Dead battery bank club

Well, I joined it.

3 years ago, I added a Deka AGM Group 31, perko switch and voltage sensing relay as an initial stereo battery for ~800w rms total draw stereo, and everything was good.

2 years ago, I added a second Deka 31 AGM, and a 3-bank Noco Genius onboard charger for longer runtimes and future proofing for amp upgrades.

1 year ago, I pulled out one of the Group 31's that wouldn't hold a charge (No warranty help from the dealer...), and replaced my trusty Deka 24M starter battery with a new Exide AGM as it was starting to give slow cranking.

This year, the other Deka 31 AGM won't hold a charge either. FML.

My maintenance routine isn't anything weird (plug in the onboard...), my wiring design is simple (parallel stereo bank, ASR+Perko separating starter battery/alternator), and while I don't trickle charge all winter (boat is stored remotely), I top up the batteries with the onboard mid-winter. Anybody have thoughts on why this is happening? Could I have a passive draw somewhere that is just sucking them down until they sulfate?

I might go back to regular lead-acids since it costs so much to replace batteries. Damn....

2 Trojan T-105 6v, Interstate starting battery, and a Pro-Mariner 50amp onboard charger, and a Perko is what I use.