EV Battery Thermal Management: Air, Liquid and Immersion Cooling Compared

Why temperature matters

A lithium-ion cell is an electrochemical system, and every reaction inside it depends on temperature. Run it too hot and side reactions accelerate: the solid electrolyte interphase grows, capacity fades and, in the worst case, the cell can enter thermal runaway. Run it too cold and internal resistance rises, available power drops and charging becomes risky because lithium can plate on the anode.

That is why a battery thermal management system (BTMS) is not an accessory. It decides how fast a vehicle can charge, how hard it can be driven and how long the pack will last.

The design targets

• Operating window: keep cells within the range specified by the cell maker, typically around 15–35 °C for best life.
• Uniformity: limit the temperature spread between the hottest and coldest cell — a few kelvin is a common target — so cells age evenly.
• Heat rejection at peak load: fast charging and track driving produce the highest heat flux.
• Heating: cold climates need the system to warm the pack, not only cool it.

Air cooling

Air cooling moves cabin or ambient air through channels between modules. It is simple, light and cheap, which is why early and low-power EVs used it. Its weakness is the low heat capacity of air: uniformity is hard to achieve and it struggles with sustained high power or fast charging.

Liquid cooling with cold plates

Most current EVs use a water–glycol coolant flowing through aluminium cold plates beneath or between cells. Liquid carries far more heat per volume than air, so the pack can stay uniform under heavy load. The loop is usually connected to the vehicle’s heat pump or chiller, which also lets it heat the battery in winter. The trade-offs are added mass, more components and the need to keep conductive coolant away from high-voltage parts.

Immersion cooling

In immersion cooling the cells sit directly in a dielectric fluid. Because the fluid touches every cell surface, heat transfer is excellent and temperature spread can be very small. It is used in some high-performance and heavy-duty applications and is studied widely for ultra-fast charging. Challenges include fluid cost, weight, sealing and long-term material compatibility.

How engineers choose

The choice depends on the duty cycle. A city car with modest charging power can work with a simple solution; a performance car or a truck that charges at very high power needs liquid or immersion cooling. Engineers size the system with electro-thermal models, validate them on test benches and check safety scenarios such as propagation after a single-cell failure.

To go deeper, explore our electromobility, batteries and hydrogen programmes, or read how motorsport engineers manage brake heat — another thermal problem with very different constraints.