[ChevLoRay]

The comment about the hydrogen gas is on the right path. When a lead-acid battery is in a discharged state, it will give off hydrogen-sulfide gas, which will remind you of a rotten egg. It is that gas which is dangerous.

In my former life, I supervised folks who drove 48-volt electric fork lift trucks. When the battery was closer to being discharged, it would heat up. At the same time, charging them will heat them up. But, we didn't have to connect jumper cables to the pos/neg posts on those batteries.

I believe that the correct way to connect jumper cables is to connect the positive and negative ends to the discharged battery, first. Then, connect the positive cable to the hot battery, and finally the negative cable to the hot battery. Some all-knowing electical gurus have told us how to do that, but we don't always follow that rule, do we?

I can tell you that a 48-volt battery that explodes will create a lot more damage than a 12volt battery....in case you wondered.....oh, they (48V)can kill a person if they are close enough.