Research and development work to solve an excessive noise problem created by an EV’s battery fast charger has led to advances in thermal management, heat dissipation, and reduced fire risks. D2H Advanced Technologies, a UK-based specialist engineering company, found that the noise occurred during the vehicle’s fast-charge cycles. It was caused by high pumping-power requirements for the coolant, essential to dissipate battery heat. Its OEM customer satisfied, D2H shifted its research focus directly to the thermal demands of series production lithium-ion batteries.

In EV batteries, heat buildup can lead to accelerated degradation or even thermal runaway. That problem may occur as greater range and faster charging become essential to persuade the public that electricity, not gas or diesel, is the future energy source for personal transport. D2H — whose clients include McLaren, Chevrolet Racing, and Formula E — is working with Croda, a specialty chemical company, to address these challenges.

D2H’s research work on the EV noise problem started in 2019 when an auto-maker contacted the company asking for a definitive cause to the problem. A 32-cell battery module for physical testing was built and subjected to a variety of potential cooling solutions including immersive and cold-plate techniques. Results were correlated via physical and computational fluid dynamics (CFD) techniques.

A Vital Extra Benefit

Use of a Croda dielectric fluid within an immersion system was identified as the answer, requiring reduced pumping power, especially in fast-charge situations. “The entire battery is immersed in an electrically non-conductive coolant,” explained Chris Hebert, D2H engineering director. “This brings significant benefits through lower peak cell temperatures, enabling higher C-rates [rate at which the battery is charged/recharged] and reduced temperature gradients within each cell, leading to longer pack life.”

But that presents a challenge because it negates using water-glycol, with its high specific heat capacity. Dielectric fluids can struggle to reach half that, Hebert explained, resulting in larger temperature gradients across the battery from inlet to outlet. However, with careful attention paid to the battery internals and the coolant flow paths — combined with Croda developing a low-viscosity, low-density fluid — “we have been able to mitigate this while still maintaining low pump power,” Hebert said.

D2H and Croda are working together, moving from cold-plate cooling to the more effective immersion cooling. (D2H)

D2H found that heat transfer across the pack was far more efficient using the immersive method with Croda fluid. “We confirmed that it has the potential to ease the thermal management challenge,” noted Hebert. “A vital extra benefit is the high flash point of low-viscosity dielectric fluids, leading to the possibility of further reduction in fire risk.”

D2H’s research into the heat generated by fast-charge cycles concerns both battery performance and longevity. Croda’s role in the work is regarded as essential, introducing a “novel” immersive fluid. “Given the benefits of the immersion cooling, development will continue,” Hebert said. “Fire risks reduce as the battery runs cooler. That immersion reduces the chance of ignition. And heat is the major cause of battery aging and performance drop-off.”

Pathways Within the Pack

A report by researchers at the University of California, Riverside cited by Hebert found that high temperatures and resistance from such cycles potentially damage batteries, resulting in accelerated wear and in extreme circumstances, fire risk. The researchers found that after 40 fast charges, the batteries had around 60% of their original capacity.

It is generally considered that for automotive applications, lithium-ion batteries require replacement once capacity dips below 80% — a level that was reached after just 25 fast-charging cycles. The report stated that at this point, there is an increased risk of electrodes and electrolytes being exposed to the air, increasing the risk of fire or explosion, especially at temperatures of 60 °C and above.

Flows of coolant around battery cells. D2H built a 32-cell battery module for physical testing. (D2H)

Despite the short timeframes involved, D2H did not just consider outright cooling performance. Credibility for every EV depends on successful packaging as much as performance and recharge time. Hebert noted immersive cooling may hold a further benefit: “Immersion designs dispense with the cooling plate and are therefore more compact, which may create the opportunity for greater space to be dedicated to the actual cells.”

While immersion on the face of it looks like a heavier option, D2H’s research suggests that careful design of the pathways within the battery pack should enable reduced gallery sizes without impeding flow, resulting in reduced coolant volume and weight.

“Enhancing thermal management, especially heat dissipation during fast-charge cycles, has the potential to enable greater power density, range, and life in future iterations — alongside reduced fire risks,” Hebert asserted. “Extended research into this area is required and our experimental work together with Croda is ongoing.”

This article was written by Stuart Birch, European Editor, SAE Automotive Engineering. For more information on D2H, visit here .


Battery Technology Magazine

This article first appeared in the September, 2021 issue of Battery Technology Magazine.

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