If memory serves me correctly, don't count on it, but I seem to recall that lead acid batteries could be desulphated by a relatively short high current charge being put into them. essentially is shocked the sulphur off the plates. That said, I don't believe that approach can be applied to SLA batteries because it would boil the acid out of the battery and could not be replaced, where as the old lead acids could and often did need, water added to the cells to replace what had been lost. I can't say that I ever attempted this technique, not successfully anyhow, but I least a did not blow up a battery, which IS a serious potential result.