If I recall the “best” way to revive nickel based cells is a slow discharge to around .4 volts then a charge and repeat until needed. More details outlined at Battery University here. I’ve done it on a number of nicads, some come back pretty good, some not so much. I want to try that high discharge trick, it seems if the purpose of refreshing is breaking up the large crystals that might help. Zapping is the process of burning off whisker shorts that pierce the separator. While it works on batteries that have a high self discharge those whiskers often reform in the same holes so the effect doesn’t last super well, I wouldn’t do it unless the self discharge is high it could do more damage than good on cells that have low capacity (large crystals) but no shorts (whiskers). Also every cell I’ve zapped (with a welder) didn’t show any improvement.