Got my new (used) batteries!! Now, how do I rejuvenate them?

Weather is no longer boiling hot, so today I will resume battery project.

I am a little concerned because, going over the thread, I see you guys told me repeatedly that voltage slump should be the same for all the batteries and I should not need to keep twiddling the pot, but that is not my experience. I have to do it anew for each battery. Does this mean something?