What fire extinguisher

[Finnster]

How small do they even make CO2 ext? I would think a 10 or 20lb ext would be the min size.

Another concern is you need to get quite close w/ a CO2. An abc is more of a stream w/ better distance. Face it, if you have a battery fire in your car, you'll want to put it out regardless to some cosmetic damage to some Alu parts. They can likely be cleaned up and polished well enough w/ timely care. Much of the car is going to be pretty fried regardless.