In December 2013 German safety officials also cleared Tesla of blame, saying "no manufacturer-related defects could be found."
A report produced by the NHTSA concluded that sudden fires starting in the bank of 7,000 batteries that power Tesla Model S sports sedans were the result of road debris that punctured the aluminum shield protecting the cars' battery packs, not a design or manufacturing defect.
The NHTSA has accepted Tesla's explanation that, under the right conditions, it is possible for objects passing under the car to get snagged on the leading edge of the plate protecting the batteries, then spike sharply upward if the opposite end digs into the pavement.
Tesla showed the NHTSA investigators a crash-test video the company shot using a three-ball trailer hitch - the same type that evidently fell from another vehicle before puncturing the battery shield of a Model S in November 2013. After passing over the debris the battery caught fire when the trailer hitch smashed through the battery shield and set off a "thermal runaway" reaction in the batteries, which overheated, caught fire, and destroyed the car.
Tesla CEO Elon Musk has recently posted clips from the crash-test videos in a blog explaining the company's offer of a solution.
The danger of serious, even catastrophic, damage from hard objects passing underneath at high speed is common to every vehicle, but isn't likely to happen more often to Tesla drivers or be more catastrophic than for other cars on the road, NHTSA investigators concluded.
Tesla's problem in the Model S car fires was damage from debris, not battery design, the company told the NHTSA.
Tesla's solution was to reinforce shields protecting the batteries, and set a higher minimum clearance level in the software that controls the air-assisted active suspension that is designed to lower Model S Teslas at highway speeds to improve aerodynamics and efficiency.
Two-thirds of 2012-2013 Tesla Model S cars - including all