Regeneration puts the battery through a sequence of deep discharges and controlled recharges, sometimes with superimposed current pulses, in order to dissolve the lead sulphate crystals formed on the plates and return the active material to the reaction.
It works when the capacity loss is due to sulfation and the plates retain their mechanical integrity. It does not work — and this is worth saying plainly — where there are shorted plates, shed active material, grid corrosion or lost electrolyte: no process recovers anything there.
The decision criterion is a prior capacity test: if the bank delivers above 50-60 % of rating, regeneration usually recovers an appreciable part; below that, replacement is the sensible route. A second capacity test at the end is what measures whether the process achieved anything.