Battery alchemy?
I still believe in carbon piles PERIOD
Unless the battery has been spilled you should not need to add acid. So far as I know only time you add acid is with a healthy battery, a minor amount to correct some imbalance which will usually be all the cells.
I haven't tried, but I don't believe in the epson salts/ other "rejuvination" stuff. You likely got one bad cell, and it bubbled / gassed the acid away, and it is done "I bet."