Thermal Runaway Explains Why Waymo Cars Burned So Completely in the Recent Los Angeles Protests
During recent protests in Los Angeles, fires triggered “thermal runaway” in several Waymo robotaxis’ lithium-ion battery packs. The phenomenon sent temperatures past 1,000 degrees Celsius, vaporized much of the cars and spewed lung-searing hydrogen fluoride
A Waymo vehicle burns in the middle of an intersection during protests in Los Angeles, California, on June 8, 2025.
David Pashaee/Middle East Images via AFP
Imagine watching a car burn until it seems to vaporize and the street itself begins to sag. That happened on Sunday in Los Angeles, when protesters torched at least five Waymo-branded Jaguar I-Pace robotaxis. When the smoke cleared, virtually the entire shell of each car—its roof, doors, hood, trunk and body panels—was gone, leaving only wheel rims and traces of aluminum lacing.
Why did the fires cause such obliteration? The answer starts with the battery. Each I-Pace can carry roughly 90 kilowatt-hours of stored chemical energy, comparable to about 170 pounds (77 kilograms) of TNT. That energy is distributed across hundreds of lithium-ion pouch cells, which are sealed in flammable electrolyte and separated by polymer films as thin as snack-bag plastic. When any one cell is punctured or overheated—or set aflame with an incendiary device—chemical reactions generate more heat than the cell can shed, and neighboring cells follow in a chain reaction. This positive-feedback loop is called “thermal runaway.” According to a 2024 study in the Journal of Power Sources, as the battery burns, its temperature can soar past 1,000 degrees Celsius.
At that point, the pack becomes its own furnace. Aluminum sections of the car’s floor surrender, liquefying at about 660 degrees C and taking the underbody with them. Magnesium parts—seat-base frames, the bracket that holds the steering column and the cross-car beam that is located behind the dashboard—flare bright white. Patches of magnesium can catch fire and burn fiercely. Plastics disappear as vapor, wheels lose their tire, and even the lidar mast on the roof quickly resembles an overcooked marshmallow. A 2025 study in Fire Technology and a 2023 study in Applied Energy noted that the placement of the battery on the floor—sometimes referred to as a “skateboard architecture”—makes the floor the hottest zone. Thus, flames radiate upward and outward, cooking everything above.
On supporting science journalism
If you’re enjoying this article, consider supporting our award-winning journalism by subscribing. By purchasing a subscription you are helping to ensure the future of impactful stories about the discoveries and ideas shaping our world today.
All the while, the battery cells vent hydrogen fluoride, a toxic, lung-searing gas documented in laboratory test burns of commercial lithium packs. Among the disturbing scenes from the recent Los Angeles protests, which erupted over federal immigration raids, are those in which protestors stood around the flaming Waymos. Historically, first responders without supplied-air protection have developed throat burns and breathing difficulties upon arriving at scenes with burning lithium-ion batteries. Depending on the hydrogen fluoride levels, an exposed person can begin coughing up blood within minutes. Whereas inhaling concentrations above roughly 30 parts per million (ppm) is immediately dangerous to health, 50 ppm may be fatal when inhaled for a half-hour to an hour, according to the U.S. Centers for Disease Control and Prevention. The Environmental Protection Agency estimates that exposure to 170 ppm for 10 minutes can be deadly. Measurements taken near electric-vehicle (EV) fires show peaks of 150 to 450 ppm, with levels during much of the fire hovering around 50 ppm.
Firefighters call such blazes “battery box fires,” and they hate them. Flame-retardant foams do little, and fire departments now favor high-pressure water lances or immersion pits. Dousing a runaway battery usually means lowering temperatures below the runaway threshold for every last battery cell—a task that, according to a 2021 interview with Thayer Smith of the Austin Fire Department, can swallow 30,000 to 40,000 gallons (about 114,000 to 151,000 liters) of water. That’s at least 40 times the amount of water required to extinguish a gasoline-car fire. If you hit the flames too lightly, stranded energy reignites hours later—a quirk the National Transportation Safety Board flagged in its 2020 report on EV firefighting hazards.
Car designers have tried to address the danger. Software monitors cell temperatures and slows the rate at which batteries charge to prevent overheating. And it automatically cuts current if anything looks amiss. Yet even the best code cannot rewrite chemistry: in 2023 Jaguar recalled more than 6,400 I-Pace cars after at least a dozen of them caught fire from overheated batteries—which had likely shorted from manufacturing defects in their pouch cells. Six of the fires happened while the car was either plugged in or within a few minutes of being unplugged. Waymo’s fleet got the update to better regulate the batteries, but software can’t help when someone smashes one of the car’s windows and lights up its interior with a “makeshift flamethrower,” as reported by the Los Angeles Times.
Could a Waymo van have burned just as thoroughly? Only with great effort. The company’s earlier Chrysler Pacifica hybrids, which were phased out in 2023, stored a tenth of their battery energy in a steel-framed shell. Steel keeps its shape beyond 1,300 degrees C, so after a typical blaze, you would still recognize the carcass. To prevent thermal runaway, Teslas have batteries that use thousands of small cylindrical cells locked inside an aluminum tray with titanium undershields and built-in firebreaks. And most brands of electric-car batteries now sit in similarly rigid aluminum or steel boxes—and are shifting toward less volatile chemistries.
Importantly, however, the scene in Los Angeles by no means indicates that electric cars are tinderboxes. A 2023 study in Finland showed that, mile for mile, they caught fire less often than gasoline cars. But when an EV does burn, the physics shift. You’re no longer fighting a puddle of gasoline on asphalt; you’re battling an energy-dense, metal-oxide battery that is determined to finish what it started—and in such cases, a single Molotov cocktail can turn a sleek robotaxi into a pool of molten alloy.
