Federal regulators closed an investigation of Tesla Motors' Model S sedans earlier this month, after concluding that three cars destroyed by fire after running over road debris didn't represent a "defect trend."
That conclusion, released March 28, does not mean the US National Highway Traffic Safety Administration (NHTSA) believes there is no safety-related defect in the design of the all-electric sports sedans.
An NHTSA report 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' 1,300-pound battery packs, not a design or manufacturing defect.
Safety officials in Germany issued a report in December clearing Tesla of blame, saying "no manufacturer-related defects could be found."
NHTSA 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.
As evidence, Tesla showed crash-test video it 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 November 13 in Tennessee, which passed over the debris and caught fire after 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 posted clips from the crash-test videos in a March 28 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.
Batteries made with energy-dense, highly flammable lithium cobalt oxide in most lithium-ion batteries are particularly susceptible to thermal runaway -- a sudden release of energy caused by short circuits or physical damage that builds up heat quickly enough to cause severe fires even from relatively small cellphone and other batteries.
Hereís an FAA training video on thermal-runaway fires:
The Federal Aviation Administration grounded all Boeing 787 "Dreamliner" passenger jets last February after several investigators found that small fires aboard the planes were due to thermal runaway from lithium-ion battery packs that were insulated so well they had nowhere to vent the heat from small short-circuits.
"Unfortunately, the pack architecture supplied to Boeing is inherently unsafe," according to an email Musk wrote to airline-business industry publication Flightglobal Jan. 29, 2013. in addition to Tesla, Musk is CEO of SpaceX, an aerospace manufacturer that makes battery packs that compete with those from Boeing that were installed on the 787s.