Battery cooling and thermal management

What is a dielectric fluid?

A dielectric fluid is an electrically non-conductive liquid that has a very high resistance to electrical breakdown. These fluids are typically used to cool and insulate electrical components.

This method of cooling is more efficient than alternative methods - such as forced air cooling - as the thermal conductivity and specific heat capacity of the liquid is higher than that of air, making the fluid more efficient at removing waste heat.

Dielectric fluids can be used in full submersion and spray systems, where electrical components are in direct contact with the fluid. They are already used to cool and insulate transformers, capacitors, high voltage cables and switchgear, providing electrical insulation and suppressing electrical discharge.

In electric vehicles, dielectric fluids can be used to cool the battery, electric-motor, power electronics and electric transmissions. The fluids must have high thermal conductivities and low viscosities to aid efficient heat transfer and pumping efficiency.

Performance criteria

For effective direct immersion cooling, a dielectric fluid must have a high density, high thermal conductivity, high specific heat capacity and low viscosity.

Other performance criteria that are important include:

• Good electrical properties - the fluid must have limited electrical conductivity to prevent arcing and also a high breakdown voltage
• Good material compatibility - the fluid must not harm elastomer seals, copper, insulation materials, or any other material found within the cooling system
• Low volatility - lower volatility reduces evaporative losses, reducing the need to top up the fluid
• High flash point - increases fluid safety and reduces fire risks in elevated temperature conditions
• Low pour point - the fluid must not freeze or become too viscous to pump at low temperatures
• High oxidation stability - the fluid must be stable at elevated temperatures and may be expected to perform over the lifetime of the vehicle

Why are new methods of cooling needed?

Air cooling is the traditional cooling approach in most vehicles today, alongside indirect water-glycol cooling. However, these methods are no longer efficient enough to manage temperatures in high performance batteries and power electronics that need to be charged at very high rates.

For higher performance applications, direct liquid cooling can be used. In direct cooling, water cannot be used as it will short circuit electronics. Water-glycol cooling systems also introduce weight and pose a risk if they leak. Direct cooling is therefore a preferred method that is safer, more efficient and more cost effective versus air and indirect cooling.

We are working with UK-based D2H Engineering to better our understanding of the difference between direct immersion cooling and indirect cooling. Using computational fluid dynamics (CFD), D2H analysed a water-glycol based cold plate cooled system and a dielectric fluid-cooled system under a simulated ultra-fast charging scenario. The data generated with the two scenarios found that:

• Maximum cell temperature was up to 20% lower when using immersion cooling compared to cold plate cooling.
• Temperature variation within a cell for an immersion cooled system was up to 57% lower than in the cold plate cooled system.
• The pumping power required to circulate the fluids was roughly equal for the two systems.

Direct immersion cooling using dielectric fluids in electric vehicles

Driven by the need to electrify, the automotive sector is now focused on creating highly efficient, power dense and lightweight electric motors, batteries and power electronics.

Effective thermal management is now the key challenge to overcome in unlocking maximum performance. Battery capacities are increasing and consumers desire even faster charging rates, increasing heat output which must be managed.

As interest in immersion cooling grows, there is a desire for fluids that can be used across the battery, motor and transmission, which requires coolants with low electrical conductivity, low viscosity, and good thermal heat transfer properties. This method of cooling is increasingly important as forced air cooling and indirect liquid cooling will not continue to meet the high-performance needs of the market.

Direct cooling of electronics in EVs will occur in the following areas:

Fast charging and high-performance use of batteries

Many electric cars are capable of being charged at very fast rates (high coulomb rates, C-rates) with further increases planned for future models. This causes the battery to heat up significantly. High temperatures reduce the lifespan of lithium-ion batteries and very high temperatures increase the risk of thermal runaway. Cooling is therefore necessary to reduce the temperature during fast charging or when demand on the battery is high.

Cooling during charging increases the efficiency of charge transfer and reduces degradation of the battery, allowing the battery to retain capacity over its lifetime so that maximum EV range can be maintained.

Electric motor(s)

E-motors are up to 94% efficient, and so small improvements in efficiency can result in large gains in EV range. Improvements in efficiency are delivered through effective cooling (reducing the effect of resistive heating) and improved lubrication.

Power electronics

The power electronics transfer power from the battery to the motor and are also responsible for controlling the current going into the battery during charging. Temperatures can increase during charging or when significant demands are placed on the battery (for example during spirited driving, or at high ambient temperatures).

High temperatures increase electrical resistance which in turn increases temperature, as a positive feedback loop. Cooling is crucial to the efficiency and longevity of the power electronics. Trends to reduce the size of power electronics produce additional challenges for cooling and dielectric fluids can overcome these challenges.

What makes our esters suitable for formulation into dielectric fluids?

We are world leaders in esters. When we set out to develop dielectric fluids for direct immersion cooling, we used our knowledge of ester chemistry to design molecules that would outperform alternatives. Our research and development programme led us to identify esters that have:

• High thermal conductivity
• Very high flash points
• Non-toxic
• Excellent dielectric properties
• Low viscosity
• Low volatility
• Halogen free
• Oxidatively stable
• Low pour points
• Biodegradable or bio-derived.

We have developed Xenitron™ 3221 - a fully synthetic dielectric ester that has been designed, tested, and manufactured to meet high standards of electrical insulation and cooling performance. The fluid can also be blended into traction fluids. With full control over our manufacturing processes, we have engineered Xenitron 3221 to be highly oxidatively stable, readily biodegradable with low viscosity, to help formulators optimise electric vehicle fluid formulations.

We have new products in development and are creating dielectric coolants that have very low viscosity coupled with high thermal performance using improved chemistries to meet the new needs of the industry. In doing so, we want to establish close partnerships with our customers to develop market leading materials for next generation fluids. Speak to us to get access to our latest development products.

A shared dielectric fluid for motors, transmissions, and gearboxes

As customer demand for electric cars grows and competition increases, there is a downward pressure on prices and manufacturers seek to reduce costs. Range is also a significant issue for EVs to be accepted in the market and one of the easiest ways to increase range is to reduce mass. A trend in electric vehicles is to combine the coolant loop from the motor with that of the lubricant used in the transmission or gearbox. This simplifies the cooling system and reduces the number of fluids required. This saves weight, complexity, and cost.

To achieve a dual-function fluid, the dielectric fluid must also have specific lubricating qualities including:

• Balanced friction and traction
• Low viscosity
• Good oxidation stability

With over 70 years experience in tribology and lubrication, our capabilities are aligned to help you meet your goals in next generation fluids.