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Fires on electric vehicles are rare, but the volatile chemistry of their batteries and the need for special training on how to extinguish them raises new safety questions as automakers are poised to dramatically increase production. Techniques for putting out burning gasoline-fueled vehicles could make worse a blaze in a battery powered one.

“We’re in uncharted waters here,” said Donald Sadoway, a professor of materials chemistry at the Massachusetts Institute of Technology. “When you start putting 70 kilowatt-hour packs in a car, it’s very different than what happens in a cellphone.”

The growing popularity of lithium-based batteries that power everything from personal electronics to bicycles has periodically been marred by outbreaks of fires. Blazes in e-cigarettes, laptops and even battery packs on one of the most sophisticated jetliners in the world, the Boeing Co. 787, have led to government restrictions and frightening headlines.

The U.S. National Transportation Safety Board has opened investigations into two recent Tesla fires, along with an earlier blaze last year. The agency charged with setting vehicle safety standards, the National Highway Traffic Safety Administration, on Thursday announced it was also gathering information on the most recent episode, on May 8 in Fort Lauderdale, Florida. Swiss police are also examining a fatal Tesla crash last week that triggered a fire.

The issue isn’t new. NHTSA also has conducted reviews of battery fires in the past, including a General Motors Co.’s Chevrolet Volt that caught fire in 2011, several weeks after the agency conducted crash tests on the vehicle. Other manufacturers whose cars have been involved in fires include the former Fisker Inc. and Mitsubishi Motors Corp.

Unlike gasoline, which needs a spark before it ignites, lithium cells contain their own ignition systems: large stores of energy that are transformed into heat and sparks when they short circuit. They also contain solvents that are powerful fuel for fires as well as oxidized metals that can feed oxygen to a blaze, complicating efforts to extinguish it.

While the battery industry has made huge strides in ensuring cells can perform safely during normal operation and recharging, there is little that can be done once cells are torn apart in a violent collision, Kumta said. In a phenomenon known as thermal runaway, a short-circuiting battery produces ever more heat, which creates a chain reaction of fire and more heat production in adjacent cells, he said.

The attention paid to the relative handful of Tesla fires compared to the thousands of other automobile fires has rankled the carmaker. The risk of a gasoline-powered car catching fire was more than four times higher than a Tesla Model S, company Chairman Elon Musk said in a 2013 blog post.

After that fire, NHTSA conducted a series of tests on Volt battery packs. Out of six simulations of various types of accidents, two of the batteries caught fire, according to the agency’s report. A search of crash records at the time found no evidence of other fires.

Not only is it difficult to compare electric vehicles to gas-powered ones, it’s also impossible to compare the likelihood and severity of fires on Teslas with those on other lithium-battery-powered cars such as the Chevrolet Bolt, Nissan Motor Co. Ltd.’s Leaf and BMW’s i3, Levine said.

Firefighters in Indianapolis who responded on Nov. 3, 2016, to a crash of a Model S that hit a tree and a building at high speed encountered what looked like a fireworks display as battery cells exploded and shot into the air, according to video shot by a news crew. The crash killed both people in the car.

When the Model X in Mountain View slammed into the side of a concrete highway barrier, the front of the car was sheared off, damaging the battery pack located under the floor. About half of the car was on fire when crews arrived, said Chief Juan Diaz. It took about two minutes to extinguish the blaze, he added.

Mountain View is located in Silicon Valley, where electric vehicles are common and firefighters trained for battery fires at Tesla’s nearby factory in Fremont in 2014, the chief said. Still, the case illustrates how fire departments may need more training on the unique issues created by battery fires.

However, foam isn’t recommended by the National Fire Protection Association for batteries. NFPA guidelines call for using copious amounts of plain water on battery fires, as much as thousands of gallons. The water helps cool the battery, which is key to halting a fire.

Concerned that the battery was continuing to generate heat and worried about the risk of electrical shock, the firefighters called in Tesla engineers – which may not be possible if accidents aren’t located as close to the company’s factory. They removed about 25 percent of the battery’s cells, Diaz said.

Fire crews accompanied the tow truck that brought the car to a salvage yard because the battery continued to pop and hiss as gas vented from the power pack, Diaz said. It didn’t reignite on the trip, but it caught fire again two more times within the next 24 hours and again six days later, Diaz said.

Scientists are working on promising lithium-ion battery formulations that will reduce the chances of fire and there may be ways to make battery cases more impact resistant, said John Warner, a consultant who is president of the National Alliance for Advanced Technology Batteries International, a trade group.