A desulfator worked for me
I had a couple of good (10 year old) Trojan deep cycle batteries that were showing about a 50% capacity when I put a cheapo "voltage pulser" on them. They got up to about 60% capacity, and I got two more years of life out of them. By then, I just wanted to have batteries that had good capacity. Everything I've seen about this technology tells me it works to some degree. (It's not at all like trickle or fast or any kind of battery charging.) But you can't restore a shorted or otherwise dead battery.