Temperature control is critical in e-vehicles. The high-voltage batteries, electric motors and power electronics all operate best within a specified temperature range, where efficiency is maintained, charging is rapid and life of the components is longer. Thermal management fluids for electric vehicles are thus more than just an old-fashioned antifreeze for a traditional combustion engine. Water-glycol coolants, special oils, coolants (refrigerants) and, in new applications, dielectric fluids are used in modern EVs to transfer heat, lubricate parts and prolong driving range.
This overview of EV thermal management fluid explained consists of an overview of the various types of fluids and their role in vehicle systems and how each fluid type has practical properties to consider when choosing or providing them.
What “Thermal Management” Means in an Electric Vehicle
Thermal management in an EV can be defined as maintaining the temperature of a number of important systems within their optimal operating range. The primary focus areas include the high-voltage battery pack, electric motor or e-axle assembly, power electronics (inverter and onboard charger), and cabin HVAC systems. These loops are designed to be interconnected in a number of ways that allow the drivetrain-generated heat to be used in winter to heat the cabin or battery, and the heat to be discharged through radiators or heat exchangers.
Why Temperature Window Matters for EV Batteries and Powertrain
Cell temperatures between approximately 20 °C and 40 °C work best for fast charging and maximum power delivery. Lower temperatures will increase cell internal resistance and reduce power output. Above that, chemical degradation will occur at a faster rate and therefore the possibility of thermal runaway will rise if the heat is not distributed evenly. Efficiency and insulations or bearings may also be damaged if motors and inverters are permitted to overheat. A well-designed EV battery thermal management fluid thus has a direct impact on the practical EV range, charging rates, and long-term durability.
Water-Glycol Coolants – The Backbone of Many EV Cooling Loops
The majority of existing EVs currently use water-glycol mixtures as their main thermal management fluids of battery packs and many powertrain parts. The fluids flow between cold plates added to battery modules, or in jackets around motors and inverters. The indirect is to heat up the component, transfer heat to the metal plate or jacket and then to the circulating cooling medium, which in turn transfers heat to a radiator or chiller.
Water-glycol coolant for EV batteries has a familiar chemical makeup from internal combustion vehicles, but with the variations that are required for electrical properties and compatibility with materials, and extended service lives depending on various loads and operating patterns.
Key Properties Required of EV Water-Glycol Coolants
When considering these coolants, buyers will consider several performance attributes that directly impact the efficiency and durability of the vehicle:
- High specific heat capacity and thermal conductivity so that large amounts of heat can be transported away by the fluid even with relatively small flow rates and relatively small pumps.
- Good temperature stability—won’t lose viscosity at low temperatures, reducing the amount of energy wasted by the pump.
- Low corrosiveness and good compatibility with aluminum, copper, polymers and elastomers found in cold plates, hoses and seals.
In some designs, the electrical conductivity is deliberately low to minimize undesirable leakage currents or safety hazards in case of leakage near high-voltage components.
Refrigerants and Heat Pumps – Integrating Cabin and Powertrain Thermal Management
Heat pump systems are able to transfer heat back and forth which makes it a key part of EV thermal architecture and hence the use of refrigerants is significant. A heat pump can remove heat from the battery, motor or outside air and bring it in to the cabin instead of dismissing it, or take excess heat and use it to precondition the battery in winter. This integration means that the use of a resistive electric heater is reduced and the overall energy efficiency is increased, directly increasing the driving range.
The EV heat pump refrigerant and coolant system then connects the refrigerant circuit to the water-glycol circuits, enabling independent control of the drive train and passenger temperatures.
Refrigerant Choices and Environmental Considerations
Common options include R-1234yf, which was used in most markets due to its low GWP, and R-744 (CO₂) for some heat pump systems which work well in cold climates. Each one has its advantages and disadvantages in terms of pressure, flammability, system complexity and serviceability. Handling these systems is different than for water-glycol coolants, and requires special certification and equipment. Refrigerants are a component of the larger thermal management equation, but safety and regulation rules are different.
Oil-Based Thermal Management Fluids – Cooling Motors and E-Axles
New passenger EVs have adopted a new method of cooling electric motors over the last few years: oil cooling. The low-viscosity synthetic oils cool the e-axle’s stator windings and the surfaces of the rotor, and they also act as a lubricant for the gear and bearing elements within the e-axle. Oil spray or channel designs inside the jacket can increase the temperature uniformity, and in many cases, provide a higher level of uniformity than jacket cooling alone. Many designs then circulate the warmed oil through a heat exchanger to allow heat rejection to a water-glycol loop.
This oil-cooled EV motor thermal management approach allows for increased continuous power in a smaller package and is evolving into more and more production vehicles.
Requirements for Oil-Based EV Thermal Fluids
When making these fluids, formulators are required to balance:
- High Dielectric strength for the oil to be able to touch windings and high voltage equipment without conducting current or shorting out.
- Enough heat capacity and heat conductivity to dissipate heat efficiently.
- Low viscosity to reduce pumping losses and drag due to churning of fluid inside the motor, yet sufficient film strength to shield gears and bearings when subjected to load.
- The fluid operates hot, in air and in contact with air and metal surfaces for long periods and excellent oxidation stability and deposit control.
Anti-wear agents, antioxidants and sometimes friction modifiers are commonly included in additive packages to meet the demands of electric drivetrain operating duty cycles.
Dielectric Immersion Fluids – Direct Battery Cooling
Direct immersion cooling involves immersing battery cells or modules in a dielectric fluid, where heat is extracted directly from the battery casing into the liquid. This can provide very uniform cell temperatures, mitigate peak temperatures during high rate charging and minimize thermal runaway spread in abuse scenarios. It is currently a fledgling solution, largely being considered for vehicles with high performance or very fast charging speeds, instead of mass market cars.
Ideal Properties of Dielectric Immersion Fluids
The fluids used for battery immersion have to satisfy strict requirements:
- Extremely low electrical conductivity and high dielectric strength to prevent any current leakage or arcing.
- High specific heat and thermal conductivity combined with low viscosity for efficient heat removal and reasonable pumping power.
- High flash point, thermal stability and chemical inertness toward cell casings, busbars, plastics and seal materials.
- Acceptable environmental profile, low toxicity and preferably low global warming potential so the fluid suits automotive scale production and end-of-life handling.
Compatibility testing and long term stability data becomes critical because the fluid is in direct prolonged contact with the battery.
Comparing Thermal Management Strategies and Fluids
Various EV models implement these strategies depending on the cost, performance requirements and packaging constraints. Air cool is only seen in small or low power applications. Today, the most prevalent way of cooling batteries is through indirect water-glycol cooling, using cold plates or jackets. The newer designs rely mainly on oil cooling for thermal management of the motor. Refrigerant heat pumps provide cabin comfort and are now becoming more widely used to support battery preconditioning. Dielectric immersion cooling remains a niche and/or next-generation, high-performance idea.
A simple overview helps to clarify the relationships:
| Thermal Management Strategy | Primary Fluid(s) | Common Applications | Key Benefit |
| Indirect water-glycol | Water-glycol coolants | Battery packs, some motors | Proven reliability, straightforward integration |
| Direct oil cooling | Low-viscosity dielectric oils | Electric motors, e-axles | Higher continuous power density |
| Refrigerant heat pump | R-1234yf, R-744 and similar | Cabin HVAC + battery preconditioning | Bidirectional heat movement, efficiency gain |
| Dielectric immersion | Engineered dielectric fluids | Emerging high-performance battery packs | Superior temperature uniformity and safety margins |
How Fluid Choice Affects System Complexity and Cost
For higher stage architectures, a combination of multiple fluid loops or immersion cooling can help to achieve better temperature control and charge rates but brings in additional pumps, heat exchangers, sensors and sealing needs. OEMs have to balance these advantages with increased bill of materials, assembly complexity and long-term servicing requirements. Knowing which fluid family is required allows the distributor or workshop to specify the appropriate fluid for a vehicle’s maintenance needs and/or to suggest a suitable replacement product.
Key Selection Criteria for EV Thermal Management Fluids
Engineers and procurement staff usually consider some of the following when choosing an option among the various families of fluids:
- Specific heat, thermal conductivity and viscosity data over the entire temperature range of interest.
- Electrical properties: Type of conductivity or dielectric strength in areas where the fluid may be in contact with live components.
- Compatibility of materials: long term interactions of metals, polymers, elastomers and potting compounds as part of the cooling circuit or battery pack.
- Resistance to chemical reactions to oxygen, hydrolysis or additive depletion during real duty cycles (stability and service life).
- Environmental and regulatory compliance: toxicity, biodegradability, GWP limits and regional rules for coolants or refrigerants.
- System level impact: Pump sizing, heat exchanger volume and vehicle energy consumption.
Questions Distributors and OEMs Should Ask Fluid Suppliers
These are the most helpful questions to ask prospective suppliers:
- Are thermal and electrical properties available with full operating temperature range of the product including cold flow?
- What materials of the battery, motor and inverter have you tested to see if they are compatible, and what are the results of the accelerated aging tests?
- How many years or how many days should the service be performed and what is the method used in the field to check the condition of the fluid?
- Does the fluid meet the existing environmental and regulatory standards in target markets, such as any low-GWP and/or regional coolant specifications?
Suppliers that can provide evidence to answer these questions, add to a buyer’s confidence in long-term performance and compliance.
Market Outlook – Why Thermal Management Fluids Are a Growth Segment
The production of EVs is continuing to grow globally, as is the need for thermal management fluids. By 2035, all of this water-glycol, oil, refrigerant and immersion fluids will be required by the electric passenger car industry alone, projected to exceed 880 million liters a year. Demand for water-glycol to cool motors will increase along with overall fleet, and the opportunity for oil-based motor cooling fluids as well as for emerging dielectric fluids is higher value. The integrated heat pump systems add to the volume of fluids, as well as to the technical complexity of the fluid specified per vehicle.
Opportunities for Coolant and Lubricant Suppliers
As more sophisticated thermal management technologies come into play, there are obvious opportunities for the manufacturers and distributors who know how to meet the technical demands. There is increasing demand for water-glycol coolants that are optimized for low electrical conductivity or longer service life; for synthetic oils that combine lubrication and cooling in high power-density motors; and for dielectric fluids for next-generation packs cooled by immersion. Formulation knowledge, application engineering support, and a consistent supply to the global market are the most desirable attributes for establishing a long-term relationship with OEMs, Tier-1 suppliers and regional aftermarket networks. However, with the continuing evolution of vehicle architectures, technical differentiation and actual field performance will be the deciding factors about which suppliers will gain significant market share of this growing market.
Thermal management fluids for EVs are a real technical challenge, but significant business opportunity, for lubricant and coolant pros. The fluids that will be successful will provide measurable gains in efficiency, safety and longevity coupled with meeting the practical needs of high volume automotive production and service.