Chemical Recipe Prevents EV Batteries From Overcooking

German researchers develop a coolant they say will extend the life of batteries in electric vehicles.

The product from the Fraunhofer Institute for Environmental, Safety and Energy Technology in Oberhausen, 25 miles (40 km) north of Dusseldorf, has been given the name CryoSolplus.

It is a dispersion that mixes water and paraffin with stabilizing tensides and a dash of the anti-freeze agent glycol. Tensides are agents that modify interfacial tension of water.

Fraunhofer Institute researcher Tobias Kappels says the advantage is that CryoSolplus can absorb three times as much heat as water and functions better as a buffer in extreme situations, such as freeway driving at the height of summer.

This means the holding tank for the coolant can be much smaller than those of water-cooling systems, saving both weight and space under the hood.

The scientists also say CryoSolplus is good at conducting away heat, moving it quickly from the battery cells into the coolant. With additional costs of just €50-€100 ($61-$122), the new cooling system is only marginally more expensive than water cooling, they say.

As CryoSolplus absorbs heat, the solid paraffin droplets within it melt, storing the heat in the process. When the solution cools, the droplets revert to their solid form. Scientists call such substances phase-change materials, or PCMs.

“The main problem we had to overcome during development was to make the dispersion stable,” Kappels says in a statement.

The individual solid droplets of paraffin had to be prevented from agglomerating or, because they are lighter than water, collecting on the surface of the dispersion. They need to be evenly distributed throughout the water.

Tensides serve to stabilize the dispersion, depositing themselves on the paraffin droplets and forming a type of protective coating.

“To find out which tensides are best suited to this purpose, we examined the dispersion in three different stress situations: how long can it be stored without deteriorating; how well does it withstand mechanical stresses such as being pumped through pipes; and how stable is it when exposed to thermal stresses, for instance when the paraffin particles freeze and then thaw again,” Kappels says.

Other properties of the dispersion that the researchers are refining include its heat capacity, its ability to transfer heat and its flow capability. The scientists' next task will be to conduct field tests, trying out the coolant in an experimental vehicle.

The researchers say overheating must be avoided if EV batteries are to have a long service life. A battery's "comfort zone" lies between 68° F (20° C) and 86° F (30° C). But even a Sunday drive in midday summer heat can push a battery's temperature well beyond that range.

Operating a battery at 113° F (45° C) instead of 30° C halves its service life.

“Thus far, conventional cooling systems have not reached their full potential,” Kappels says. “Either the batteries are not cooled at all, which is the case with ones that are simply exchanged for a fully charged battery, or they are air-cooled.”