What Elon Musk says about Tesla battery fire

The CEO of electric car company Tesla said Friday that a battery in a Model S that caught fire this week was apparently impaled by a metal object.

Elon Musk gave more detail in a blog post about the fire that had became an Internet video sensation and unsettled Tesla investors. He also defended the car's battery technology.

Musk wrote in a blog post Friday that fires are more common in conventional gas-powered vehicles.

"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," Musk wrote.

The CEO said a curved metal object on the road was apparently to blame for the fire Tuesday. He says the large object's shape led to a powerful hit on the underside of the vehicle, punching a 3-inch hole through an armor plate that protects the battery under the passenger compartment.

The car properly contained the blaze in one section of the battery, the company said. The driver was able to exit the highway in the Seattle suburb of Kent and get out of the vehicle before flames engulfed the front of the sedan.

Of the estimated 194,000 vehicle fires in the U.S. each year, the vast majority are in cars and trucks with gasoline or diesel engines. Electric vehicles make up less than 1 percent of the cars sold in the U.S.

Firefighters struggled to extinguish the Tesla fire, finding that the flames reignited. After dismantling the front end of the vehicle and puncturing holes in the battery pack, responders used a circular saw to cut an access hole in the front section to apply water to the battery, according to documents. Only then was the fire extinguished.

Musk wrote:

'It is important to note that the fire in the battery was contained to a small section near the front by the internal firewalls built into the pack structure. At no point did fire enter the passenger compartment.

Had a conventional gasoline car encountered the same object on the highway, the result could have been far worse. A typical gasoline car only has a thin metal sheet protecting the underbody, leaving it vulnerable to destruction of the fuel supply lines or fuel tank, which causes a pool of gasoline to form and often burn the entire car to the ground. In contrast, the combustion energy of our battery pack is only about 10% of the energy contained in a gasoline tank and is divided into 16 modules with firewalls in between. As a consequence, the effective combustion potential is only about 1% that of the fuel in a comparable gasoline sedan."

Musk said in his blog post that a "road crew" at the scene of the fire identified the curved metal piece, which fell off a tractor-trailer, as the apparent culprit. However, state and local officials have not been able to say what debris, if any, was found in the area.

The blog also includes an email from Robert Carlson, the owner of the Tesla S (and a shareholder) that caught on fire, including this statement:

"I completely agree with the assessment to date. I guess you can test for everything, but some other celestial bullet comes along and challenges your design. I agree that the car performed very well under such an extreme test. The batteries went through a controlled burn which the internet images really exaggerates.'

Shares of Palo Alto, California-based Tesla Motors Inc. fell sharply Wednesday and Thursday after a video of the car fire circulated on the Internet. The stock recovered somewhat Friday, rising $7.67, or 4.4 percent, to $180.98. The shares still finished the week with a loss of $9.92, or 5.2 percent, but they're still up nearly 400 percent this year.

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