Thermal management in EVs key to optimal battery performance

As more players enter the electric vehicle market, automakers need something to help them stand out from the competition. For many, the answer is vehicles with longer driving ranges, greater power output and faster recharge times.

Pushing batteries further and further has consequences, however. One of them is excess heat generation.

Thermal management has become a hot topic for automakers and suppliers. That's because optimal battery performance depends on keeping the battery pack from getting too warm — or too cold.

"We're trying to keep the battery temperature ... at about human [comfort] level," said Dana Nicgorski, Bosch system engineering lead for thermal management of electric vehicles. "Batteries are happy at basically human temperatures."

Batteries heat up during driving, when the electricity is flowing out, and during charging, especially fast-charging, when power is flowing in. Ambient conditions around the pack can also heat the battery.

"Over time, at high temperatures, you lose capacity, you lose reliability, and if you have a thermal runaway, you can actually have fire," said Sanjay Misra, senior scientific principal at Henkel, a provider of thermal interface materials.

"Your gas tank doesn't get hot when you're filling it. Your gas tank doesn't get hot when you're draining it, when you're running the car. So EV batteries, lithium ion batteries, are kind of special 'gas tanks' because if you fill them, if you charge them, they get hot, and the faster you charge them, the hotter they get," Misra said at an October Shift conference in Detroit.

"We're talking about an explosive system with very tight temperature limits, all packed tightly in a vehicle, and you're sitting in it, so it's a challenge."

Which cooling method is best depends on the automaker and the vehicle architecture. Options include air cooling, direct or indirect liquid cooling, refrigerants or even "phase change materials," which absorb and release heat by melting and resolidifying.

"In the next several years, we really think that liquid cooling — basically your glycol-water-mix cooling plate — I think that's going to be the predominate way to cool it," Misra said.

Driving range is to a great extent based on the energy density of the pack. But you need to be able to pull the heat out as fast as energy is put in. Without adequate thermal management, fast-charging can cause batteries to get so hot that the cells get damaged.

"People expect their vehicles to regain range relatively quickly," Nicgorski said. "Fast-charge is limited partially by how well we can thermally manage the battery."

If the cells heat up too much, they can initiate the same reaction in adjacent cells, causing a domino effect, or "propagation" reaction. That's thermal runaway.

Some suppliers are looking at venting as an answer.

Gore Automotive Products created a low-pressure evacuation vent for battery packs, which uses a flexible, waterproof membrane that expands and releases extra pressure that occurs from overheating.

Donaldson Co.'s dual-stage battery vent first equalizes pressure while preventing contaminants from entering the battery, then expels heat and gas from inside the battery cell quickly and in a controlled direction.

DuPont's approach to thermal management includes cell-to-cell insulation separators, preventing propagation of heat from the initial cell to the neighbor cell, and lightweight, thermal insulating flame barriers.

Some suppliers are examining alternate battery-pack materials.

NeoGraf Solutions recommends using heat spreaders that are made of graphite, rather than aluminum, to cool the cells more quickly.

"Going to graphite, you can help battery-pack manufacturers meet their goals: their fast-charge goal, their cell lifetime goal, their miles-per-charge goal and whatever safety anti-propagation goal they have," said applications engineering manager Bret Trimmer.

For all of these solutions, thermal management comes down to improving EV safety, range and the driver experience. "The message is to implement the thermal systems which enable the fast-charge that the users expect for the vehicle to meet their needs," Nicgorski said.

But Denise Gray, CEO of LG Chem Michigan Inc. Tech Center, cautions that the industry is still in the early stages of solving the thermal management problem.

"I still want to push for more innovation in that area," Gray said at the Shift event.

She said that includes looking at the issue from a cell chemistry perspective. "What are the chemistry tricks we can do to make it more thermally resistant, if you will? ... I'm looking for that next, next technology."