MYSTERY SOLVED: They are overvolting your batteries! 15V is too high! It should be max 14V with 13.5 to 13.8V nominal! Whether those devices are meant to desulfate or not, they are overcharging your batteries which is shortening their life!
Yes and no
. Yes, charging to over 14v for an extended period of time will cook a battery. No, it's not a limit that should never be exceeded. Step 1 is to read the datasheet or data plate on the battery, there is too many plate compositions, pure lead, antimony or calcium added, etc. to make a general statement that covers all battery types, So a suggested float and cyclic charge voltage printed on the battery is a good bet. But for a generalization with many weasel words, 13.6-14.0v is a typical float voltage for continuous/standby charging. No overcharging at all, but if you only ever charge to 13.8, you're going to have sulfation build up as there is no overcharging (not sure if "overpotential" would be a better word here?) To deal with the sulfation. A 14.0-14.5v (depending on the plate material, read the datasheet/dataplate! ) as used on a cyclic charge will take care of normal very mild sulfation formed during use (or while sitting not fully charged for a short period of time), and to fully top off the battery. A 15v charge is more aggressive overcharging, will take care of more sulfation than a 14.5v charge. The overcharging will make the cells gas, and it also equalizes a battery by letting undercharged cells get a chance to take a full charge. This should only be done every few months on cells that are refillable, or have a recombination catalyst. I.e. Some of the hawker cyclon cylindrical cells that are sealed and can take significant pressure. But not done on AGM unless the manufacturer reccomends it, and pretty much never on gel cells.
You have to use the right tool for the job. A 15v battery equalizing/desulfating charger isn't for daily use. A battery tender meant to live on a battery, and just float it at 13.8v isn't for charging a battery subject to cyclic use and storage.
The problem is that lead acid is very easy to charge, very hard to charge well. Plus given the typical failure mode is fairly safe if overcharging isn't severe, chargers are usually pretty far from the "very well" area.
I've seen an automotive starter battery last 9 years (replaced the car, not the battery! ) because GM (shockingly! ) made the alternator charge the battery to 14.5v after starting, and then float at 13.8v afterwards, so it wasn't cooking the battery the entire time the engine was on.
Xantrex even sells alternator regulators that do the same thing to convert the alternator from a battery cooker to a proper charger.
Know your tools and when to use them, and everything is happy
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