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[See updates below, including commentary from Tesla CEO Elon Musk.]

Since October, three Tesla Model S electric vehicles have caught fire after they ran into something—first a chunk of metal, then a concrete wall, and then a trailer hitch. Now the U.S. National Highway Traffic Safety Administration is investigating whether two of the fires (the third happened out of its jurisdiction in Mexico) are the result of a safety defect. No one has been hurt in the fires.

With relatively few electric cars on the road there’s a danger that electric cars will get a reputation for being unsafe.

Tesla CEO Elon Musk says not to worry. He says the rate at which the cars are catching fire is actually very low compared to gasoline powered ones. Is this actually the case?

Not if you’re only looking at fires caused by collisions. And that’s what the NHTSA wants to know–whether the Model S is particularly vulnerable to catching on fire when it collides with something.

Tesla has been comparing the rate its Model S catches fire (1 in 6,333 so far) with the rate with which cars in general catch fire (1 in 1,350 per year). But these figures are really comparing apples to oranges. Only four percent of vehicle fires are caused by collisions—the rest are largely the result of mechanical and electrical failures, which isn’t surprising when you remember that a large fraction of the cars on the road are old and wearing out.

When you look at the number of fires in collisions, (the numbers come from here and here) it comes to one in 32,603 registered vehicles. That’s far less frequent than one fire per 6,333 Model S’s.

That said, we’re only talking about three car fires—that could still be in the realm of bad luck. Based on the limited data, Musk probably isn’t justified in making a strong claim that the Model S is less likely to catch fire. It’s also probably too early to make the reverse claim—that the Model S is more likely to catch on fire–based on the numbers I give above. [UPDATE: It may actually be possible to get statistical significance at these small numbers. But it depends on the assumptions made.]

Of course, it doesn’t make sense to just wait around for the numbers to get larger, which is why it’s good that NHTSA is looking into the situation. It’s also good that Tesla sent out a software update that adjusts the Model S air suspension so that the car has a higher clearance and is less likely to hit road debris. That might be enough to prevent future fires in similar situations.

It’s important to note that no one has been hurt in these fires, which suggests (again, we’re dealing with just a few cases) that the battery design is working properly. It was designed to prevent fires from spreading into the passenger area, and so far that design has worked. Also, a warning system gets drivers out of the car in plenty of time (see “How Tesla is Driving Electic Car Innovation”). [Update: In a phone conversation this afternoon, Musk noted that the key concern isn’t whether a car catches fire or not, but whether the incident hurts people or causes economic harm. No one has been hurt. And Tesla has agreed to cover the cost of fires in its warranty.]

Any vehicle can be made safer. What else could be done to make the Model S and other electric vehicles less vulnerable to fires in the future?

One promising option is to replace the flammable liquid electrolyte used in conventional lithium ion batteries—that is mostly what’s burning in the fires. Nonflammable electrolytes exist, although there are challenges to making batteries that use them cheap and reliable enough for electric vehicles.

Toyota and a start-up called Sakti3 are working on solid-state batteries that don’t use flammable liquid electrolytes (see “Solid-State Batteries). Several researchers that are part of a new program at the Advanced Research Projects Agency for Energy are also looking at alternatives. Using a less flammable electrolytes would not only make batteries safer, it would also decrease the need for costly cooling systems. In the long term, such electrolytes might also make it possible to use batteries as structural materials, which would reduce vehicle weight and the total amount of energy needed to propel a car (see “Building Cars Out of Batteries Isn’t as Crazy as it Sounds”).

In the near term, it might come down to engineering. Conventional cars have been engineered to make fires from collisions relatively rare, even though gasoline is of course highly flammable (and the cars run by setting it on fire). Electric vehicles store less energy, in part because they’re far more efficient, and there’s no combustion or high temperatures involved propelling the car. So there’s reason to expect they will ultimately prove less prone to fires, even with existing battery materials.

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Tagged: Energy, Tesla Motors, Model S, fire, statistics

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