I just found this here thread and thought I would give my 0.02 cents worth. Last summer I was toping up the fluid level in my TT battery, and I noticed the date code of the battery in my brand new TT showed it to be already 3 years old. The age of the battery and my thoughts about the dealer are another matter which I won’t go into here. What I find in common with the OP is the battery condition I see under observation this winter. In my case, testing with a hydrometer shows 3 cells on the edge of the red zone, 2 cells in the middle of the white zone, and 1 cell in the green zone. My estimation from this data is the battery has only about 60% of its specified reserve capacity. The low hydrometer reading is due to sulfation of the plates inside. Here is where I see the parallel to the OP……. when I charge this battery, the one good cell boils profusely, and the others hardly boil or not at all. My smart charger thinks the battery needs more charging, so that one cell very quickly drops in fluid level, and the others don’t. I would guess the OP had a couple of cells with sulfate deposits on the plates, and the good ones quickly boiled off to being dry. My plan to recover my own battery is to use a product called Battery Equaliser. Have a look at this link……
I applied the Battery Equaliser additive today, so time will tell if the Hydrometer readings will recover to normal levels. Batteries with hydrometer readings below 1.150 are apparently beyond hope, according to the maker of this additive.
Sulfation is where the sulfur content of the acid electrolyte fluid drops out and is deposited on the plates. This acts as an insulator on the plates and lowers the strength of the acid. This condition will be accelerated if the battery is left idle for a few weeks or months. The rate depends on the ambient temperature, with hotter equaling faster.