NHTSA opened the probe after two U.S. reports of battery fires in Model S sedans after the cars ran over road debris that damaged the battery pack under the vehicle.
Tesla earlier changed the software that raises and lowers the car based on its speed to increase the default ground clearance.
Documents posted on the NHTSA web site said that the agency "believes impacts with road debris are normal and foreseeable. In this case, Tesla's revision of vehicle ride height and addition of increased underbody protection should reduce both the frequency of underbody strikes and the resultant fire risk."
It said that "a defect trend has not been identified. Accordingly, the investigation is closed." But it added, "The closing of the investigation does not constitute a finding by NHTSA that a safety-related defect does not exist, and the agency reserves the right to take further action if warranted by new circumstances."
Tesla CEO Elon Musk, in a blog posting noted that in the two incidents in the U.S., plus another in Mexico, the drivers walked away without injury and wrote that, based on these fires, the odds of a fire in a Model S crash are lower than a typical gasoline car.
Related story: Feds review Model S after third fire report
He also noted that the company already had updated the software in all Model S sedans "a few months ago to increase the default ground clearance of the Model S at highway speeds, substantially reducing the odds of a severe underbody impact."
Musk said, however, that the company decided "to bring this risk down to virtually zero to give Model S owners complete peace of mind" so it began adding the extra shielding to vehicle bodies being built as of! March 6.
Tesla is not recalling cars already on the road, but will "retrofit the shields, free of charge, to existing cars upon request or as part of a normally scheduled service," Musk wrote.
The battery already is protected by a quarter-inch aluminum plate and to the plate has been added:
A "rounded, hollow aluminum bar" designed to deflect objects entirely or absorb impact and push the object into the trunk area ahead of the battery.A titanium plate to protect "sensitive front underbody components."An angled aluminum shield that will absorb additional energy and cause "the Model S to ramp up and over the object if it is essentially incompressible and immovable."
Musk said the the additional components should not affect ride, handling or aerodynamics and would have only a 0.1% effect on the car's electric range. Musk added short videos of testing with various debris. The post and videos can be seen here.
Karl Brauer, senior analyst at Kelley Blue Book, believes the additional shields should close the fire issue. "Tesla has added substantial protective measures to the underbody of the Model S," says Brauer. "This combination of upgraded materials should further reduce the chance of a battery pack puncture. Given that no punctures have been reported over the past 4 months, even without the added protection, this fix should eliminate the potential of a battery-pack fire under all but the most extreme circumstances."
Tesla says the regular Model S can sprint from zero to 60 miles per hour in 5.4 seconds and has a top speed of 125 mph. (Photo: Tesla)View Fullscreen
Tesla Model S electric car, which has a range of 208 miles to 265 miles depending on the size of battery and price. (Photo: Tesla)View Fullscreen
The Tesla Model S electric car dashboard is devoid of the usual switches and knobs -- most car functions are handled via the 17-inch touchscreen in the center. (Photo: Tesla)View Fullscreen
The Model S has active air suspension and using the touchscreen in the dash, the drive can raise and lower the ride height for road and weather conditions. (Photo: Tesla)View Fullscreen
Tesla Model S. (Photo: Tesla)View FullscreenLike this topic? You may also like these photo galleries:Replay
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