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== See also ==
== See also ==
* [[Battery energy storage system]]
* Battery energy storage system
* [[Photovoltaic system fire]]
* Photovoltaic system fire
* [[Warehouse fire]]
* [[Warehouse fire]]


Latest revision as of 22:32, 2 November 2026

An electric vehicle fire is a fire in a car or other vehicle that is driven wholly or partly by electricity stored in a battery pack, usually a lithium-ion battery. The term covers fully electric cars and plug-in hybrids, which combine a battery pack with a fuel tank.[1][2] What sets such a fire apart is the battery pack. Once it is involved, the fire can sustain itself through thermal runaway and reignite until the stored energy has been used up.[3]

In many respects an electric car burns like a conventional one. Reviews summarised by the Netherlands Institute for Public Safety (NIPV) found similar peak heat release rates of about 6 MW and similar temperatures of about 800 °C.[4][5] The differences lie in the course of the fire. A battery fire develops more slowly and lasts longer, can produce small explosions and flare flames, and releases more hydrogen fluoride. It is also very hard to extinguish, because water cannot reach the cells inside the sealed pack.[6][7] These features matter most in an indoor car park fire, where the possibilities for firefighting are already limited.[8]

Battery packs and how they fail

The battery pack of an electric car is built up in stages. Single battery cells deliver about 4 V; cells are connected into modules of up to 60 V, and the modules form a pack of 300 to 1,000 V. Battery packs in passenger cars can now store 100 kWh or more.[9] The battery is monitored by a battery management system, which watches the individual cells. If threshold values are exceeded it can cool the pack, stop charging or disconnect the pack completely.[4]

The central failure mechanism is thermal runaway. NIPV defines it as a failure mechanism that leads to self-heating in a battery or battery cell and that can result in fire.[3] Each battery pack has a safe operating window. When this is exceeded, thermal runaway can start at temperatures that depend on the battery type and range from about 70 to 250 °C.[3] Lithium-ion is an umbrella term for several chemistries, and elsewhere in the same report the onset range is given as 50 to 270 °C.[1]

During thermal runaway the heated cell releases flammable gases, which ignite if there is enough oxygen and the ignition temperature is reached. The heat of one cell warms the neighbouring cells, so that the runaway spreads as a chain reaction through the pack.[3] If the gases are trapped, they can escape suddenly when the casing or a pressure valve fails. In the right mixture they can cause a flare fire or a gas cloud explosion, and burning fragments can be ejected from a damaged pack.[3] If the released gases do not ignite at once, an explosive mixture can build up in a confined or poorly ventilated space.[10] The underlying chemistry is treated in the article on lithium-ion battery fires.

Cells of about 4 V form modules of up to 60 V and a pack of 300 to 1,000 V; one overheating cell heats its neighbours and vents gas.
Structure of an electric vehicle battery pack and the chain reaction of thermal runaway between cells. Based on Van der Graaf et al. (2021/2023), pp. 21–22.

Battery failures can have internal and external causes. Internal causes include design or manufacturing errors in the electrodes, separator or electrolyte, and failures of the battery management system that lead to overcharging. External causes include heat, misuse and damage in a collision.[4] Published failure probabilities differ widely, from one in a million or lower to about one in 100 million. NIPV concludes that battery failure seems relatively rare, but notes that no lithium-ion battery is intrinsically safe.[4]

How an electric vehicle fire develops

Temperatures in electric and conventional car fires are similar. Tests on both types of vehicle measured about 800 ± 100 °C over periods of 5 to 30 minutes. Other tests measured flame temperatures of 798 °C, 807 °C and 863 °C for batteries at 50%, 75% and 100% state of charge.[4] The heat radiation from an electric car with a fully charged battery is similar to that from a conventional car.[2]

The peak heat release rate is also similar. In the review by Sun and co-workers, both types reached about 6 MW. The time to the peak ranged from 6 to 40 minutes for electric cars and from 6 to 35 minutes for conventional cars.[5] Electric cars with 15 to 25 kWh batteries reached about 4 to 6 MW. With batteries of more than 100 kWh the peak rose to about 8 MW, against an average of about 6.8 MW for a mid-range conventional car.[5] These values are of the same order as the reference value of 8.5 MW for a burning car used in Introduction to Fire Safety Engineering.[11]

Dot plot of peak heat release rates: electric cars 4.2 to 7 MW, plug-in hybrids 6 to 8 MW, conventional cars 2.1 to 10.8 MW.
Peak heat release rates in full-scale fire tests of electric, plug-in hybrid and conventional cars. Based on Sun et al. (2020), as cited in Van der Graaf et al. (2021/2023), p. 24.

The course of the fire differs. A petrol or diesel fire develops quickly, because a plastic fuel tank can fail and release its fuel all at once. Battery cells first have to warm up and release gas before they burn. In the first minutes the temperature therefore rises slowly, by about 0.2 °C/min, and only at thermal runaway does it rise by more than 10 °C/min.[12] A normal car fire burns for just over half an hour and peaks at about 5 MW after 10 to 15 minutes. Without intervention, an electric car burns out in about 90 minutes, and after extinguishing it can keep reigniting for several hours.[12]

Two timelines: conventional car peaks at about 5 MW after 10 to 15 min, out after about 30 min; electric car burns about 90 min, may reignite.
Typical course of a conventional car fire and an electric car fire without intervention. Based on Van der Graaf et al. (2021/2023), p. 26.

Fires in electric cars can also include repeated small explosions, parts flying about and flash fires.[6] Because the battery pack burns for a long time, nearby vehicles are exposed to heat radiation for longer. NIPV therefore expects electric cars to make "travelling car fires" more likely: fires that pass from vehicle to vehicle through a car park.[6] The opposite path also exists. A conventional car burning next to an electric car can heat its battery pack and trigger thermal runaway there.[13]

Energy content of the battery

The battery adds energy that is released slowly and is hard to remove. NIPV, following Sun and co-workers, estimates the heat released by a burning battery pack from its storage capacity. The stored electrical energy in kWh is converted to GJ and multiplied by a factor of 7, which Sun and co-workers assume for the heat of flaming combustion. The factor accounts for the difference between the energy stored and the energy released when the pack materials burn:[5]

E=f⋅0.0036⋅C

Symbol Description Unit
E heat released by combustion of the battery pack GJ
f factor for the heat of flaming combustion, assumed as 7 none
0.0036 conversion from kWh to GJ GJ/kWh
C energy storage capacity of the battery pack kWh

For a battery of 90 kWh, which gives a range of about 400 km, the estimate is 90 × 0.0036 × 7 ≈ 2.3 GJ. A conventional car needs a 30-litre petrol tank for the same distance, and at about 32 MJ per litre this holds about 1.0 GJ.[5] On this basis NIPV concludes that an electric car fire requires a larger firefighting effort, while noting that many fuel tanks hold more than 30 litres.[5] A 100 kWh pack gives about 2.5 GJ by the same estimate.

Bar chart: 90 kWh battery about 2.3 GJ, 30-litre petrol tank about 1.0 GJ; whole cars 4.7 to 8.5 GJ electric and 3.3 to 10 GJ conventional.
Estimated heat released by a 90 kWh battery pack and a 30-litre petrol tank for a range of about 400 km, with the total heat of whole cars. Based on Van der Graaf et al. (2021/2023), p. 25.

The factor of 7 is an assumption from a single review and applies to the battery pack only, not to the whole car. The total heat released by complete electric cars in fire tests ranged from 4.7 to 8.5 GJ, against 3.3 to 10 GJ for conventional cars.[5] For comparison, Introduction to Fire Safety Engineering gives a fire load of 6,000 to 12,000 MJ for a car.[14] NIPV notes that the heat release rate rises with battery capacity, so its values have to be updated as battery packs change.[15]

Toxic and corrosive products

Burning battery packs release corrosive substances from the electrolyte and products from the burning plastics. Besides methane, carbon monoxide, carbon dioxide and hydrogen, these include toxic gases such as hydrogen fluoride (HF) and hydrogen chloride.[12] Measurements found about 20 to 200 mg of HF per Wh of battery capacity, so a 100 kWh pack can emit 2 to 20 kg of HF, depending on its state of charge.[12] Comparative tests found about 1.8 times more HF from an electric car than from a similar conventional car, and hardly any difference in the other toxic gases.[12]

The effect on escape is limited by the fact that smoke from any car fire is already severely toxic. NIPV, citing Swedish research, considers the effect of the extra HF on escape safety to be nil. Anyone who inhales smoke from a vehicle fire inhales a harmful mixture of toxic gases, however the vehicle is powered.[7] HF is reactive and settles on floors, walls, ceilings and clothing. Its concentration in smoke falls by more than half within 10 to 20 minutes, while firefighting clothing becomes more contaminated.[12]

Water used on an electric car fire also needs attention. Extinguishing water has hardly more environmental impact than water used on conventional car fires and may be drained to the sewer.[6] Water in an immersion container is different. Lithium oxide in the burnt pack reacts to lithium hydroxide, so the pH keeps rising, and this water may not be discharged directly to the sewer.[6][16]

Firefighting

Firefighting tactics for electric vehicle fires distinguish between putting out the flames and stopping the battery. Putting out the flames needs no more water or crew than for a conventional car. Long-term cooling of the battery pack to prevent reignition needs much more water, depending on the pack and its state of charge.[17] Fire services match the flow rate to the expected heat release rate.[18] As rules of thumb, a high-pressure jet of 125 L/min is generally sufficient for up to 2.5 MW, and a low-pressure jet of 450 L/min for up to 10 MW.[19]

The battery pack itself is very difficult or impossible to extinguish. It is sealed against moisture and dirt, so water cannot be applied directly to the cells.[3][7] Once the flames are out, the pack can reignite within a short time and requires continuous cooling and temperature monitoring for a long period.[17] At present the best method for long-term cooling is to immerse the vehicle or its battery in a water-filled container. Until the car can be removed, a mobile sprinkler is the best available way for the fire service to keep the pack cool.[17]

Several common methods do not work as expected. A blanket of foam is not useful, because the pack can reignite under the foam: laboratory studies show that thermal runaway can restart even with little oxygen.[16] A sprinkler system cannot extinguish a burning battery pack, because the droplets cannot reach the seat of the fire. It can, however, reduce growth and spread to adjacent vehicles and structures.[20]

Other hazards are specific to electric cars. The high-voltage system means a risk of electric shock, although this is negligible when firefighters extinguish from a distance.[21] A burning electric car may start to move, and firefighting clothing needs decontamination because of the HF released.[7] No additional personal protective equipment is needed compared with a conventional car fire.[16] In the Singelgarage fire in Alkmaar in 2020, a police robot towed a burning electric car out of the car park because its battery kept reigniting. The car was then cooled for a long time in an immersion container.[22]

Electric vehicles in car parks and tunnels

Charging adds causes of fire that do not exist for conventional cars. These include damaged cables or charging stations, failures of the battery management system, collisions with charging stations and cars with an unnoticed defective battery entering the car park.[6] The probability of fire in a parked electric car is unlikely to be higher than in a parked fossil-fuelled car. The charging process, however, is expected to raise the probability of fire, partly because the technology is still in a phase of teething problems.[23]

For indoor car parks, NIPV proposes a range of measures rather than a fixed package. Examples are a single facility that cuts the power to all charging points, and charging bays close to the vehicle entrance and at street level. NIPV also advises keeping such bays away from pedestrian escape routes and ventilation inlets.[24][25] Sprinklers or water mist are recommended because they keep the fire small, which matters more when the fire is harder to fight.[26] A plan for removing a burnt car from the car park is also advised, because it must be taken out to stop it reigniting inside.[25]

Electric vehicles also use road tunnels. The sources available for this wiki do not treat electric vehicle fires in tunnels specifically. Tunnels do not fit building regulations in several respects, so a fire safety engineering approach is applied, and they are usually provided with a smoke control system; this is discussed under tunnel fire.[27]

Knowledge gaps and common misconceptions

A common misconception is that electric cars are much more likely to catch fire than conventional cars. In 2021 there were no reliable datasets on the probability of electric vehicle fires, either when parked or when charging. The role of production defects and ageing is still unknown.[8][15] In 2019 about 218,000 electric cars were registered in the Netherlands, about 2.5% of all passenger cars, so experience is still limited.[28]

A second misconception is that an electric car fire is far more intense. Peak heat release rates and temperatures are similar; the main differences are duration, reignition and the difficulty of cooling the battery.[4][12] Ageing may even reduce the intensity of thermal runaway, because an older battery reaches a lower maximum state of charge and produces fewer flammable gases.[23]

NIPV lists several open research questions. These include the exact process of thermal runaway, the thermal load of electric car fires on car park structures and travelling fires involving several electric cars. Further questions concern the best detection, the effect of different suppression systems, external cooling of the pack and salvage after a fire.[29] Manufacturers are testing ways to get water into the pack, such as a sprinkler inside the battery, a hose connection on the casing and a marked spot where the casing can be pierced.[30]

See also

Notes

  1. ↑ 1.0 1.1 Van der Graaf et al. (2021/2023), p. 18.
  2. ↑ 2.0 2.1 Van der Graaf et al. (2021/2023), p. 24.
  3. ↑ 3.0 3.1 3.2 3.3 3.4 3.5 Van der Graaf et al. (2021/2023), p. 22.
  4. ↑ 4.0 4.1 4.2 4.3 4.4 4.5 Van der Graaf et al. (2021/2023), p. 23.
  5. ↑ 5.0 5.1 5.2 5.3 5.4 5.5 5.6 Van der Graaf et al. (2021/2023), p. 25.
  6. ↑ 6.0 6.1 6.2 6.3 6.4 6.5 Van der Graaf et al. (2021/2023), p. 29.
  7. ↑ 7.0 7.1 7.2 7.3 Van der Graaf et al. (2021/2023), p. 30.
  8. ↑ 8.0 8.1 Van der Graaf et al. (2021/2023), p. 11.
  9. ↑ Van der Graaf et al. (2021/2023), p. 21.
  10. ↑ Van der Graaf et al. (2021/2023), p. 27.
  11. ↑ Van Rossum (2027), §4.5.
  12. ↑ 12.0 12.1 12.2 12.3 12.4 12.5 12.6 Van der Graaf et al. (2021/2023), p. 26.
  13. ↑ Van der Graaf et al. (2021/2023), p. 32.
  14. ↑ Van Rossum (2027), §4.4.
  15. ↑ 15.0 15.1 Van der Graaf et al. (2021/2023), p. 54.
  16. ↑ 16.0 16.1 16.2 Van der Graaf et al. (2021/2023), p. 47.
  17. ↑ 17.0 17.1 17.2 Van der Graaf et al. (2021/2023), p. 46.
  18. ↑ Van Rossum (2027), §8.6.
  19. ↑ Van der Graaf et al. (2021/2023), p. 45.
  20. ↑ Van der Graaf et al. (2021/2023), p. 37.
  21. ↑ Van der Graaf et al. (2021/2023), p. 43.
  22. ↑ Van der Graaf et al. (2021/2023), p. 12.
  23. ↑ 23.0 23.1 Van der Graaf et al. (2021/2023), p. 28.
  24. ↑ Van der Graaf et al. (2021/2023), pp. 38–39.
  25. ↑ 25.0 25.1 Van der Graaf et al. (2021/2023), pp. 41–42.
  26. ↑ Van der Graaf et al. (2021/2023), p. 5.
  27. ↑ Van Rossum (2027), §11.2.
  28. ↑ Van der Graaf et al. (2021/2023), p. 10.
  29. ↑ Van der Graaf et al. (2021/2023), pp. 55–56.
  30. ↑ Van der Graaf et al. (2021/2023), p. 56.

References

  • Van der Graaf, J., Hessels, T., & Rosmuller, N. (2023). Fire safety of indoor car parks accommodating electrically powered vehicles. Nederlands Instituut Publieke Veiligheid. (Original work published 2021)
  • Van Rossum, E. (2027). Introduction to Fire Safety Engineering. Fire Safety Academy.
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