Inquired: Accidents Involving Electric Vehicles

  09. November 2016

How do batteries in an electric vehicle react in an accident with or without fire? Dr Dietmar Schelb, Head of the Research Centre for Fire Protection (FFB) at the Karlsruhe Institute of Technology (KIT), spoke about this topic at the 5th Fire Chief Forum of the International Fire Academy in Vaduz. We asked: What are the most significant findings for fire services?

What are the peculiarities of electric vehicles?

Dr Dietmar Schelb: In modern electric vehicles the battery itself can burn because the electrolyte – different hydrocarbon compounds – and components of the anode such as graphite are flammable. Also, the cathode can release oxygen under severe heat and hence the fire can be maintained without an external air supply. This is the essential difference between the modern lithium-ion batteries and other batteries. 

What should be considered in an accident involving such a vehicle?

Dr Dietmar Schelb: If the chassis is deformed in an accident, there is the possibility for the battery cell to be deformed in such a manner that a short circuit and fire will start, not immediately but with a time delay. There are usually no outward signs. In such a case a fire can occur 24 hours after the accident or in the most unfavourable case even later.

This would also apply when another vehicle or vehicle part penetrates the battery, even without deformation of the chassis or the battery.

How do I have to imagine the fire development?

Dr Dietmar Schelb: The fire will not abruptly be noticeable in the form of an explosion but begin moderately. The battery starts to burn – usually clearly audible because successive short circuits take place. If there is still work being done on the vehicle at the scene, means of self-protection should now be considered. If there is still rescue work being done for trapped victims, this will be a difficult balance for the fire service.

What should be done after such an accident, if it did not cause a fire at the scene?

Dr Dietmar Schelb: The current recommendation is to park the towed vehicle separately. In extreme cases, a fire watch would be a possibility, but it is a big hassle. And the problem is: where to begin with a fire watch? Evidently not when only a bump at a parking area occurred, but clearly if you cannot recognise a vehicle due to the deformation. Where is the line between? We do not know. Therefore, the recommendation to park the accident vehicle at a distance from other fire loads.

What to do when the battery has begun to burn?

Dr Dietmar Schelb: The battery fire may take a long time, say one hour. Of course, at some point without extinguishing measures, the rest of the vehicle will be on fire, and you have a "normal" full car fire. You can then put plenty of water on it and contain the fire. This is simultaneously an air pollution control measure. But it can be that a fire service cannot extinguish the battery fire because usually, you cannot access the battery. Ideally, a stationary water supply is set up because of the long burning life. If possible, the used extinguishing water should be retained because usually, the batteries contain substances of the water pollution class 3.

A conceivable unfavourable situation could arise if in a highly deformed vehicle the battery starts to make noises and / or signs of smoke or showing only a small flame. In this case, a tentative extinguishing attempt with only little water can cause internal shorts and aggravate the situation, because the cells when shorted strongly heat up and the electrolyte may leak as a gas. If extinguishing is done with water, then plenty has to be used and for a long time. 

How is the situation when the vehicle is burning and the battery is exposed to a flame?

Dr Dietmar Schelb: If a lithium-ion battery is exposed to a flame –  e.g., after an accident or due to a fire in the vicinity of an electric vehicle – I have to expect that the battery behaves like any other liquid or solid combustible with a high heat release rate. A rule of thumb for a first estimate of the fire load is to divide the weight of the battery by 10, and you have the equivalent of litres of petrol – plus / minus 50%. The battery weights are somewhere between 10 kilos for a plug-in hybrid and up to nearly a tonne for pure electric vehicles. It corresponds to the filling of the tank of a petrol lawnmower for the plug-in and the tank capacity of a large car for the 1000-kg battery.

For electric vehicle fires, the rule is: always use self-contained breathing apparatus. Additionally, to the substances that are released during the vehicle fire, substances are released from the battery. This includes also an unknown quantity of hydrofluoric acid.

After such an operation, the rules for contaminated gear should be followed precisely. Worn gear must be professionally cleaned.

A third scenario: An electric vehicle is parked in an underground parking, which is flooded after a heavy rain. Is this a problem?

Dr Dietmar Schelb: According to the official licensing requirements, the batteries do not have to be water tight. According to the information available to us, all previously tested batteries are water tight, because the electronics in the battery has to be protected as well.

Consider the risk of electricity, we presume – as compared to the electrical installation installed in the garage –the additional risk by electric vehicles or charging stations to be low.

If water penetrated the battery of an electric vehicle, it would create short circuits and thus the outgassing of combustible electrolyte fumes and probably hydrogen. If the vehicle is at this time still under water, then it will blow bubbles and smoke.

If the vehicle is then out of the water, we recommend treating the vehicle as after an accident: after towing the vehicle out of the garage to park it separately for at least 24 hours. 

Doctor rer. nat. Dietmar Schelb was born in 1965, received his PhD at the Institute of Combustion Technology at the DLR Stuttgart, and since 1 November 2014 is head of the Research Centre for Fire Protection (Forschungsstelle für Brandschutztechnik). The Research Centre is part of the Department of Combustion Technology at the Engler-Bunte-Institute, working together closely with the Landesfeuerwehrschule Baden-Württenberg, fire services and the various institutes of KIT, for example in the areas of battery technology and hydrogen safety. Contact: dietmar.schelbanti spam bot@kitanti spam, Tel. +49 721 608 44450.

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