Bosch’s flexible thermal unit. Consolidation of elements for packaging is important, as is the ability to react quickly to various thermal demands. (Bosch)

Addressing thermal management for EVs

Batteries and humans like to be at room temperature; thermal-management systems make sure that happens — while maximizing efficiency.

When General Motors unveiled the 2025 Cadillac Escalade IQ, the electric sport utility vehicle with a 24-module Ultium battery pack providing more than 200 kWh of energy that contributes to an estimated 450 miles (724 km) of range and up to 750 hp, the company enumerated an array of technology features, ranging from the 55-in. total diagonal LED display powered by the Qualcomm Snapdragon Platform to the Four-Wheel Steer capability, which reduces the turning diameter of the SUV by more than 6.5 feet (to 39 feet, four inches). And there is a suite of other features.

But among all that there was another feature perhaps not as glamorous as the sort of equipment in a luxury EV that will sticker at an estimated $130,000: A heat pump.

The Escalade IQ is equipped with what GM calls “Ultium Energy Recovery.” The purpose of the thermal system is to allow “energy to transfer between the battery/power electronics and the cabin.” The reason behind Ultium Energy Recovery’s development: “using every Watt possible to optimize [driving] range and minimizing the power from the high-voltage battery used for five-zone climate control.”

In other words: not wasting energy that can be used to propel the Escalade IQ while still providing the utmost comfort in the cabin. Lawrence Ziehr, Ultium energy recovery program manager for GM, explained the desire to maintain the Ultium battery pack in its optimum thermal range: “The biggest challenges with cooling are getting even cooling across the pack, which we do by connecting cells to a common heat exchanger to allow balanced flow through the pack and across the modules. Achieving a cooling/heating across the entire battery is a priority for us and ensures we are able to get the most out of our batteries and ensure performance and range.”

To that end, Ziehr added: “We do an extensive amount of battery cooling where we connect the system to our refrigerant/heat pump and cool well below ambient temperature.” This approach is being taken for all of GM’s Ultium-powered vehicles.

Heart of the Vehicle
As Jason McClymont, head of the North American business unit for Electric Drive Systems, Vitesco Technologies USA, notes, “Thermal management is the heart of the vehicle now. Without thermal management, it is difficult for all the other components in an electric vehicle to operate in the conditions for which they are built.”

His colleague Gerhard Eser, principal expert, Thermal Management, amplifies that by noting, “With an EV, the only energy we have for heating and cooling is basically the battery.”

Owners, of course, are primarily interested in achieving the utmost range from their EVs. But there are a couple of additional factors that come into play: nobody wants to ride in a vehicle that is too cold or too hot – plus, the temperature of the battery must be managed to keep it operating at its optimal state. As Bosch’s Andreas Douglas, director of engineering at its Waltham, Mass. operation said, “Batteries are like humans; they want to be at room temperature. So we want to make sure the device itself is comfortable. Otherwise, it is less efficient overall.”

This situation means thermal management — which Douglas describes as “the underpinning of everything” — has several functions to perform in an EV, from maintaining the comfort in the cabin to handling the thermal needs of the batteries and associated onboard electronics.

Clever Tech
Which results in a situation where engineers must develop what Douglas calls “clever technologies.” An internal-combustion engine generates waste heat that can be used to condition the cabin in the winter. In the summer, things are different. “Air conditioning has never been free on the engine,” said Harry Husted, chief technology officer at BorgWarner. “When the car is sitting at idle and the air conditioning compressor kicks in, extra fuel must be dumped in to generate the torque to spin the air conditioning compressor.”

Generally, that fuel is not considered to be a huge loss. Husted points out that when eV owners fully charge their battery pack, they want the vehicle to go to the stated range. But there are more demands on the battery than just providing energy to produce torque.

This means that action must be taken so that the use of energy drawn from the battery is kept to a minimum for non-propulsion needs and any waste heat is efficiently used. “The more energy we pull from the battery, the less we have for traction,” said Vitesco’s Eser. “That’s the reason why we have to connect all the waste energy in the system — from the e-machine, from the battery, from the electronics — and put it all together. The major difference between an ICE and a BEV is that we have to be really careful with our precious energy.”

Bosch’s Douglas says that whereas in ICEs “components were largely thought of thermally in a discrete manner,” the approach they are taking at Bosch for EVs is “looking at it holistically: what are demands of subsystems and developing products and technologies to move energy as efficiently as we can from one to the other.” In other words, they are developing systems that determine where there is a need for or an excess of thermal energy and apply it to or extract it from those subsystems.

What’s more, attention is paid to the ambient temperature and using it when possible. BorgWarner’s Husted says, for example, “If you are trying to cool the battery pack, you might be lucky enough if the battery is on the warmer side and the ambient air is on the cooler side, so you can cool the coolant with outside air.”

At Bosch, Douglas said, the company even is considering solar energy as an input to help with the energy load.

The Packaging Challenge
But there is a limiting factor that is being faced by those developing thermal management systems for EVs: packaging.

Douglas said that for EVs, packaging is a key performance indicator. Consider frunks, he said. Although there is space freed up from under the hood of a vehicle compared with when there was a large internal-combustion engine in that space, there still are power electronics, inverters, motors, etc. So whatever is being developed for thermal management still needs to be comparatively compact.

Going back to that possibility of using air to cool the battery, Husted points out, “It is elegant in one way, because you eliminate coolant, its weight and the plumbing. But the problem with air cooling is the heat transfer, which is nowhere as good. So the ducts need to be larger and there need to be fans. But because of the compactness needs in automotive and the power levels of propulsion – we’re talking 100 kW, way more than someone’s house uses, more like a neighborhood of power – we go to liquid cooling.”

BorgWarner acknowledged the effectiveness of liquid heat exchange. It has developed a line of compact, high-voltage coolant heaters that warm the battery pack as well as the cabin. Battery warming can be important, Husting points out. “The battery pack is more efficient at delivering energy when it is warmer; it enables the battery to be [more effectively] charged. When the temperature gets too low in a lithium battery, we don’t want to charge it very hard – or at all, depending on the temperature – because of the lithium plating potential on the anode side. So a warm battery for charging and for power and energy delivery is a good thing.”

At Vitesco, the company has developed a coolant thermal-management module that McClymont described as “an integration of multiple pumps and multiple valves into one unit,” adding, “We simplify the entire architecture of the thermal-management system into one modular solution, which makes it a lot easier for installation and easier in terms of packaging – there’s one central unit.”

There are multiple ports into and out of the unit, Eser said, adding that in terms of location in the vehicle, the location should be where the lines can be as short as possible (thereby reducing the amount of coolant required, which means less weight) and likely on the side of the vehicle, “because with an EV, you want a frunk.”

Similarly, Bosch developed flexible thermal units that are combinations of heating and cooling system elements combined in compact packages. Douglas describes them as “clever ways to use coolant, refrigerant, oil – whatever medium to move thermal energy.” And, he added, “You don’t want to waste energy in this movement.” He said that in the ICE days, routing things in comparatively long distances wasn’t an issue. But any losses with an EV make a big difference.

All of which leads to what he described as a complex engineering challenge of ensuring that the elements of the propulsion system — the battery, the power electronics, the motor — are at temperatures that allow them to operate at their best while providing a comfortable environment inside the cabin.

“Consider what it takes to perform fast charging while someone is sitting in their vehicle in Arizona,” Douglas offered.

What about immersion cooling for batteries?
Keeping battery cells at the right temperature for various conditions — cool when under load, warm when the ambient conditions aren’t — typically is addressed by using a liquid-cooled cold plate at the base of the battery pack and/or running liquid-coolant channels between the cells in the pack.

But what about the concept of immersing all of the cells in a dielectric fluid? Not only could this improve thermal consistency throughout the pack, but also eliminate components such as the cold plate, thermal interface materials and more.

But James Edmondson, principal technology analyst at IDTechEx.com, pointed out there are more than a few challenges to the immersion approach, which, while beneficial, may be limited to applications in high-performance, high(er)-cost EVs.

“The industry has much less experience with this approach and hence manufacturing has not been optimized for high volumes,” he said. “This itself raises costs, but having to seal the battery modules to avoid leakage is a challenge that also increases costs. For these reasons, at the moment, the higher-end, lower-volume vehicles are the ones that can afford to implement this approach.” “Additionally,” Edmondson added, “immersion cooling typically requires spacing between the cells for fluid flow and there is the weight of the fluid itself, hence you reduce the energy density of the battery pack. This means that for an application where you want to pack as much battery capacity in a given volume, then immersion might not be the ideal solution. Immersion is well suited for an application where you need to pack as much power into a given volume – this means higher-end motorsport/performance car applications,” he continued.

Also in 2021, Stuttgart, Germany-based Mahle said it was developing battery immersion cooling, specifically citing the design’s ability to reduce DC fast-charging times a well as the potential to reduce EV battery size and cost. And Tier 1 supplier Valeo said at the 2023 IAA auto show that it entered a partnership with France's TotalEnergies to develop an immersion-cooling system with a dielectric fluid that "will also be smartly used to cool all VALEO power electronics products in future."

According to Edmondson, during the first half of 2023, 96% of EVs used cold-plate cooling. While IDTechEx estimates have it that there will be an increase by as much as nine-fold per year between 2026 and 2033 for immersion cooling, it still will be “a relatively small part of the overall automotive thermal-management market.”

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