Tesla Model S (autoblog.com)
Federal safety investigators said Friday they have found no apparent defect trends in the Tesla Model S, an electric luxury car that has its share of fans and critics.

The National Highway Traffic Safety Administration closed a four-month investigation into whether the car's batteries were at heightened risk for underbody strikes and subsequent fires.

Even though no defect trend was found, Tesla Motors said in a written statement it would further protect the bottom of its cars with a reinforced underbody shield. The California-based automaker started adding the shields on March 6 and will retrofit older models free of charge. It will also increase the ground clearance of new cars.

"This fix should eliminate the potential of a battery-pack fire under all but the most extreme circumstances," said Karl Brauer, a senior analyst at Kelley Blue Book.

Although there are more than 194,000 car fires in the United States every year, NHTSA investigators began a preliminary investigation into potential trouble on Nov. 15, 2013 following two Model S fires, one in Tennessee and one in Washington. No one was injured or killed in either fire.

Roadway debris punctured the battery pack near the front of the compartments in both cases, damaging the batteries' lithium ion cells that then began "thermal runaway" events. The opposite ends of the debris had dug into the pavement and jutted upward. Tesla called this a "piking effect."

The company said it has conducted 152 vehicle level tests, and that the new shields prevented any damage or penetration of the new shields. The first is a rounded, hollow aluminum bar that's followed by a titanium plate. The third layer of protection is made of solid aluminum. (There's video of the new plates in action on the Tesla Motors website).

"We have tried every worst-case debris impact we can think of, including hardened steel structures set in the ideal position for a piking event," Tesla Motors founder Elon Musk wrote on the company's blog," essentially equivalent to driving a car at highway speed, into a steel spear braced on the tarmac."

Approximately 15,805 vehicles are eligible for the upgrades, which NHTSA says should reduce the frequency of underbody strikes and subsequent fire risks. But the agency did note that the closing of the investigation did not necessarily constitute a finding that a defect does not exist.

"The agency reserves the right to take further action if warranted by new circumstances," it said.

Tesla has created headlines since first entering the marketplace with the all-electric sport sedan. The Model S has been hailed as the best car ever tested by Consumer Reports and is enthusiastically regarded by many of its car owners. But its method of selling cars directly to consumers instead of via traditional dealerships has upset established car dealers, who have worked to have sales of Teslas banned in three states.

Pete Bigelow is an associate editor at AOL Autos. He can be reached via email at peter.bigelow@teamaol.com and followed on Twitter @PeterCBigelow.

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