by postfach » Tue Nov 06, 2007 8:58 pm
Lead acid batteries (what most people have) give off hydrogen gas, which is flammable, generally this is not an issue, but if you have the battery in an enclosed space the gas will build up, and could possibly ignite, so when you relocate the battery to the inside of the cabin, it has to be in a sealed box with a vent going to the outside of the car (not sure if that is just for motorsport since i know r33 skylines have the battery behind the back seat).
Sealed gel type batteries don't give off any gasses so don't have to be in a sealed box.