Alright thanks for the explanation. I was charging an older battery and about an hour in I was getting a bubble here and there out of 2 or 3 cells and I realized that I didn't quite know what I was looking for. I figured that all cells needed to bubble but then the question popped in my head "well what constitutes bubbling"
I would categorize my fish tank aerator more as a boil then a bubble but then again its not a small pump. I get what your saying MexicoWanderer.
So can a accurate hydrometer reading be taken while the battery is under charge?