Batteries have to be babied. That’s because batteries simply perform better when they don’t have to deal with extreme heat or cold. When it comes to batteries for electric vehicles, so-called thermal management systems that ensure batteries operate within a certain temperature range, will be crucial to helping electric cars drive greater distances for a longer period of time.
However there is a debate raging, over whether automakers should turn to air or liquid to regulate battery temperature, and the choice has provided fodder for some green car makers to toot their own horns or slam competitors.
Tesla Motors CEO Elon Musk has derided Nissan’s battery pack, which uses an air cooling system, as “primitive” compared with the sophistication of even Tesla’s first prototype, which uses liquid cooling. As a result, the LEAF pack will have temperatures “all over the place,” claimed Musk, causing it to suffer “huge degradation” in cold environments and basically “shut off” in hot environments. General Motors for its Volt and Ford’s line of hybrids and EVs, have also opted to use liquid for battery temperature regulation. Coda Automotive, meanwhile, uses an air cooling system for its Coda Sedan.
According to Matt Keyser, a senior engineer with the National Renewable Energy Laboratory who focuses on battery thermal management systems, there are advantages to both technologies. Air cooling systems can be less complicated and lower cost, while liquid systems will generally take up less space and let you “drive the battery a little harder,” Keyser explained. Liquid cooling allows the battery to handle a larger “pulse” of power (i.e. more kilowatts, which in electric cars are like horse power).
Of course, there are trade-offs, too. Among the weaknesses of a liquid cooling system are the potential for fluid to leak (which can cause an electrical short), said Keyser. Maintenance and repair can also be costlier and more complicated for liquid-cooled systems, which require more components and may weigh more than air systems. The battery pack can be “totally isolated” from liquid by an aluminum shell, however, which ensures that fluid (a 50-50 mix of water and glycol in the Tesla Roadster system) doesn’t “get to the battery itself,” said Keyser.
Air systems, meanwhile, are generally less effective at maintaining a uniform temperature (key for battery longevity) within and between cells in a battery module, and they can’t carry as much heat away from the battery as quickly as a fluid-based system. A similar idea is emerging around liquid cooling for servers in data centers: liquids are much better than air at transferring heat.
Why is that? Think of an empty cup versus a cup filled with liquid, Keyser suggested. The liquid simply has more mass with which to take heat to another medium. Huge amounts of air can be blown through a battery with a fan to help transfer more heat, but this can result in large pressure drops and inefficiency, said Keyer. An air cooling system could in theory work about equivalent to a liquid system, he said, but then the fan would have to be so big that it would need to draw power from the engine (in a hybrid) or run on its own battery (in an electric car).
Keyser emphasized that he still considers air cooling systems a viable option. “I’m trusting OEMs,” he said, to design the thermal management system so that the battery “won’t be abused,” even in the most strenuous drive cycles. To do that, according to Keyser, automakers may opt to “give you slightly less performance so they can use air cooling.”
Alternatively, since a bigger battery pack can deliver a certain amount of power with less current, and therefore less heat, than a smaller pack, automakers could use a slightly larger battery pack with an air cooling system. With plug-in hybrid and all-electric vehicles, said Keyser, liquid cooling appeals in part because “the battery pack is fairly large as it is,” Keyser noted.
For example, the 33.8 kWh battery pack in Coda Automotive’s upcoming electric sedan will weigh about 700 pounds and deliver 100-120 miles of range, Coda Chief Financial Officer Dan Mosher said earlier this year. For GM’s upcoming Chevy Volt, which will have a 16 kWh battery and a gas engine that kicks in after about 40 miles of electric range, the automaker’s executive director of global electrical systems, hybrids, electric vehicles Micky Bly has said that 200 kilograms (a little over 440 pounds),“is the rough estimate within the realm of a few digits for battery weight.”
Put another way, you can get higher performance (e.g. more power) out of a smaller battery pack with pricier liquid cooling than you can get out of an air-cooled pack of the same size. So the decision for car buyers, said Keyser, is ultimately whether you want to “invest your money in a bigger battery, or in the cooling system.”
Image courtesy of Tesla.
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