There’s little remaining doubt that the main reason why electric vehicles (EVs) catch fire is because their batteries overheat. Some lithium-ion vehicle batteries are prone to catching fire if they are poorly manufactured, handled or improperly installed. That should be no surprise. EV battery packs contain hundreds of small individual cells linked together. EV manufacturers are continually exploring ways to develop new batteries that can store more energy. From: Using Research From Various Sources. by Kent Fuller-October 2022

But the greater the energy density of an EV’s battery pack, the greater the risk there is of one of its many storage cells overheating. If one overheats it can lead to a cascading heat progression (a domino effect) and a major fire. External damage to the battery pack, such as in a crash, or a penetration, may cause it to rupture, leading to a fire. Saltwater damage is a newly recognised and previously unreported hazard. In Florida just this month, saltwater contact has been linked to dozens of electric vehicle fires that occurred during Hurricane Ian.

A Tesla fireworks display in Florida.

The spectacular brick red to purple-tinged flame is typical of a fire where lithium is present. A thermal runaway or domino effect is the most dangerous outcome of battery overheating.
When an EV battery cell reaches a critical temperature, from whatever cause, it begins to release heat rapidly. This causes the surrounding cells to also quickly reach their maximum operating temperature. Those cells then begin to release heat at an accelerating rate. It becomes a chain reaction that results in a massive buildup of heat inside the car’s battery pack which can then rupture, spilling the electrolyte liquid. The electrolyte is usually a flammable petroleum product and this leakage compounds the fire problem. What can follow is rapid combustion of the entire battery pack and a violent explosion. Thermal runaway makes it very difficult to extinguish a battery fire. The packs are hard to access as well as very dangerous for emergency services crews. Even after the fire appears to be successfully suppressed there is a strong likelihood of re-ignition.

Battery Charging
When an EV battery is on a charger and fully charged, or nearly so, is the time when its fire risk is greatest. It’s also the most dangerous time because that’s when most energy is stored in the battery.
The battery is a chemical storage device. It delivers energy to the motor by a chemical process that releases electricity to propel the vehicle and run its electronics. When its charge is exhausted, charging reverses that chemical reaction inside each battery cell. You can think of normal driving as being like water in a tank slowly draining out, and charging like the water being pumped back in again. However charging is not a fully efficient process. As a result, when EV batteries are being charged, some waste heat is generated within the cells. In the case of a battery pack in good condition slow charging over many hours (say overnight), this heat should escape to the surrounding atmosphere. However, under fast charging conditions the battery will invariably become much warmer, and so the risk of a fire starting will be greater, especially in hot climates.
Two points worth remembering in this context. Firstly, there is a lot of pressure on EV battery manufacturers to make their products smaller, lighter and able to hold higher energy densities (be more powerful for their size). This is to increase a car’s range, its mileage between charges. However, inside each individual cell it means reducing the space between the anodes and the cathodes, making them more vulnerable to an internal short circuit. That can lead to an instant catastrophic discharge. In addition, even healthy lithium batteries do not operate well when they become too hot (or cold), so some form of external thermal regulation (refrigeration or heating) is required in many climates. This consumes battery energy and reduces vehicle range.
Overcharging
Two of the major factors contributing to the increasing numbers of electric car fires are problems with overcharging and the use of poorly regulated charging equipment, including that found at some charging stations. US data show that overheating due to overcharging and excessively fast charging is one key fire cause. The charger pours too much current through the battery and as a result, the battery overheats and is damaged. Consumer demand for shorter and shorter charging cycles at roadside facilities seems likely to exacerbate this situation.
If the damage is severe enough, the battery is ‘fried’ and will no longer hold any charge at all.
But a complete failure may not happen immediately while at the charging station. Later, on the road the battery may quickly discharge itself completely. This means it will no longer supply power to the motor. At this point, the car stops working and all control is lost. In some extreme cases, the battery may explode. This usually happens after one of its cells catches fire. You do not want to be in the car at that time.

Tesla Catches Fire Spontaneously

The United States Consumer Product Safety Commission (CPSC) is said to be continually working with “stakeholders” to reduce the number of fires caused by electric vehicles. They have issued warnings about the dangers of using some rechargeable batteries. They have also warned consumers against buying cheap replacement batteries from eBay and Amazon. Without specifying the likely origin of these batteries they say they are often “counterfeit” and unsafe.
There is said to be no uniformity in EV battery packs, their placement in cars or their shape.
They all differ between car manufacturers and even between different models from the same car maker. This is said to present a major headache for emergency services trying to extinguish EV fires. Most battery packs form part of the floor pan of an EV and are very difficult to access.
A Chevrolet Bolt receives it battery pack (arrowed). No two EV models share the same pack.
All EV car makers have suffered fires and had major recalls connected with battery fire risk; GM, Ford, Hyundai, BMW, Tesla, Mitsubishi, Audi et al. The cost of these recalls in August 2021 to just 3 of these manufacturers was reported to have exceeded US $2.2billion.  Below a video of an Audi EV lighting up spontaneously:- https://www.youtube.com/watch?v=pmfHACYwelY
As an aside it’s also worth noting that installation of an original equipment manufacturer’s replacement battery may cost a substantial portion of the price of a new car. For a Chevvy Bolt EV a replacement battery is currently US$15,734 plus tax, with a quoted life of around 8 years or 100,000 miles. The price of a new Bolt starts at US$25,600. So at the end of a short life the car is therefore a throwaway item. How green is that?
Conclusions
EV advocates claim that electric car fires happen far less often than conventional gasoline powered cars. I suggest this misdirects focus away from EV safety. To validate the claim one would have to evaluate the statistics on a level playing field; allowing for mileage driven, vehicle age, the incidence of fires during refueling versus recharging, insurance records of fire involvement that results from crimes (such as deliberate arson), and so on. There has been a frantic rush to get EVs out on the market and so no such figures exist. It is worth noting that petroleum vehicle fires can usually be readily extinguished, but that is not so with EVs.
Some of the issues facing firefighters are explained in this video:- https://youtu.be/J_bZu1YN2CY
Given that several ships have been lost or severely damaged due to EV blazes, one might be somewhat skeptical of working or living in a high rise office or apartment building where people are charging electric vehicles in the basement car park.

The burnt out car ferry Felicity Ace above is seen here on its way to Davey Jones locker in March 2022. The fire was started by an EV car battery. As well as the ship itself, around 4,000 vehicles, including VWs and Audis, and some high-end Porsches, Bentleys, and Lamborghinis were lost. The value of the cargo alone was over $400 million.
There is talk that solid state batteries can alleviate fire problems, but that remains to be seen.
These do not exist yet for cars and the experimental ones suggest they will contain lower energy density and so provide a shorter vehicle range. No one seems able to discuss their likely ability to accept a fast charge, which is defined as being half an hour or less.
Other images
Then there was the car transport Grande America which sank in 2019 with 2,000 new cars. The fire was started by an EV battery. In 2019, two other car transport vessels experienced car fires, causing severe damage to the vessels and cargo, and the loss of 5 lives.

The Tesla, while it was charging in the garage, almost took this owner’s house with it. It might have also worried the neighbours somewhat.
And Finally
This massive fire at an amusement park in China started with one lithium battery powered bus. It eventually consumed 55 buses that were parked side by side.
Footnote:- Chevrolet make two car models with similar names and that can be confusing. The model called a Volt is a hybrid vehicle (petrol and battery powered). The Bolt is an EV.

Very informative article from by Kent Fuller