Battery overcharging.

[Andy Bennett]

And of course that means battery acid is squirting/bubbling out of the fill caps. I had that once with a problem charge and acid literally squirted up about 18 inches through the small air hole in the cap. Pure luck I wasn't looking down on it at the time otherwise I would have had a face/eye full. So, apologies if stating the obvious but whatever you do don't get close when it's charging 12A.
Andy