In http://www.straightdope.com/mailbag/mbattcharge.html, SDSTAFF Ken says:
I beg to differ. While a defective car battery can get very hot, indeed hot enough to start a fire and/or explosion, this is not why it was (and still sometimes is) necessary to add water to car batteries or other lead-acid batteries. Indeed, while a properly operating and installed car battery might get warmer from use, it shouldn’t get hot enough to “boil” the acid. The “boiling” that commonly occurs in lead-acid batteries is due to electrolysis, not heat. Charging a lead-acid battery involves reversing the chemical reaction that creates the electricity in the first place, by pumping electric current back into the battery. There is a limit to how much energy can be stored in this fashion. Once the battery is fully charged, pumping more juice into it causes the water in the battery acid to break down into hydrogen and oxygen gas, a process known as electrolysis, which then bubble out of solution and float away into the atmosphere. (I did this as a school science experiment in 8th grade, and I bet many of you did too. The reverse reaction is the basis for modern fuel cell technology.) So batteries “boil” when they are over-charged, but not because they are hot. I’ve often observed lead-acid batteries “boiling” while being overcharged. They are typically quite warm, but you can still touch them with your bare hands. The gas bubbling out is a corrosive and explosive hydrogen/oxygen mixture, not water vapor, which is why you don’t want to charge lead-acid batteries in a sealed compartment.
The reason you can get away with using modern, sealed car batteries is because charging systems are so much better than they used to be, so that over-charging is not much of a problem in today’s cars. A good voltage regulator turns off the charging current when the battery is fully charged. Modern “sealed” car batteries have pressure relief valves that are supposed to let the explosive hydrogen/oxygen gas mixture out if overcharging does occur due to a defect in the charging system. But of course the battery is then ruined because you can’t put any water back in.
By the way, a “boiling” battery is just “un-burning” water. See Cecil’s classic column Water contains hydrogen and oxygen. Why doesn’t it burn?. You have to add water to the battery afterwards to replace that which was “un-burnt”.