Hi Marvin Most gel cells are damaged from overcharging and not sulfating( no more water inside ). Most cases of sulfating can be recovered with a slow, low current charge. You need to have just enough current to overcome the normal leakage but not enough to generate excess gas. I've not seen, personally, enough difference between these types of battery rejuvenators and just using a trickle current to justify there purchase. About 5 years ago, we took two batteries that did not recover after a trickle charge. Neither of the two recovered after using a device, similar to the one described with high frequency pulses. All the other batteries recovered. These were auto sized and we used a current of 200ma. Dwight