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In the past two months, three Tesla Motors Model S electric cars have caught fire after their lithium-ion battery packs were damaged. Last week the National Highway Traffic Safety Administration said it would investigate whether Tesla’s Model S needs to be modified to prevent further fires.

In two cases, the cars ran over large metal objects at highway speed; the third car hit a concrete wall. No one was hurt: a warning system allowed the drivers to pull the car over and get out before smoke started coming from the battery pack, and the design of the battery pack slowed the spread of the fire, which never made it into the passenger compartments. Tesla has said it will cover fires in its warranty, so the cost won’t be felt by owners. And Tesla founder Elon Musk argues that the fires are still very rare.

Even so, the incidents have drawn attention to the safety of the batteries used in electric vehicles (see “Early Data Suggests Collision-Caused Fires Are More Frequent in the Tesla Model S than Conventional Cars”). They are also just the latest examples of lithium-ion battery fires in electric vehicles—we’ve seen fires with the Chevy Volt and Fisker Karma plug-in vehicles. Boeing’s 787 Dreamliner was grounded because of problems with its new lithium-ion batteries.

There are inherent risks when you store enough energy to propel a two-ton car at 75 miles an hour for hundreds of miles. After all, thousands of gasoline-powered cars catch fire in collisions each year. In principle, those risks can be managed through structural design and cooling. But could the lithium-ion battery cells themselves be made safer?

Electric-vehicle battery packs are made of hundreds to thousands of battery cells, each of which contains a flammable liquid electrolyte. Managing the risks of lithium-ion battery fires comes down to two things: keeping the electrolyte from catching fire, and keeping a fire from spreading if it does happen.

However, lithium-ion battery cells themselves can sometimes generate enough heat to ignite the electrolyte in a process known as thermal runaway. Short-circuits between the two electrodes in a battery cell, for example, can heat up the electrodes. If these electrodes get too hot, the heat can trigger chemical reactions that quickly generate more heat until the electrolytes burst into flame. This seems to be what happened in the Tesla fires, when damage to the battery packs caused short-circuits leading to thermal runaway.

Short circuits can be the result of manufacturing defects, but battery makers have become very good at preventing those. When batteries are used as intended, there’s only one fire for every 100 million lithium-ion battery cells out there, says Jeff Dahn, professor of physics and chemistry at Dalhousie University. Tesla also guards against thermal runaway events with an extensive liquid cooling system designed to cool the cells so fast that if one cell catches fire, its neighbors won’t.

If, however, multiple cells are damaged, the cooling system might not be enough. “If the Tesla pack is abused severely by a large metal object thrust through the pack, it will probably have a fire in most instances,” Dahn says.

Tesla further protects the battery pack with a quarter-inch-thick plate of hardened aluminum. In many cases, this seems to work. The Model S earned the highest safety ratings from NHTSA after crash tests. But the protection didn’t prove to be enough in the case of the fires.

Tesla also built a firewall between the pack and the passenger compartment. “That firewall is designed so that even if the pack does go into thermal runaway, it does not penetrate the passenger compartment,” Musk says.

Since the accidents, Tesla sent out a software update that changes the settings on the Model S suspension so that the battery pack is higher off the ground at highways speeds, making it less likely to hit the sort of chunks of metal that caused two of the fires.

Beyond making battery packs safer, it’s also possible to make the cells themselves safer by switching to electrode materials that store less energy but can withstand higher temperatures before thermal runaway starts. Some other automakers have done this, but the resulting battery packs cost more. It’s not clear that Tesla has selected electrode materials to improve the safety of individual cells.

Musk says it would be possible to increase the thickness of the aluminum plating that protects the battery pack. But that would add considerable weight that would hurt the car’s performance and lower its range on a charge. He says that he doesn’t think this will be necessary, since the current safety measures have protected drivers, and he expects that there won’t be many fires.

Ultimately, it might even be possible to replace the flammable electrolyte with a nonflammable one, but such batteries have proved difficult to develop (see “Solid-State Batteries,” “Building Cars Out of Batteries Isn’t as Crazy as It Sounds,” and “A Trick for Making Batteries Safer Could Also Make EVs Affordable”). 

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Credit: AP Photo | Tennessee Highway Patrol

Tagged: Energy, Tesla Motors, Elon Musk, lithium ion, thermal runaway

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