(mods please sticky??)
After reading some rather scary posts about people putting lead acid batteries in the cabins of their vehicles, i decided to do this write up.
A lead-acid battery uses a combination of lead plates or grids and an electrolyte made of a diluted sulphuric acid to convert electrical energy into potential chemical energy and back again. The electrolyte of lead-acid batteries is hazardous to your health and may produce burns and other permanent damage if you come into contact with it. Right off the bat you know that, if you've ever seen under and old car's hod, an old battery that is corroded is bad for you. Sealed gel batteries don't corrode like this:
When lead acid batteries (which use the sulfuric acid) are charged they produce hydrogen gas and acidic aerosols. This leaves us with two seperate problems. For one, it is poisonous, and if it's inside the cabin of the vehicle, and the gases builds up, it could be damaging to your health.
There is another, bigger problem, though. When hydrogen and oxygen build up inside the cabin, the chances of an explosion increase tremendously. Concentrated oxidants AND fuels are in close proximity to one another which is what causes the hazard (thanks wettnudel's). When it is on the exterior of the vehicle, it's not too big of a deal, because the gases get a chance to escape before they can build up and pose any sort of threat. That's why hooking a set of jumper cables directly to the battery of a car that's jumping another car is a bad idea.
That's also why you don't hook up the dead battery first and hook the leads directly to the charging battery. This can cause the batteries to spark and ignite the toxic gas. Imagine what would happen if the acidic lead battery were in the trunk of your car or rear hatch of your suv.
Then acid will fly all over, which is very bad for your skin, and can cause health problems. Plus, there's an exploding battery, you know nothing good can come from that.
That's why sealed gel batteries or AGM batteries:
Are highly recommended if there has to be batteries in the cabin of the vehicle. They cannot spill, can be mounted in more positions, and don't corrode. There is little chance of a hydrogen gas explosion when using these batteries; these are basically the safest batteries you can use.
So next time you go battery shopping, and want something that will be used inside your vehicle, make sure it is NOT lead acid.