Q-4: "Several years ago I saw an article that gave tips on how to rejuvenate old Ni-Cd cells," recalled one reader. "I can't find that article today. Is there a technique that you are aware of that would bring old batteries back to functionality?"
A-4: I recall the article but didn't bother trying to look it up. The following answer reflects my thinking on this subject. When a pack goes down (indicates lower-than-normal rated capacity), it is usually because one or two cells in a multiple-cell pack is/are starting to go bad. So the first problem is to identify the bad cells. To open a pack and search for the bad cells is a time-consuming process. After they are finally identified, you either substitute new cells or, as you suggested, try a technique to rejuvenate them.
There might be some tricks to revitalize those cells, but I wonder how long the process will last. When you are finished with any such process, you may still have a battery pack with a variety of cell characteristics. The bottom line: Do you really think it is worth spending the time? When a pack starts to lose capacity noticeably, I prefer to discard it in favor of a new pack. When I consider the cost of a new one compared to the time it takes to repair an old one, I'd rather be out flying with that new pack.