You may have a bad one then.
I know mine has brought basically useless batteries back to usable life. I am not certain about that voltage change though. It seems to me that I keep desulphating for days and the voltage does not rise, but then when I charge the battery it does hold the charge far better than before. Like some batteries totally died in 24hrs sitting there, then I run desulphate mode for several days or a week on them, still dead but after charging they stay charged. It has been awhile since I used it that way, I was getting junk batteries from a recycler and doing this.
It does seem strange to me it worked without charging, since the sulphates on the plates are the problem and caused by discharge normally, and disolveing the sulphates back off the plates and into the water is normally what happens durring charging.
The problem is when the sulphates won't dislove back off the plates.
So I don't really know what is going on in there at the time, maybe it is just softening the sulphates some how so that charging removes them as it should, but not actaully putting them back into the acid like charging does?
Anyway, I'll see tommorow/today what happens to 2 trojans. Just checked them, they were sitting unused since before I went to Canada (about 8 weeks at least) and I don't know what state of charge they were before I left. But I checked then with a meter and it showed the pair at only 11.5V and when I put the charger on 40amp setting it dropped back to 15amps charging and kept going down quick to about 12amp so I stuck it into desulphate mode for the night.
Another set about the same size was showing 12.45V and was taking a 42amp charge in the 40 amp mode so my charger was working correctly. These are about 360amphr trojans each.
Got them at scrap prices from a recycler, 10 batteries for about $50, 6 came back good and I got alot of use from them. 2 of those died in my truck on a long trip, not sure what happened to them, but I still got 4 great ones, 2 maybes, 4 junk I still try things on. Not bad for $50 investment really. I love junk batteries and desulphating
I've got a bunch of other types also.