For the third time in six weeks, a Tesla Model S was destroyed by a fire after its battery was damaged, prompting new questions over the design of its advanced lithium-ion batteries.
The accident occurred on Wednesday afternoon on a highway in Smyrna, Tenn., near Nashville, after the car struck a tow hitch lying in the roadway, sparking an electrical fire, according to the Tennessee Highway Patrol. The driver pulled the car over to the emergency lane and was unhurt, authorities said.
The National Highway Transportation Safety Administration said it would investigate the accident. The agency “will contact the local authorities who are looking into the incident to determine if there are vehicle safety implications that merit agency action,” it said in a statement on Thursday.
Shares of the all-electric car company tumbled more than 7 percent to close at $139.77. Tesla’s high-flying stock, which hit $194 earlier this fall, has been under pressure recently as the automaker has cautioned that it faces higher costs on research and a shortage of the batteries.
The accident was similar to a fire that broke out on Oct. 1 when a Model S struck debris on a highway in Kent, Wash., outside Seattle. The driver in that incident was also unhurt, and federal safety regulators determined, after a delay of more than two weeks because of the partial government shutdown, that the fire was not the result of a defect in the car’s design.
The second fire occurred in Mexico on Oct. 18 after the driver crashed into a wall and a tree; federal safety regulators did not investigate that fire because it took place outside the United States.
The safety administration has investigated battery fires in the past. Two years ago, when a Chevrolet Volt burst into flames three weeks after a government crash test damaged its battery and cooling system, investigators found no design defects in the car.
Tesla said it would conduct its own inquiry into Wednesday’s fire. “Our team is on its way to Tennessee to learn more about what happened in the accident,” Tesla said in a statement.
The battery in a Tesla is a long, flat slab under the passenger compartment. It uses the same lithium-ion technology that is increasingly found in advanced products because of its high power and long life. Computers, cellphones and the electrical systems of Boeing 787 Dreamliners all use the technology now, and have been susceptible to fire as well.
In the case of the Dreamliner, two incidents prompted regulators to ground Boeing’s fleet of the new jets for several months this year until a more secure steel case could be devised to enclose the battery. But the Dreamliner’s battery design, with eight large cells, is different from the Model S battery, which has many hundreds of individual cells. And the Dreamliner battery fires started without any outside impact, unlike those in the Model S batteries.
Safety experts said that the Model S fires raised questions about its vulnerability to being punctured as it travels down a road. The battery in the low-slung Model S is protected by a reinforced metal plate.
Clarence Ditlow, executive director of the Washington-based Center for Auto Safety, said that Wednesday’s incident showed that the first fire was not a fluke.
“The initial failure mode is puncture of the battery pack from road debris,” Mr. Ditlow said. “The obvious engineering fix would be to add a safety shield. With Tesla, there is some protection there. They just need a better shield.”
Ralph J. Brodd, a battery consultant in Henderson, Nev., said after the Oct. 1 fire that the shielding might not have been strong enough to prevent the impact from causing a short in the battery.
A stronger metal plate would add weight to the car, reducing its efficiency and the long driving range that the Model S is known for.
Tesla did not say whether it would consider changing the design of the battery.
Dan Edmunds, director of vehicle testing at Edmunds.com, said that it was too soon to tell exactly what role the car’s lithium-ion battery played. But he questioned whether the Model S’s low clearance, a design element that supports its aerodynamics, made it easier for the car to strike road debris.
“Is it too low?” he said. “It’s hard to say. If you hit something big enough, it could disable any car.”
Analysts said that the most recent fire, coming so soon after the first two, raised a perception issue for the Model S, which has earned high marks on safety from government regulators and independent automotive publications like Consumer Reports.
“The problem here isn’t that the cars lack effective safety technology, as all three Tesla accidents and fires have resulted in no injuries to the drivers,” Karl Brauer, senior analyst at Kelley Blue Book, said in a statement. “The problem is that we have three fires in six weeks.”
The firefighter who put out the blaze on Wednesday said it was contained to the front of the car.
“The interior of the vehicle had very minimal damage,” said Tim Cole, a firefighter with the Almaville Volunteer Fire Department. Mr. Cole said that the flames were extinguished within a couple of minutes and that it wasn’t different from other car fires.
Analysts also said that accidents like the one Wednesday were bound to become more common as Tesla sells more cars. Last quarter, 5,500 Model S sedans were delivered to customers, and this week its chief executive, Elon Musk, said in a conference call with analysts that constraints on the supply of batteries was holding back production of the cars.
“Fires happen when cars crash,” said Michelle Krebs, senior analyst for industry researcher Edmunds.com. “It’s a high-profile company, and the more cars on the road, the higher the odds of an incident.”
Correction: November 9, 2013
Because of an editing error, an article on Friday about a third fire involving the battery of a Tesla Model S misstated the circumstances of a fire involving a Chevrolet Volt, a plug-in hybrid car. The Volt burst into flames three weeks after suffering damage to its battery and cooling system in a government crash test, not during the test.