I have 3 batteries, 1 starter and 2 service. But they are in the same compartment and seperated from everything else.
I've seen some accidents with exploding lead acid batteries, so I'm kinda paranoid about safety. There are also many fuses close to the batteries to minimize the danger of a cable-fire. And never put anything metallic in there - a good battery can deliver 500+A ;)
Here is an example of a ship battery, which did explode:
The owner was lucky: He still has his eyesight, only slight chemical burns of the lungs and he had to throw up because of the fumes from the sulfuric acid ...