The US National Transportation Safety Board (NTSB) has called for improved safety measures following an investigation into the risks to emergency responders from lithium-ion battery fires in electric vehicles.
According to the NTSB, fires in electric vehicles powered by high-voltage lithium-ion batteries pose the risk of electric shock to emergency responders from exposure to the high-voltage components of a damaged battery. A further risk is that damaged cells in the battery can experience uncontrolled increases in temperature and pressure (thermal runaway), which can lead to hazards such as battery reignition/fire. The risks of electric shock and battery reignition/fire arise from the “stranded” energy that remains in a damaged battery.
The NTSB identified two main safety issues: the inadequacy of vehicle manufacturers’ emergency response guides for minimising the risks to first and second responders (firefighters and tow operators) posed by high-voltage lithium-ion battery fires in electric vehicles, and gaps in safety standards and research related to high-voltage lithium-ion batteries involved in high-speed, high-severity crashes.
On the basis of its findings, the NTSB made safety recommendations to the National Highway Traffic Safety Administration (NHTSA), the manufacturers of electric vehicles equipped with high-voltage lithium-ion batteries, and six professional organisations that represent or operate training programmes for first and second responders.
As part of the investigation, the NTSB studied three electric vehicle crashes resulting in battery fires and one non-crash fire. The NTSB says the crashes caused extensive damage that extended into the protected area of the cars’ high-voltage battery cases, rupturing the cases and damaging battery modules and individual cells. All three crash-damaged batteries reignited after firefighters initially extinguished the vehicle fires and the vehicles had been stable for several minutes.
The non-crash electric vehicle fire was caused by an internal battery failure that occurred during normal vehicle operations and did not reignite.
In one of the crash incidents, the report says a 2016 Tesla Model X caught fire after crashing into a garage attached to a house. When firefighters arrived, both the SUV and house were on fire. The fire under the SUV appeared to be extinguished but reignited an unspecified number of times. Firefighters removed the SUV from the garage to assess the fire and identified the fuel source as the SUV’s high-voltage battery pack. After the vehicle was pulled onto the driveway, the battery reignited. The fire was quickly extinguished using water, after which the vehicle remained stable for about 45 minutes. The Tesla then started to emit heavy white smoke, ignited again, and burned in what firefighters described as a “blowtorch” manner.
Firefighters applied water at up to 200 gallons (757 litres) per minute but that did not extinguish the flames. The crew allowed the vehicle to burn freely to eliminate as many interior combustibles as possible.
Firefighters later propped the SUV on cribbing blocks to expose the underside and applied more water, at a maximum rate of 600 gallons (2271 litres) per minute for about 45 minutes to cool the battery. Applying water to the underside of the SUV extinguished the fire.
One of the responding battalion chiefs told investigators that the fire was “very severe” and more difficult to extinguish than firefighters expected.
A vehicle towing service was called and as the SUV was loaded onto the tow truck, it again began to emit smoke and the battery fire reignited. Firefighters applied water, but they had difficulty directing the water under the vehicle because it was resting on planks (its wheels had been displaced in the crash). The SUV was lowered from the flatbed and firefighters again extinguished the fire. The tow truck driver suffered minor burns to his arms while lowering the SUV from the flatbed because the controls were near the sides of the burning vehicle, and he had to remove his wet, slippery gloves to operate them.
Firefighters continued applying water at about 300 gallons (1135 litres) per minute to cool the battery. The SUV was towed from the scene, approximately six hours after the crash.
In another incident, about 20 minutes after a 2017 Tesla Model X that caught fire after crashing had arrived at an impound yard, a California Highway Patrol (CHP) officer heard popping sounds coming from the wreckage and called the fire department, which monitored the SUV with thermal cameras but took no fire suppression action. Firefighters were dispatched again to the impound yard less than an hour later when the battery reignited. Crews monitored the vehicle, but the fire went out on its own and firefighters took no fire suppression action.
The battery reignited again five days after the crash. A security guard reported a fire at the impound yard after seeing smoke coming from under a tarp that had been placed over the vehicle wreckage. CHP investigators had inspected the vehicle earlier that day and in the course of removing electronic equipment, had stood on parts of the vehicle near the front of the battery case.
Firefighters arrived and reported flames coming from the right front side of the Tesla before suppressing the fire using water and foam. The Battalion Chief on scene told NTSB investigators that firefighters were concerned about the risk of electricity travelling up the water stream and had been cautiously starting and stopping the application of water.
Firefighters telephoned the manufacturer’s engineers, who advised them to apply foam and that electricity “should not be a big risk”. After about five minutes of foam application, no further smoke or fire was observed.
Combining both initial and secondary responses, around 1400 gallons (5299 litres) of water and foam were used to extinguish the battery fires.
In the third crash-related fire, a 2014 Tesla Model S erupted in flames after hitting a wall at high speed. When firefighters arrived, heavy flames were coming from the front of the car and the heat was intense, according to the Battalion Chief on scene. After suppressing the fire in the car’s interior, which took less than a minute, firefighters focused on the fire at the car’s right front corner. Firefighters reported that the heat was intense and that they could see electrical arcing. The Battalion Chief told investigators that the fire came from a lithium-ion battery module that was lodged under the car’s A-pillar. The module had ruptured and the individual battery cells inside were visible. Firefighters estimated using 200 to 300 gallons (757 to 1136 litres) of water and foam to stop the flames and electrical arcing.
Two large pieces of the battery had separated from the vehicle and were found in the street, although the pieces did not appear to be on fire. The main part of the high-voltage battery was still connected to the car but had dropped to the ground.
As part of the clean-up, the vehicle and debris were loaded onto tow trucks. The operator of one truck told NTSB investigators that he had attended a Tesla training class and was familiar with the company’s vehicles. He said that while he was winching the car onto his truck, the entire battery case separated from the vehicle. The battery reignited, but a brief application of water and foam extinguished the fire. The operator said that the battery briefly reignited again when a chain passed over the battery case while the battery and two piles of debris were being loaded onto a separate truck, but it self-extinguished.
While being unloaded at the tow yard about three hours after the crash, the battery case and modules briefly arced and smoked, but the battery self-extinguished once again, without any fire suppression being performed.
The full report can be read here, and the NTSB also produced an accompanying video: