Tesla Model S Fires Might Be a Big Deal—But Not For the Reasons Some Are Saying

2012 Tesla Model S

For the third time in six weeks, it has become apparent that a Tesla Model S, somewhere, somehow, caught fire. Consequently, for the third time in six weeks, the automotive world has entered a “Tesla Cycle,” which plays out as follows:

News breaks that a Model S has caught fire, either via pictures posted online (LOOK! EXPENSIVE NEW CAR BURNING!) or through local reporters.
Tesla issues statement to the effect of “Our cars are safe. Nobody was hurt. It was just road debris/concrete wall that punctured the battery pack and caused the fire.”
Mainstream press picks up the story, presenting Tesla’s statement not to be alarmed without dissent, but still treating the fire as though there’s reason to be alarmed.
Voices of reason emerge and remind that conventional cars are loaded with gasoline and frequently catch fire—especially when they drive over sharp objects or through concrete walls as the three fiery Teslas had.
Tesla’s stock price takes a hit and mainstream press covers it, as though the stock wasn’t tremendously overvalued and due for some kind of correction anyway.
Americans become hyper-vigilant in looking for burning Teslas to photograph.

This is a fatuous little feedback loop, but it overlooks entirely the million-dollar question: How did two Tesla Model S sedans catch fire from impacts with road debris?

We’re not asking this because, golly gee, cars should be impervious to the occasional bit of scrap metal on the highway. (They should be, but that’s beside the point.) No, what’s confounding are the actual logistics of how two cars’ battery packs were pierced. Every Tesla Model S’s battery pack is protected by a quarter-inch-thick metal plate. This doesn’t seem terribly secure at first glance. Some industry commentators have even posited that having only a single layer of metal wrapping leaves the Model S more vulnerable to battery-pack punctures than other EVs, which have several layers of metal around their batteries.

There is, however, a hangup. Exactly what type of metal is used for the protective plating, Tesla hasn’t said. But one assumes that the company went with the conventional choice when one needs a strong, durable, low-cost, lightweight metal: steel. Quarter-inch-thick steel is so strong that it’s used for Level III body armor—so-called bulletproof vests rated to stop a 7.62 NATO round fired from a reasonable distance. For the non-gun-enthusiasts in the audience, we’re talking about body armor so strong it’s capable of stopping a bullet shot by a military-grade rifle. Quarter-inch thick steel is commonly used to “up-armor” vehicles or building walls to be, at the minimum, bullet-resistant. A private security company recently published the results of a test in which steel slightly less than 0.25-inch thick flat-out stopped bullets of the kind used in most AK-47s.

We mention all of this not because being bulletproof sounds tough, but because road debris under a Tesla is literally a projectile. The parameters of scrap metal hitting the underside of a Tesla are, of course, different from the physics of an assault rifle’s bullet. But these distinctions probably favor the Tesla’s odds of escaping unscathed. Road debris doesn’t have the slick shape of full-metal-jacket ammunition, and it likely strikes the battery’s protective plate with less force than a bullet would. Most important, a piece of metal in the road, whether  it’s static or propelled upward by a front tire, is almost definitely not hitting the plate at a perpendicular angle. That means the object has to pierce through more than a quarter inch, as it’s traveling on an angle through the protective plate.

Absolutely none of this should be construed to suggest that we think some kind of conspiracy is underway, or that “road debris” is cover for “aliens with IEDs.” The point is that based on the facts available to the public, the explanation for the two road-debris fires doesn’t add up.

An Alternative Explanation—A Frightening One

Even after something pierces that quarter-inch-thick steel plate over the battery pack, it’s unlikely that a Model S would be engulfed in flames. In fact, it probably shouldn’t be. Although a puncture into a lithium-ion battery can create a significant fire, chemical engineers with whom we spoke say that Tesla has done an outstanding job of designing its lithium-ion systems to retard and contain the heat generated by chemical reactions that would lead to fire through a process called “thermal runaway.” The Model S has plastic barriers separating cells and sections, as well as an intumescent material, which slows the spread of catastrophic heat. Describing the first Model S fire, Tesla said that these safeguards meant the “thermal runaway” fire that happened was limited and controlled. If you look at pictures of the car, nothing about the fire seems limited; it looks like an inferno.

A quick, immense fire in a Model S battery pack might be better explained by the more intense type of thermal runaway created by an “arc discharge,” says theoretical physicist Lewis Larsen. In essence, an arc discharge would just be a spark jumping from one cathode to another within the battery pack. Larsen says such occurrences are not infrequent in many batteries—not just lithium-ion—but in the old-fashioned lead-acid battery used to start conventional cars, the effects are minimal because the battery structure is physically spread out. To give electric cars like the Model S such great range required increasing the density of their batteries, and that in turn means that everything inside an EV’s lithium-ion battery is shrunken and compressed. With cathodes only 20 to 30 microns apart, Larsen explains, an arc of electricity can traverse the physical barriers. It’s incredibly hot. Hot enough to set the electrolyte on fire, hot enough to burn in a vacuum at 1800 degrees celsius or higher. When it starts, a vehicle occupant might hear the kind of clunk or pop they’d normally assign to driving over some debris in the road. And unlike driving over debris, an arc-discharge-created thermal runaway would set a car aflame vigorously and quickly.

There is no official rate at which lithium-ion cells are known to fail. Designs vary wildly between laptop batteries and smartphones and cars, and every manufacturer takes a different approach to design and construction process. Industry sources with whom we checked put the common failure rates at somewhere between one per million and one per 10 million cells. Infinitesimal odds! But consider the scale we’re talking about here: each Model S contains 7000 cells, meaning any given car doesn’t have a one in x-million chance of a cell failure, it has a 7000 in x-million chance. Put differently, there are 19,000 Model S sedans on the roads worldwide, and that means Teslas, cumulatively, are sitting atop 133 million cells. Most cell failures in lithium-ion batteries burn out harmlessly. But a small number will cause problems. As of now, there’s no way to know whether the number and severity of any problems would be worse than, say, the risks of driving a car containing a tank of gasoline and a gasoline-explosion device called an “engine.”

Instrumented Test: 2013 Tesla Model S
First Drive: 2014 BMW i3
Prototype Drive: 2015 BMW i8

To be clear, we aren’t saying that any of Tesla’s cars underwent the scenario described above. We couldn’t say that. We can’t even say that it’s likely. What we offer is a scenario that, according to a theoretical physicist whose expertise includes lithium-ion battery design, is a plausible occurrence and appears to fit with what little Tesla and others have said about the fires. Larsen also stressed how impressed he is that despite what looked like substantial fires in all three Model S incidents, the occupants were able to get out of the cars unscathed. We agree completely. Cars—even ones with wait-lists—are replaceable.

You can view Dr. Lewis Larsen’s full—and technically complex—exploration of field failures in lithium-ion batteries in this slideshow.

About Justin Berkowitz