First, you need to understand the difference between combustion-based and battery-based fires. They might look the same, but they’re actually very different. Fires in vehicles with batteries larger than a typical car battery (anything hybrid and up, in terms of electrification) can rage for much longer than normal fires.
If an electric car uses lithium-ion batteries (most do) and catches on fire, the batteries are at risk of a process called ‘thermal runaway’. This happens when the cells inside the battery pack short circuit after being punctured (as in, after an accident) or due to a manufacturing defect and start producing ‘Joule energy’, which is when current passing through a conductor produces thermal energy.
This extra heat energy is often hard to dispell faster than it’s being produced, which in turn induces a chemical reaction to generate more heat, creating the vicious cycle called thermal runaway.
Ultimately, this makes it incredibly difficult to stop an electric vehicle fire, because even if the flames get put out, the battery is unstable enough to keep producing heat and re-igniting the blaze.
Fire services need to drop thousands of litres of water over a burning EV to stop the flames and, sometimes they resort to submerging the vehicle in cold water for days to keep the batteries cool. The only other option is let the fire burn itself out because chemical-based extinguishers don’t work on battery fires.