OK, some observations by me.
I will pretty much see champagne sized bubbles, albeit, very very small volume of them, coming to the surface on my T-1275 by the time it will only a 7 or 8 amp charge rate from the Mega Watt S35-12, when set at 15.0v. I have never seen the T-1275 have much of anything resembling a roily boil.
I have seen a much higher volume of champagne bubbles on my old manual Sear 12V 6 amp charger when doing an equalization charge, mainly because it will put out 16.0V, and the battery will take it, but only for a short while before the voltage starts dropping off back to 15.7 or 15.6V.
Someone should do a video of batteries, and bubbling observations based on state of charge. A picture or a video from an expert is worth a thousand words.
I know Mex Wanderer now has a video camera feature on his smart phone. All he'd need to do is host it somewhere like YouTube after setting up a free account, or on PhotoBucket.