...AT 9:27 A.M. ON MARCH 23, 2018, just as the morning rush hour was starting to ease in Mountain View, California, a violent collision threw a ho-hum Friday commute into chaos......For reasons still unexplained, a Tesla Model X SUV, traveling at 70 mph down a flat, straight stretch of the 101 freeway, abruptly drifted left and slammed into a concrete median that divided the freeway from an offramp.
The crash resulted in a complex and dangerous accident scene. The impact tore the front end off the vehicle’s frame, ripping open the Tesla’s 1,200-pound, 400-volt lithium ion battery and scattering energized cells across the road. By the time units of the Mountain View Fire Department arrived, just after 9:30 a.m., the badly damaged battery was shooting flames five feet into the air. Firefighting crews readied their gear and got to work.
If there was a fire department anywhere in the world equipped, trained, and prepared to handle the situation on the 101 that morning, it was the Mountain View Fire Department. The Bay-area city of 80,000 is in the heart of Silicon Valley and home to a who’s who of technology behemoths, including Tesla itself.
As they’d been trained, firefighters shot copious amounts of water directly at the flaming battery, and extinguished the fire in a couple of minutes. After eight more minutes with no reoccurrence of fire, they turned off their hoses. But the severed battery continued to hiss and pop, sounds Diaz likened to “slamming a hand on a kitchen table.” Firefighters feared the vehicle’s frame could be energized, but they lacked tools to test it. They also lacked the proper protective equipment to handle or remove the battery’s energized lithium ion cells, and there was no way for responders to drain the massive amount of energy still clearly trapped in the unstable battery. The tow company refused to load the hissing car onto a flatbed for fear of electrocution. Firefighters found themselves in the middle of a busy freeway with a battered EV and few options for what to do next.
Faced with these hazards, the Mountain View firefighters resorted to one of their few remaining options: call Tesla.
About two hours into the event, a team of Tesla engineers arrived at the crash site, dispatched from company headquarters just down the road. The engineers began the laborious task of disassembling the damaged battery cell by cell, dropping each into a bucket of water. The six-lane 101 remained closed for six hours as Tesla employees removed the exposed portion of the battery and isolated the exposed high-voltage wiring, while firefighters provided water as necessary to keep the fire from reigniting. With about a quarter of the battery removed, it was agreed that the car was safe to transport to an impound yard in nearby San Mateo. A Mountain View fire captain and fire truck accompanied the tow on the roughly 20-mile trip. During the ride, what was left of the car’s battery continued to pop like a firecracker.
At the impound, the vehicle’s battery ignited twice within the first 24 hours, and the salvage yard operator had to call San Mateo firefighters to assist. The following Thursday, six days after the initial crash, the battery reignited yet again. Tesla engineers eventually removed the remaining battery from the vehicle and de-energized it by submerging it in a vat of salt water.
The primary challenge faced by Mountain View firefighters that day was stranded energy, a widely overlooked issue with little scientific literature, according to researchers interviewed for this story. Stranded energy is any scenario where electrical energy remains in a battery without an effective means to remove it. This typically happens when the battery is damaged—by force, a coolant leakage, heat, or water intrusion—and normal function ceases. This can also lead to thermal runaway. How much energy remains in the battery when it’s damaged can greatly affect the severity and duration of the reaction.
