Federal Safety Agency Ends Its Investigation of Tesla Fires

Federal safety regulators said on Friday they had closed an investigation into whether the design of Tesla Motors’ all-electric car was making it prone to catch fire, after the automaker said it would provide more protection to its lithium-ion batteries.

The addition of a titanium underbody shield and aluminum deflector plates, along with increased ground clearance, “should reduce both the frequency of underbody strikes and the resultant fire risk,” the National Highway Traffic Safety Administration said on its website.

Tesla’s share price rose $5.05 on Friday, or 2.44 percent, closing at $212.37. It had fallen 2.7 percent on Thursday.

Twice last year in the United States, the Tesla Model S sedans burst into flames after running over debris.

The first accident occurred on Oct. 1, when the car struck a metal object and burned on a highway exit in Kent, Wash., damaging one of the modules in its liquid-cooled battery pack, which extends in a long, flat slab on the underside of the car. The second in the United States happened on Nov. 6 on a highway in Smyrna, Tenn., after a Model S ran over a tow hitch in the roadway.

The drivers were not hurt, but after the second accident, regulators opened their investigation, and Tesla, to reduce the chance of hitting any objects, adjusted the air suspension so that the cars do not automatically lower themselves as much at highway speeds.

A third Model S also caught fire last year, in October, but the crash took place in Mexico, which falls outside of the scope of federal regulators.

In a statement on Tesla’s website Friday, Elon Musk, its chief executive, acknowledged that the company had to address consumers’ concerns over the cars, which start at about $70,000.

“We felt it was important to bring this risk down to virtually zero to give Model S owners complete peace of mind,” Mr. Musk said. “Starting with vehicle bodies manufactured as of March 6, all cars have been outfitted with a triple underbody shield.”

He called last year’s United States-based car fires “extremely unusual” and stressed that the occupants were safe.

He also said that Tesla would, upon request or as part of normally scheduled auto service, retrofit the shields, free of charge.

In a conference call with analysts in February, Mr. Musk acknowledged that the fires had led to a “significant drop” in demand, but that its efforts to communicate with customers through social media had helped demand recover.

The fires posed a test for the upstart carmaker, which has won high marks from independent publications like Consumer Reports, and for safety from regulators.

But the regulators’ investigation of Tesla is not the only safety issue that the company has encountered. In January, Tesla announced a recall for about 29,000 wall adapters for its 2013 Model S after the adapter cord or wall outlet caught fire in a garage in Irvine, Calif.

Tesla said the adapter change would be performed with an “over the air” software update — similar to the way it adjusted the ground clearance of the car at highway speeds — so there was no need for owners to bring their vehicles into a dealership.

The company is also facing resistance in some states to selling its cars directly to consumers through its company-owned stores rather than through independently owned dealerships. Tesla won a reprieve in New Jersey this week when regulators gave automakers until April 15 to comply with the state’s franchise rules, an extension from an original deadline of April 1, according to a notice that was sent to new car dealers.