Although the main point of the discussion is the mechanics of ventilation, and the potential problems of water, the sheer weight of a car battery should be thought about. Back in the days of Super-8 cine cameras I helped someone who wanted to make a film of the Surrey mines. The only practical lighting he could come up with then was a set of motor halogen bulbs run off a 12V car battery. The mines are basically horizontal dry passages, and on most trips, carrying the battery around was a challenge. We had it in a sturdy canvas bag, in which it fitted snugly. The strap was long enough to pass over a shoulder. In crawls we usually pushed and pulled it along the floor, which is smooth, with few protusions to cause problems. Sometime we had the services of the late Chris Sayle, who was built like an ox, and just hung the bag around his neck as he crawled along on hands and knees!
However, when we wanted to film in a remote section, which under normal circumstances would take a couple of hours to reach, we took the battery in a few days earlier, so we could get there quickly when we were filming.
I don't think we would have used a car battery if we had been taking it into somewhere more cave-like or vertical. The risk of dropping the thing and smashing the casing is one thing, and the sheer inconvenience of dragging it through awkward places might have put us off.
Nowadays I think there might be more convenient ways of getting 12V into a remote spot. Depending on the drain on the system, perhaps smaller 12V batteries might do the job. My lads little electric play car uses sealed 12V (I think) batteries which you could carry about far more easily than a car battery.
