I've heard of draining the battery, then adding an epsom salt solution to break down the sulfation. That gets drained and rinsed, then new (or reconditioned) acid gets poured back in. I've brought unchargeable stuff back to life just by adding distilled water, but none of those batteries was ever very good after that.
A hydrometer used to be a normal mechanics tool. You could pop the caps off, measure the electrolyte in each cell and top them off. Then most car batteries became "maintenance free", but I'm not sure how different the construction is.