Don't spark around that boiling battery. I had a bit of an explosion a couple years ago. Blew the top right off the battery, thankfully that's when we had to wear those face diapers and it absorbed most of the acid. I was trying to charge some battery's quickly and didn't turn off or unplug the charger before unhooking the clamps at the battery. It sparked and **** hit the ceiling. In almost 30 years of charging all sorts of 6-12 volt batteries and knowing/hearing about what could happen, I finally saw it first hand. If you overcharge a battery enough that it starts to stink gtf outta there and kill the power to the charger. That's far from my proudest moment but don't want to see someone blow their face off doing something like that.