-life handling and repair economics, and OEM engineering organisations have little accumulated experience with any of it.
IDTechEx’s assessment is that immersion will grow strongly but from a small base, with dielectric fluid demand for EVs forecast to rise roughly ninefold between 2026 and 2033 while remaining a minority share of automotive thermal management. The technology’s near-term home is where heat flux is extreme and volumes are modest.
What Engineers Should Watch Next
Three signals matter over the next 24 months. One: Caterham Project V industrialisation. If XING Mobility’s cell-to-pack immersion architecture reaches genuine series production in a sports car, it becomes the reference point that every OEM advanced engineering team benchmarks. Two: the fluid chemistry race. With fluorinated fluids retreating under PFAS pressure, the lubricant majors’ ester and hydrocarbon formulations will decide the cost and weight equation. Three: megawatt charging. As charging power climbs, in trucks and in concepts such as the Shell Triple 10, the thermal case for direct cell contact strengthens, and the same logic applies to the power electronics covered in ATN’s traction inverter guide.
Frequently Asked Questions
Battery immersion cooling places lithium-ion cells in direct contact with a non-conductive dielectric fluid, removing the cold plate and thermal interface materials used in conventional packs. Heat leaves through much more of each cell’s surface, giving more uniform temperatures and higher heat extraction.
Single-phase systems pump a liquid dielectric fluid past the cells and reject heat in an external heat exchanger. Two-phase systems use a fluid that evaporates on the hot cell surface, absorbing its latent heat, then condenses within the enclosure. Nearly all automotive applications to date are single-phase.
Yes, in low volumes. The McLaren Speedtail was the first production road car with immersed cells, and the Mercedes-AMG ONE cools all 560 of its cells directly with a non-conductive fluid. Caterham’s Project V, using XING Mobility’s immersion cell-to-pack technology, showed its production prototype in January 2026.
Published comparisons show immersion holds cells cooler at high charge and discharge rates than cold plates. TotalEnergies reports its immersed Renault Megane E-Tech demonstrator halved 0 to 80 per cent charging time versus the standard car, a company figure consistent with peer-reviewed testing.
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