NTSB says Tesla battery reignited twice after fatal accident [Updated]

NTSB investigators are looking into a Tesla battery fire that twice reignited, after a high-speed crash caused major damage to a Model S sedan in Florida earlier this year. According to the National Transportation Safety Board, the incident required emergency services to extinguish the fire three times in total, after the driver apparently lost control while doing 116 mph in a 30 mph limit zone.

The four-person NTSB team was dispatched to the scene of the crash on May 9, where two of the car's occupants died and a third was injured. In the report, issued this week on the accident, investigators not that, after the initial fire that destroyed the car, the battery pack reignited twice.

The first time the battery caught fire again was after the Tesla was placed on top of the wrecker to remove it from the accident scene. That second blaze was extinguished, and the car was carried to a storage yard. The battery pack caught fire again after the car was deposited at the storage yard, and had to again be extinguished.

This isn't the first time that batteries inside an EV have caught fire after an accident, of course. Chevy had a similar issue with a Volt undergoing crash testing. The battery in that car caught fire twice, three weeks after the initial test, and led Chevy to fit Volts with extra bracing and more isolation in the battery pack.

The NTSB says it's still working on figuring out what caused the batteries to reignite, and the procedures currently being implemented around EV fires. When emergency services first reported to the scene, the Fort Lauderdale Fire and Rescue Department used 200–300 gallons of water and foam to douse the car, which was "fully engulfed in flames" according to the preliminary report. "The investigation will also include examining the procedures used to extinguish the battery fire and to remove and store the car postcrash," the Board says.

Tesla's own "Emergency Response Guide" for First Responders advises considerably more than 200-300 gallons should be used. "If the battery catches fire, is exposed to high heat, or is generating heat or gases, use large amounts of water to cool the battery," the latest version of the guide suggests. "It can take approximately 3,000 gallons of water, applied directly to the battery, to fully extinguish and cool down a battery fire."

Post-extinguishing, Tesla says, thermal imaging cameras can be used to identify lingering hotspots that might not be visible to the naked eye. "There must not be fire, smoke, or heating present in the high voltage battery for at least one hour before the vehicle can be released to second responders (such as law enforcement, vehicle transporters, etc.)," the guide says. "The battery must be completely cooled before releasing the vehicle to second responders or otherwise leaving the incident. Always advise second responders that there is a risk of battery re-ignition." It's not stated in the preliminary report as to how long after the first fire was extinguished that the car was first moved.

In a statement made back in May when the crash took place, a Tesla spokesperson promised full cooperation with the investigation and pointed to the automaker's own findings about post-crash fire safety. According to Tesla's data, for instance, gasoline cars in the US are five times more likely to experience a fire than a Tesla vehicle.

"Our thoughts are with the families and friends affected by this tragedy. The family who owned the car has been a close friend of Tesla for many years, and this hits us particularly hard. We are working to establish the facts of the incident and offer our full cooperation to the local authorities. We have not yet been able to retrieve the logs from the vehicle, but everything we have seen thus far indicates a very high-speed collision and that Autopilot was not engaged. Serious high-speed collisions can result in a fire, regardless of the type of car. Tesla's billions of miles of actual driving data shows that a gas car in the United States is five times more likely to experience a fire than a Tesla vehicle. This doesn't change how devastating an event like this is for our customer's family and friends, and our hearts are with them" Tesla spokesperson

When fires do occur because of battery pack rupturing, the automaker suggests, it's typically only in the case of very severe incidents. With this particular Model S impacting the wall at 86 mph, according to the car's own data records, that probably counts as severe. Tesla also points to the fact that its batteries are designed to burn more slowly, so as to allow occupants time to escape should that be necessary, and that the architecture of the battery packs themselves means one broken section needn't necessarily mean the whole battery assembly will ignite.

Arguably most important, Tesla also says it has been expanding its efforts to deliver EV-specific training to first responders. It seems clear that, with the amount of water and foam used in this particular crash an order of magnitude lower than Tesla's recommended amount – not to mention the fact that, as the First Responders Guide cautions, "battery fires can take up to 24 hours to extinguish" and Tesla even suggests that emergency services consider "allowing the battery to burn" itself out – the firefighter response fell short of what EV electrical fires require to be rendered entirely safe. Though the NTSB's report this week is only preliminary, it already appears the organization will be making that an area of focus.

[Update: We've updated this post to clarify that the NTSB investigation in question focuses on the lithium-ion battery in the Tesla, and not the Autopilot system, for the sake of clarity. "The NTSB does not, at this time, anticipate autopilot being a part of this investigation," the Board said back in May.]

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