By Deepa Seetharaman
DETROIT (Reuters) - Elon Musk, the chief executive of Tesla Motors Inc
"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 conventional gas-powered vehicle, Musk said in a blog post on Tesla's website.
Tesla shares fell more than 5 percent this week, the sharpest weekly decline since mid-August, after images and a video emerged Wednesday of a Model S on fire after an accident near Seattle Tuesday morning.
The fire occurred after the driver struck a large metal object on the highway. The object punched a hole three inches in diameter through the quarter-inch armor plate protecting the battery pack with a peak force of about 25 tons, Musk said.
The alert system onboard the Model S instructed the driver to pull over and he left the vehicle before the fire broke out. There were no injuries.
The video of the burning car was posted online Wednesday by auto blog Jalopnik and has been widely disseminated by other media. News of the accident sparked a sharp decline in the company's stock on Wednesday and Thursday.
Shares rebounded Friday and were up 4.4 percent at $180.98 on the Nasdaq.
The accident that led to the fire stemmed from a "highly uncommon occurrence," Tesla's head of sales and service, Jerome Guillen, said in an email exchange with the owner of the damaged Model S that was included with Musk's post Friday.
Guillen said the Model S drove over a "large, oddly shaped metal object" which hit the leading edge of the car's undercarriage and then rotated into its underside. Guillen described this as the "pole vault effect."
The driver, identified by the emails as Rob Carlson, said he was a Tesla investor and that the battery underwent a controlled burn that was exaggerated by images online.
"I guess you can test for everything, but some other celestial bullet comes along and challenges your design," Carlson wrote in the email.
A curved metal object that fell off a semi-trailer likely caused the damage to the car, Musk said. If a gas-powered car hit the same object on the highway, "the result could have been far worse," he added.
"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," Musk wrote in his blog post.
Musk did not say if Tesla would make any changes to the Model S battery design as a result of the accident. A Tesla spokeswoman did not immediately respond to an emailed question asking if such changes were under consideration.
The fire was the latest in a string of problems for lithium-ion batteries, which are used heavily in electric cars sold by various automakers. Such batteries are lighter and more powerful than lead-acid and nickel-metal hydride batteries, but they pose an increased safety risk.
This was Tesla's first battery fire. General Motors Co's Chevrolet Volt and Mitsubishi's <7211.T> i-MiEV have faced similar problems in the last couple of years. Boeing Co
Emergency officials at Tuesday's accident said firefighters struggled to extinguish the flames in the Model S.
Musk said in his blog post that firefighters were right to use water to douse the fire, but they erred in puncturing the battery's metal firewall. Doing so created holes that allowed the flames to vent upwards into the front section of the car.
(Reporting by Deepa Seetharaman; additional reporting by Ben Klayman; editing by Matthew Lewis and Andrew Hay)