When is the best time to regenerate the batteries?

The best regeneration time of used battery will be that the total battery capacity should be remained for 70%~50% after a full charge.  And 40~50% at least after a full charge.  Generally, battery cannot be used up to 0%. (Maximum Use : Car battery -  up to SoC 40%, Other types - up to SoC 20%)   If the battery charging state is lower than 50% (For Car battery) ~ 30% (For Deep Cycle), the plates can be severely damaged and pastes of plate’s activation material can be broken away from the plates.  In this case, please note that the regeneration will be almost impossible.
The PRIME regenerator removes sulfation to raise the gravity of electrolyte and activates sulfation to restore battery life span and capacity like new battery conditions.