I had some dead 12v 12Ah batteries from an APC smart ups 1000 that were quite old (md 2009) and very dry. I have replaced them with a set of new 18Ah batteries. I thought I would try my hand at getting these old batteries going again.
One battery was showing ~12 volts but could draw no current what so ever and the other was shown only ~10 volts
charging at 15.5 in a desulfication phase