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Batteries that have failed

 A fatal accident in Texas demonstrates the fire safety risks posed by electric vehicles.

On April 17, shortly before midnight, a 2019 Tesla Model S careened off a Houston-area road, collided with a tree, and caught fire. The crash claimed the lives of two men.

While much of the media coverage has focused on whether the vehicle’s autopilot system was engaged or not—responders report that no one was in the driver’s seat at the time of the accident, but Tesla CEO Elon Musk says company data indicates the autopilot was not engaged—the incident also serves as a reminder of the fire hazard posed by electric vehicles (EVs), as well as the importance of continued research in this area.

Tesla and other electric vehicles are propelled by massive battery packs comprised of thousands of lithium-ion battery cells. The Tesla Model S sports car’s newer models, in particular, are powered by battery packs capable of generating up to 100 kilowatt-hours of energy—enough to power the average American household for approximately three days.

“The incident over the weekend demonstrates that we still have a lot of work to do,” said Victoria Hutchison, a Fire Protection Research Foundation research project manager. “Over the last few years, there have been significant advancements in the architecture and power of modern EV battery technologies. Fire service guidance must be updated to reflect the current state of EV battery technology.”

‘Never been involved in a crash like this’

According to media reports, firefighters spent approximately four hours on the scene of Saturday’s crash, extinguishing the flaming batteries with more than 30,000 gallons of water. By comparison, the world’s largest firefighting aircraft can carry only about 20,000 gallons of water.

“Our office has never encountered a scene like this,” Mark Herman, a constable with Harris County (Texas) Precinct 4, told Houston’s KHOU News. “Normally, when the fire department arrives, they are able to extinguish a vehicle fire within minutes, but this took hours.”

Palmer Buck, fire chief of the Woodlands Fire Department, confirmed in a subsequent interview with Car and Driver that the initial fire was extinguished in a matter of minutes. However, smaller restarts continued to occur, posing a challenge to firefighters. The flames did not extinguish until they lifted the charred remains of the Model S into the air and sprayed water directly on the bottom—where the battery pack is located.

“The good news is that the most basic firefighting tool is to saturate it with water. That is a directive that many firefighters appreciate. Having said that, this was our first encounter with a large-scale lithium-ion fire on a large scale.”

Unlike gasoline, which can be drained from a vehicle’s tank, there are no foolproof methods for recharging a car’s lithium-ion battery following a crash. As a result, energy is trapped inside the battery, and a condition known as thermal runaway can occur, in which the battery essentially continuously overheats and overpressurizes, increasing the risk of fires, arc flashes, off-gassing, and occasionally explosions.

“To retard or control a reaction, you must remove the heat,” Hutchison explained. There have been numerous instances of EV fires being extinguished only to rekindle days later, after the vehicle has been removed from the scene.

“The inability to detect the amount of energy stranded in a damaged battery unit, combined with a lack of guidance on how to safely remove the stranded energy, creates significant challenges for firefighters responding to EV incidents and creates uncertainty about when the scene is safe,” Hutchison explained.

The National Transportation Safety Board (NTSB) announced that it will investigate the recent Texas incident, making this the NTSB’s 28th investigation into a Tesla crash.

The board released a report on EV fire safety in January, concluding that manufacturer guidance and federal safety standards are inadequate for emergency response to EV fires.

“While the emergency response guides provided by electric vehicle manufacturers are adequate in some ways, they fall short in others,” NTSB board member Jennifer Homendy stated in a video about the report.

To close safety gaps, the report recommends that EV manufacturers such as Tesla improve their vehicles’ emergency response guides, that the National Highway Traffic Safety Administration consider the quality of those guides when assigning new car ratings, and that organizations such as the National Fire Protection Association continue to educate their stakeholders about the fire risk associated with EVs.

Since 2012, NFPA has provided training to first responders on how to respond to emergencies involving alternative fuel vehicles, including EVs, and Hutchison said the foundation is hopeful to secure federal funding soon to launch a new research project focused on EV firefighting techniques and technologies. That research, she explained, could help inform future iterations of the training.