Finding
Fast charging, not driving, sets the size of an EV's battery cooling, and the different temperature limits of each subsystem force several loops
At highway speed a pack delivers about 20 kW and its internal losses are a few hundred watts. At 250 kW on a DC fast charger the same pack generates heat on the order of 8 to 10 kW, which a chiller has to pull out through cold plates while holding every cell within a few degrees of the others. The motor and inverter peak during hard acceleration and towing; compute adds a steady load that never switches off.
The temperature limits explain the architecture. Lithium-ion cells age fastest above about 40°C and plate lithium when charged cold, so the battery loop runs near room temperature and is often heated before charging. Silicon carbide MOSFETs tolerate junctions up to 175°C and Class H motor windings up to 180°C, so the drive loop can run warmer and reject heat to a radiator without a compressor. Cabin climate is the only load that cares about people. Integrated systems such as Tesla's octovalve, Hyundai's E-GMP heat pump and BYD's e-Platform 3.0 route heat between these loops with one refrigerant circuit and a multi-port coolant valve, harvesting motor and inverter heat for the cabin and battery in winter.
The same parts - aluminum cold plates, interface materials, electric pumps, chillers - now cool BESS containers and liquid-cooled DC fast chargers, and the cold plate is also the core of direct liquid cooling in AI data centers. Who makes them is covered on Thermal System Supply Chain.
Loads
Thermal loads by subsystem
Each subsystem has a different limit and a different worst case.
| Subsystem | Main challenge | Limit | Worst case | Typical cooling |
|---|---|---|---|---|
| Battery pack | Cell temperature rise and cell-to-cell spread | LFP 15-35°C, NMC 20-40°C in normal operation; spread under 5°C | DC fast charging; cold-soaked charging | Liquid cold plates under or between cells; refrigerant chiller; immersion in development |
| Traction motor | Copper and rotor losses under sustained load | Winding insulation class (Class H, 180°C); magnet demagnetization | Towing, grades, repeated launches, track use | Water-glycol jacket; oil spray on windings; oil-cooled rotor |
| Inverter | Heat flux at the semiconductor junction | SiC junction 175°C; Si IGBT 150-175°C | Peak acceleration; high switching frequency | Pin-fin baseplates, direct substrate cooling, low-resistance interface stack |
| Onboard charger and DC-DC | Heat in a small enclosure | Magnetics and capacitor temperature | Long AC charging sessions in hot weather | Shared coolant loop, conduction paths, gap fillers |
| Compute and sensors | Continuous load in dense electronics | SoC junction around 105°C; throttling | Hot soak in summer with autonomy stack active | Cold plates, vapor chambers, liquid-cooled compute (Tesla AI4) |
| Cabin | Heating and cooling people in all weather | Comfort, defog and defrost rules | Winter heating at -20°C | Heat pump with resistive (PTC) backup; seat and wheel heating |
Hardware
Cold plates and cooling methods
The cold plate is the common part; its construction trades cost against heat flux.
| Type | How it works | Advantage | Tradeoff |
|---|---|---|---|
| Stamped tube-and-plate | Coolant in formed tubes bonded to a conductive plate | Low cost, proven | Weaker heat spreading than channel designs |
| Extruded channel plate | Channels inside an aluminum extrusion | Structural, repeatable; common under battery modules | Channel geometry limited to the extrusion profile |
| Brazed or friction-stir plate | Machined or stamped channels sealed by brazing or friction-stir welding | Flexible geometry, robust joints | Higher cost and process control |
| Microchannel plate | Many small channels for surface area | Highest heat flux; power modules and AI chips | High pressure drop; sensitive to contamination |
| Die-cast integrated plate | Cooling channels cast into a structural part | Fewer parts; cell-to-body packs | Expensive to change once tooled |
| Domain | Hardware | Bottleneck | Main method |
|---|---|---|---|
| Traction motor | Stator, rotor, gearbox | Rotor and end-winding hot spots | Jacket cooling, oil spray, integrated e-axle loop |
| Traction inverter | SiC or IGBT module, gate drivers, busbars | Junction-to-coolant thermal resistance | Pin-fin or direct-cooled baseplate |
| Onboard charger and DC-DC | Switches, magnetics, control boards | Mixed heat sources in a small box | Shared liquid loop and conduction frames |
| Autonomy compute | SoCs, memory, power stages | Sustained chip power | Liquid cold plate or vapor chamber |
| Infotainment and cockpit | Display and application processors | Steady consumer-electronics load | Spreaders, pads and airflow |
Fluids
Coolants, refrigerants and controls
The fluid sets corrosion, electrical safety and compliance; the controls decide where heat goes.
| Fluid | Role | Use | Key consideration |
|---|---|---|---|
| Water-glycol | Liquid heat transport | Battery, inverter and motor loops | Low conductivity coolant and material compatibility; freeze protection |
| Transmission or e-axle oil | Direct cooling and lubrication | Motor windings, rotor, gears | Dielectric strength and copper compatibility |
| R-1234yf | Refrigerant for heat pump and A/C | Most EVs worldwide | EU PFAS proposal; flammability class A2L |
| R-744 (CO2) | High-pressure natural refrigerant | Some VW ID and Chinese models | Works well in deep cold; system pressure above 100 bar |
| Dielectric fluid | Direct-contact cooling | Immersion packs and compute in development | Cost, weight, fluid life and serviceability |
| Function | What it does | Effect |
|---|---|---|
| Battery preconditioning | Heats or cools the pack before a charging stop, often triggered by navigation | Shorter fast-charge sessions, especially in winter |
| Loop prioritization | Allocates heating and cooling capacity among battery, drive and cabin | Stable performance while towing or fast charging in heat |
| Waste heat recovery | Routes motor and inverter heat to cabin and battery; some OEMs run the motor inefficiently on purpose to make heat | Less battery energy spent on heating |
| Fault detection and derating | Detects pump, valve, sensor and exchanger faults | Controlled power reduction instead of component damage |
BESS and EVSE
BESS and charger thermal
Stationary systems run at high power for hours, outdoors, and pay for cooling in parasitic load.
| System | Challenge | Effect | Cooling approach |
|---|---|---|---|
| BESS containers | Cell temperature and uniformity across a full climate range | Cycle life, usable capacity, fire risk | Liquid cooling has replaced forced air in new LFP containers (Tesla Megapack, CATL TENER, Sungrow PowerTitan) |
| BESS power conversion | Inverter and transformer heat | Limits continuous output | Liquid-cooled power stages or forced air |
| BESS auxiliary load | Chillers and fans run continuously | Lowers round-trip efficiency | Free cooling when ambient allows; setpoint control |
| DC fast charger cabinets | Aggregate heat from rectifier and DC-DC modules | Derates output on hot days | Liquid-cooled modules or hybrid air-liquid cabinets |
| Charging cables | Resistive heat at high current | Caps current near 500 A without cooling | Liquid-cooled cables and connector temperature sensors; required for the Megawatt Charging System |
Sources
Sources and scope
- Scope
- EV Thermal Systems
- Sources
- OEM technical disclosures (Tesla, Hyundai, BYD); semiconductor datasheets (SiC and IGBT junction ratings); IEC 60085 insulation classes
- Updated
- October 2026