Unless you’ve been living under a rock, you already know that electric vehicles (EVs) are becoming increasingly popular as people look for more sustainable and affordable transportation options. However, as with any new technology, there are certain risks associated with their use. One of the most significant concerns related to EVs is the potential for fires and the difficulties in extinguishing them. The industry is changing rapidly, and what was true just a few years ago has already changed. It's essential for people to be aware of how to reduce the risk of fire, what to do in case of a fire, and what the future holds for this technology and the concern with vehicle fires.
Given the vastness of the topic, we have decided to present a three-part series on EV battery fires. The first part will examine what causes them, the second will focus on innovations in the industry to address concerns and improve battery safety, and the final part will look at the future fire risks of electric vehicles, and how we predict the technology will change.
Our goal is to clarify misinformation and provide accurate and comprehensive knowledge of the issue to our readers. So, without further ado, let's get right to it!
What causes a fire in an EV battery?
There are several reasons that can cause a fire in an EV, but the majority of cases are due to a fault or defect in the battery design, abuse of one or more battery cells (by overheating, crushing, penetration, or overcharging), or as a result of a collision. A fire starts when a damaged or abused battery cell is short-circuited, triggering a chemical reaction that generates toxic and flammable gases, and a significant amount of heat. This heat can lead to a chain reaction called “thermal runaway”.
Hot Thermal Runaway works:
Electric vehicle battery packs are typically composed of hundreds, if not thousands, of individual battery cells that are densely packed together to occupy as little space as possible. When one or more of the battery cells are damaged or abused, the resulting heat can affect the battery cells adjacent to the first, causing a failure, more heat, and gases in an uncontrollable exothermic chain reaction.
As thermal runaway progresses, the separator structure within the battery cell collapses, causing the electrodes to touch. This creates an internal short circuit that generates even more heat. Eventually, the pressure builds up and the gases within the cell are vented, either through blast caps in cylindrical and prismatic cells or by the bursting of pouch cells. During this process, heavy metal dust particles from the cathode can form a dark cloud, which is followed by a white vapor cloud as the gases carry fine droplets of the solvent.
As oxygen mixes with the vapor cloud & heat continues to build, the battery cell may ignite, causing surrounding cells to do the same. In rare cases, the vapor cloud can explode without warning. Luckily, this is unlikely to occur. Roughly 11% of the cases that have been reported to Australian research company, EVFireSafe, described a vapor–cloud explosion.
How often do EVs catch fire compared to other vehicles?
It is important to note that while fires in electric vehicles are much less common, the biggest risk is when fires happen. In ICE vehicles, fires usually occur during operation of the vehicle and this results in little damage to external property or life as the vehicle is located in an area away from buildings and people.
However in an electric vehicle, the fires that do occur are often while the vehicle is parked and charging. This means that damage and destruction of other nearby vehicles or the physical structure (garage, house, parking facility) can occur. In some cases, owners houses have been completely destroyed as a result of a fire starting while they were sleeping.
What makes EV fires different from other vehicles?
In ICE vehicles, fires are most commonly caused by fuel leaks, electrical system failures, or overheated engines. Naturally, EVs don’t have to worry about most of those causes, but EVs are drastically different from ICE vehicles once a fire is already present.
In an ICE vehicle, starving the fire of oxygen will usually extinguish it. However with an electric vehicle, this will not work. That is because most battery cells already contain everything needed to sustain a fire, no outside material or oxygen is needed. The cathode of the battery cell often provides a source of oxygen, and commonly used battery chemicals are highly flammable, even at room temperature. This means that even if you submerge a burning battery cell, it will continue to have a thermal event until the temperature of the cell is reduced significantly.
Additionally, the location of EV battery packs (often located under passenger compartment) make accessing the source of the fire very difficult for emergency responders. If a battery cell goes into thermal runaway, this positioning makes it very difficult to direct a stream of water onto the battery pack to cool it.
EV fire suppression
Eventually, an EV on fire will burn out completely on its own. In some cases though this could take a very long time and secondary ignition is a risk (we’ll get to this more later).
The most common method currently used on EV fires is to apply large amounts of water to the vehicle for an extended period of time. This has the effect of cooling the battery cells and bringing them out of a thermal runaway event. This method requires emergency responders to spend hours, and many tens of thousands of gallons, to get the fire under control so that the scene can be cleared.
Other methods are to jack one side of the vehicle up to allow better access to the battery pack (not recommended by some manufacturers), and a third common method is to submerge the vehicle entirely in a large container of water.
All three of these methods are used to cool the battery pack. Once again, there is no way to starve the fire of oxygen.
Secondary ignition, also known as re-ignition, occurs when nearby battery cells that were damaged in the initial incident or even cells in other modules go into thermal runaway and ignite after a battery cell has already caught fire and been suppressed. Although the original cell does not typically reignite, other cells can ignite at different times, leading to reignition of the EV traction battery. This can happen without warning and in some reports happened more than two months after the initial event. Naturally, this can pose a significant risk if proper precautions were not taken to protect surrounding property.
According to some fire agencies around the world, the best practice is to allow the EV traction battery to burn out completely while protecting the surroundings. However, this may not be practical in many cities and towns where road closures and long firefighting operations may not be feasible.
In these cases, monitoring the battery for a period of time (15-30 minutes) using thermal imaging cameras and listening for audible signs of thermal runaway, such as popping or hissing noises, is suggested before clearing the vehicle for towing.
While EV fires are different from ICE vehicles and require different methods of suppression, they are also incredibly rare. Nonetheless, there is still room for improvement and innovation to make EV batteries even safer.
In the upcoming articles in this series, we will explore various topics such as making safe EV batteries, the role of Battery Management Systems (BMS) in preventing fires, how battery state of charge affects fire risks, and technology being designed and implemented to reduce the risk of battery fires. We will also discuss fireproof EV battery pack designs and what we can expect in the future. Stay tuned for more information on how the industry is working to make EVs even safer!