Also, fuel fires are easier to deal with. Just spray a bunch of foam everywhere and you're pretty much good.
You seem to seriously underestimate the amount of science that goes into extinguishing fires. The average crash tender carries five completely different systems for fighting various fires, and every fire department has a sixth one ready. Then again: the stuff that they carry isn't designed for the kind of danger a high power car battery poses. But, expect larger L2 firefighting systems becoming standard on your average fire truck real soon now.