Tesla’s Model S sedan has wowed reviewers, aced crash tests, invoked great admiration from fans, but within five weeks two remarkably similar encounters with road debris in Washington and Tennessee resulted in serious fires.

Whether the Oct. 2 and Nov. 6 incidents will prompt a recall is still being determined, but according to the executive director of an influential non-profit automotive safety group in Washington, D.C., it should if investigators prove both fires were caused by battery compartment punctures.

And it does appear this could be the case. In both incidents, an 85-kwh Model S was traveling on a highway, and both drivers reported striking sturdy metal objects.

Just The Facts

The Model S uses high capacity 18650 lithium-ion cells supplied by an internal arm of Panasonic called the Automotive & Industrial Systems Company.

Tesla did not respond to our inquiry for this story, but in the batteries’ raw state, their chemistries are believed to be anywhere from mildly to much more flammable than batteries in the Chevy Volt or Nissan Leaf, for example.

According to Altairnano engineer and Senior Director of IP & Technology, Jay Akhave, Tesla has the highest energy in its packs which he estimated at 146 watt-hours per kilogram.

Exactly how combustible these are compared to li-ion batteries in mainstream cars mentioned is not exactly clear, but Tesla Chairman Elon Musk minimized their fire potential compared to gasoline cars in a blog post Oct. 4.


“In contrast, the combustion energy of our battery pack is only about 10-percent of the energy contained in a gasoline tank and is divided into 16 modules with firewalls in between,” said Musk. “As a consequence, the effective combustion potential is only about 1-percent that of the fuel in a comparable gasoline sedan.”

Model S battery packs also include fire-retardant gel, and protecting the underside is a quarter-inch-thick aluminum cover.

In his post following the first fire, Musk implied damage believed to be caused by a curved section that fell off a semi-trailer was an anomaly:

“The geometry of the object caused a powerful lever action as it went under the car, punching upward and impaling the Model S with a peak force on the order of 25 tons,” said Musk. “Only a force of this magnitude would be strong enough to punch a 3-inch diameter hole through the quarter-inch armor plate protecting the base of the vehicle.”

Musk said government records show Americans drive about 3 trillion miles per year, which “equates to 1 vehicle fire for every 20 million miles driven, compared to 1 fire in over 100 million miles for Tesla.

“This means you are 5-times more likely to experience a fire in a conventional gasoline car than a Tesla!” concluded Musk, “For consumers concerned about fire risk, there should be absolutely zero doubt that it is safer to power a car with a battery than a large tank of highly flammable liquid.”

But five weeks after that deduction, another fire almost like it happened in Smyrna, Tennessee.


The driver – who wrote a testimonial saying he felt safe and the car saved his life – said he ran over a three-ball trailer hitch and this is believed also to have ruptured the battery.

“I felt a firm ‘thud’ as the hitch struck the bottom of the car, and it felt as though it even lifted the car up in the air,” wrote Juris Shibayama, MD. “My assistant later found a gouge in the tarmac where the item scraped into the road. Somewhat shaken, I continued to drive.”

Was this another anomaly?

The National Highway Transportation Safety Administration last reported it is talking to the Tennessee Highway Patrol, and offered no further info when asked yesterday.

“NHTSA is in close communication with Tesla and local authorities gathering information about the incident to determine if additional action is necessary,” it said.

Helpful Suggestions?

Among remedies suggested by Tesla fans are a “cow catcher” like a railroad train would use to clear debris, or an entire redesign of the underbody shielding outlined in an article written by a Model S owner for Green Car Reports.

Such suggestions even from the sympathetic may show why those less sympathetic are sitting back wondering if Tesla is innocent until proven guilty – or worse, guilty until proven innocent.


Not counting a recent Mexico Model S fire that happened under different circumstances, what has seemed apparent to observers in both U.S. fires is:

1) The battery pack’s fire protection has so far worked well enough to let cars come to a stop and occupants escape safely.
2) Once set on fire, the cars still burned to a crisp before the fires were contained – and Washington firefighters showed first responders may not know how to extinguish the EV blaze as fast as possible.
3) The Achilles’ heel appears to be the shielding Musk said was so robust.

Not An Outlier?

Yesterday in a phone interview, Clarence Ditlow, executive director of the Center for Auto Safety in Washington said he has doubts about the odds of two similar incidents in quick succession for a car that only has 19,000 units on the road.

The CAS was founded in 1970 by Consumers Union and Ralph Nader and has frequently challenged NHTSA in court on issues, while in other rulings siding with it, and has long played an influential role in consumer safety.

Characteristically tight-lipped NHTSA has said nothing new lately, but Ditlow said if the battery shielding in Tennessee failed and resulted in fires, the answer is clear.

“One road-debris-caused fire may be an outlier, but two is not. In other words, I would not expect to see two road debris fires in such a small volume vehicle,” said Ditlow.

Ditlow added that the 5-foot-wide, 9-foot-long underside is tantamount to the proverbial barn door.


“You clearly have a large flat target. So the potential for road debris hitting the vehicle … is higher than in, say a gas-tank vehicle,” said Ditlow suggesting a vulnerability with the Model S compared to internal combustion cars that Musk did not.

Further, “road debris is a common hazard” he said, and odds are it will happen again.

Conservatively figuring 10,000 miles driven per year by owners of 19,000 Model S sedans now on the road equals 190 million road miles annually driven, and this is increasing.

“The fact that you had two impacts shows it’s not an exceedingly rare event like being hit by a meteor so you need to upgrade the shied,” said Ditlow adding investigators still must state the cause for the record but looking ahead should this be the case, added, “Most shields that are genuinely designed for road debris are made of steel.”

This would mean a recall would be in order, he said.

“And I understand the issue of weight on an EV,” he said. “But if an aluminum shield is not going to do the job, you have to get something that is.”

Ditlow said many light-duty trucks like Jeeps use one-eighth-inch steel, and suggested this could work for Tesla’s large flat, 4,800-pound or so car riding little more than five inches off the pavement.

He also acknowledged more data would be needed for a precisely engineered solution.

Steel? On a Tesla?

The bumpers on a Model S are about the only steel you’ll see on its elegant aluminum-intensive skateboard chassis.

According to Michigan-based steel industry executive, Ron Krupitzer, automotive engineering to encounter all likely underside crash events is a lot like evaluating crash-impact data to engineer a bumper.

As vice president for automotive applications for the Steel Market Development Institute, a business unit of American Iron and Steel Institute, Kruptizer says he has a team that regularly creates solutions for automotive applications.

Steel is three-times denser than aluminum, and while heavier, he said much less of it could be used to create a stronger shield with comparable weight to aluminum.


A steel sheet the same square area as a quarter-inch-thick aluminum sheet would be only one-twelfth of an inch thick to weigh the same. However, steel’s tensile strength and puncture resistance can be so high, it might not take much – and this also shows the present aluminum “armor” under a Model S is only equal to one-twelfth of an inch, or .083-inch-thick steel.

Unknown is the exact metallurgy of Tesla’s armor, however, but good, better, and best steel is available too.

Krupitzer added his family includes major Tesla fans, he’s followed the issues, and has been approached by other reporters on the topic already.

He said it would be interesting to build a rust-resistant shield that meets critical weight requirements by using less material, and offers far more effectiveness.

“I’m confident we could have a steel selection that’s at least four- to five-times stronger and even more,” said Kruptizer.


“If Elon wants to work with the North American Steel Industry so we can obtain the model data for the underbody package we could work with him in designing a successful steel skid plate,” he said.

Or, as Green Car Reports mentioned, in question are design patents Tesla already holds for a more robust “ballistic shield” with a crush zone built in.

It’s unclear whether this aluminum and carbon fiber design has already been utilized or is waiting in the wings, said the report.

No Conclusions Yet

Krupitzer acknowledged it is still premature to talk about any solutions – such as GM voluntarily chose with its Volt battery – unless Tesla expresses the need.

A recall imposed by NHTSA could however create that need. Musk was recently quoted saying “there’s definitely not going to be a recall” but as Bloomberg and several other articles quickly noted, “it’s not up to him.”

Presently federal authorities are having to apply criteria to a car not quite like any they’ve designed protocols for.

Forbes cited Allan Kam, a former senior enforcement attorney with NHTSA, who said investigators must assess whether a component is failing under foreseeable circumstances, and whether it’s safety-related.

On Tuesday Musk said the dialogue is ongoing.

“We literally are in constant contact with them,” said Musk. “NHTSA has real problems to deal with where people die or are seriously injured. Their time is preoccupied with that, not with fictional issues created by the media.”

This is true, but also fiction-free is questions do remain and not just in the minds of dispassionate watchers, but even among Tesla’s most ardent fans.


But not all of them. Others say demands for safety can be frothed to excess and side with Elon, such as a Volt-owning plug-in car fan and engineer on the GM-Volt forum who goes by “kdawg.”

“There are few accidents compared to ICE cars. The people were able to control their cars and exit them. No deaths. And the amount of energy stored in the battery is only a fraction of that in a gasoline tank,” he said. “Should Tesla make a beefier plate for the bottom of their car? I’m indifferent. It’s sort of how I felt when GM added that extra plate on the Volt’s battery due to side impacts. As an engineer, you can’t protect for 100-percent of possible scenarios … Until then, when is enough enough?”