The metal object that caused last week's Tesla battery fire reportedly tore through the bottom of a car, leaving a gaping hole in the armor plate that protects the battery.
"The geometry of the object caused a powerful lever action as it went under the car, punching upward and impaling the Model S with a peak force on the order of 25 tons," Tesla CEO Elon Musk wrote in a blog on Friday. "Only a force of this magnitude would be strong enough to punch a 3-inch-diameter hole through the quarter-inch armor protecting the base of the vehicle."
Click on the image below for a closer look at the Model S.
The Model S has a flat battery in the undercarriage of the vehicle, which acts almost like a structural member. It keeps the center of gravity low, makes the car handle well, and provides a huge trunk in the front and rear.
(Source: Design News)
The accident occurred last week after the Tesla Model S, which was being driven in an area near Seattle, Wash., struck a curved section of metal that fell off a semi-trailer. The car exited the highway, stopped at the bottom of an exit ramp, and was eventually engulfed in flames after the driver escaped. A passerby caught the fire on video, causing the fire to become an international news story.
Experts who spoke with Design News agreed that the circumstances, as described by Musk, were probably impossible to anticipate and impractical to design for, even if they could be foreseen. "It was way beyond the normal expectations of any accident," David Cole, chairman emeritus of The Center for Automotive Research, told us. "You can't design for everything."
"As a practical matter, you would never design for anything as bizarre as this," added Steven Eppinger, professor of management science and innovation at MIT. "In principle, you can design for every accident that might ever happen. But in reality, even if you could think of them all, it's impractical to design for every possibility."
Battery experts said that the fire was caused by one of two failure mechanisms -- internal short-circuiting of cells or spillage of electrolyte. "Either the metal cut through a plurality of cells creating a massive short that caused the temperature to rise above the ignition point, or the metal breached the case of the cells, which leaked their volatile, flammable electrolyte," Donald Sadoway, the John F. Elliott Professor of Materials Chemistry at MIT, wrote in an email to Design News. "Then, if the metal were dragging along the road and sparking, that could have started a fire."
"If you break the cells, then it's possible for the electrolyte to leak out," added Ralph Brodd, founder of Broddarp of Nevada, a consultancy that specializes in lithium battery technology. "And the electrolyte is almost like gasoline. It's very flammable."
In his blog, Musk stressed that the fire was effectively contained, and was prevented from entering the passenger compartment. He also argued that the Model S performed far better than a gasoline car could have. "Had a conventional gasoline car encountered the same object on the highway, the result could have been far worse," he wrote. "A typical gasoline car has only a thin metal sheet protecting the underbody, leaving it vulnerable to destruction of the fuel supply lines or fuel tank, which causes a pool of gasoline to form and often burn the entire car to the ground."
Experts who talked to Design News emphasized that they made their assessments based on Musk's description of the incident, and on reports from firefighters at the scene. Based on that, they concluded that Tesla's design probably couldn't be faulted. "There's no such thing as absolute perfection," Cole told us. "As a designer, you just hope you can get fairly close to it."
Warning: Video contains profanity.