Ok, Ok - I'll admit that I have done an operation that has somewhat restored electricity holding power and for a relatively short time elongated the life in a few wet-cell truck batts.
By, taking my big charger and hitting the batt with high voltage start-amps repetitively.
First leave start-amps on for about 30 seconds to get batt plates to wake up (i.e. good n' hot)! Then switch it off for about a minute. Then hit it with about 30 consecutive jolts of start-amps for about 5 seconds each with 10 to 15 second rest times between each 5 second start-amp surge. BUT
- be careful - the old batt has potential to explode while doing this operation. I hook it up to charger in gravel field where fire is not possible and stand some 50 or more feet away to plug/unplug the extension cord.
In 1960's I was instructed to do this by an old gas station mechanic/owner. I've found from doing so that some (but not all) virtually dead batts can still become usable for less than a year. Is it worth the trouble... well yes, maybe, in an easy to access Batt area of a road vehicle... if you feel like trying your best to keep a worn out ol' batt alive for a while longer. Otherwise - bite the bullet and purchase new.
As far as marine batts - NO - I will not bother doing this at all! Way too much trouble hauling batts out, taking to safe place to perform start-amp surge process and then to put back into boat... for limited time before the batt says F-it and really quits for good. IMO, much easier, safer, and more pleasing to simply purchase and install new wet-cell marine batts that can last up to 10 + years if charged and otherwise maintained correctly!
Happy Batt-Doings daze! - Art