How dangerous are burning electric cars?

A dry bang, then it starts: A battery module of an electric car is on fire in the Hagerbach test tunnel. A video of the test impressively shows the energy contained in such batteries: Meter-long jet flames hiss through the room and produce enormous amounts of thick, black soot. Visibility in the previously brightly lit tunnel section quickly drops to zero. After a few minutes, the battery module has burned out. Ash and soot have spread throughout the room.

Target group: operators of parking garages and underground garages

The trial, which was funded by the Swiss Federal Roads Office (Astra) and involved several Empa researchers, already took place in December 2019. Now the evaluation is available. "In our experiment, we also had private and public operators of small and large underground garages or parking garages in mind," says project manager Lars Derek Mellert of the company Amstein + Walthert Progress AG. "All of these existing underground structures are increasingly being used by electric cars as well. And operators are asking the question: what to do if such a car catches fire? What are the health hazards for my employees? What effects will such a fire have on the operation of my facility?" But until now, there was hardly any meaningful technical literature, let alone practical experience for such a case.

Mellert developed three test scenarios with the support of battery researcher Marcel Held and corrosion specialist Martin Tuchschmid from Empa. Experts from the Hagerbach AG test tunnel and the French "Centre d'études des tunnels" (CETU) in Bron were also involved. "We mounted test surfaces in the fire room on which the soot settled," explains Martin Tuchschmid, corrosion and fire damage specialist at Empa. "The surfaces were chemically analyzed after the test and also stored in special rooms for several months to track down possible corrosion damage."

Scenario 1: Fire in a closed room

The first scenario involves a fire in a locked parking garage without mechanical ventilation. A parking area of 28 x 28 meters in area and 2.5 meters in floor height was assumed. Such a parking floor would have 2000 cubic meters of air volume. The fire of a small car with a fully charged battery of 32 kWh power is assumed. For reasons of experimental economy, everything was scaled down to 1/8: A fully charged battery module with 4 kWh capacity was thus set on fire in a room with 250 cubic meters of air volume. It was investigated how the soot settles on tunnel walls, surfaces and on protective suits of firefighters present, how toxic the residues are and in what way the fire site can be cleaned after the event.

Scenario 2: Fire in a room with sprinkler system

Scenario 2 deals with chemical residues in the extinguishing water used. The test setup was identical to that in scenario 1, but this time the smoke from the battery was directed under a water shower resembling a sprinkler system with the aid of a metal sheet. The soot water that rained down was collected in a catch basin. The battery was not extinguished in the process, but also burned out completely.

Scenario 3: Fire in a tunnel with ventilation

This scenario was about the effect of such a fire on a ventilation system. How far does the soot spread in the exhaust air ducts? Do substances settle there that lead to corrosion damage? In the experiment, a 4 kWh battery module was again set on fire, but this time a fan blew the smoke at a constant speed (approx. 1.5 m/s) into a 160-meter ventilation tunnel. At distances of 50, 100 and 150 meters from the fire site, the researchers had mounted metal sheets in the tunnel, on which the soot settled. The chemical composition of the soot and possible corrosion effects were analyzed in Empa's laboratories.

The results of the trial were published in early August 2020 in a Final Report published. On the one hand, project manager Mellert can give the all-clear: A burning electric car is no more dangerous thermally than a burning car with a conventional drive. "The pollutant emissions of a vehicle fire have always been dangerous and, under certain circumstances, fatal," the final report states. Completely irrespective of the form of drive or the energy storage system, the primary goal must be for everyone to get out of the danger zone as quickly as possible, it says. In particular, the highly corrosive, toxic hydrofluoric acid is often discussed as a special hazard in burning batteries. In the three tests in the Hagerbach tunnel, however, the concentrations remained below the critical range.

Conclusion: A state-of-the-art tunnel ventilation system can cope not only with burning gasoline cars but also with electric cars. Increased corrosion damage to the ventilation system or tunnel equipment is also not to be expected based on the results now available.

Fire departments also don't have to relearn anything because of the tests. Firefighters know that the battery of an electric car cannot be extinguished and can only be cooled with large amounts of water. Thus, the fire can possibly be limited to a few battery cells, and part of the battery will not burn out. Admittedly, such a partially burned-out wreck must be kept in a pool of water or a special container to prevent it from reigniting. But this is already known to the specialists and is already being practiced.

The extinguishing water is toxic

On the other hand, the extinguishing and cooling water produced when fighting such a fire and storing a burnt-out battery in a water bath poses a problem. The analyses showed that the chemical contamination of the extinguishing water exceeds the Swiss limit values for industrial wastewater by a factor of 70, and the cooling water is even up to 100 times above the limit value. It is important that this highly contaminated water does not run into the sewage system without proper pretreatment.

Professional decontamination necessary

After the tests, the room was decontaminated by professional fire cleaners. Subsequently taken samples have confirmed that the methods and the time required are also sufficient for the cleanup after the fire of an electric car. But Mellert warns private owners of underground garages in particular, "Don't try to clean up the soot and dirt yourself. The soot contains large amounts of cobalt oxide, nickel oxide and manganese oxide. These heavy metals cause severe allergic reactions on unprotected skin." So fire cleanup after an electric car fire is definitely a job for professionals in hazmat suits.

And here's the video.