Just for giggles I went and took a photo of those two boat batteries and checked their voltages. I have them in a plastic tub in case one would explode or something.
The inverters were pulling 82 amps at the time I took the photos and the wind turbines were putting out 27 amps. So the net load on the bank was 55 amps. Theoretically each battery in the bank (26 batteries including these two) should be delivering 4.2 amps. I don't know if this pair is or not. I suspect not because I got them hooked up with a piece of 10 gauge extension cord with a 30 amp fuse on it. Being these are way smaller their voltage will drop quicker than the big ones under load, so they don't put out as much amps as the big ones, I don't think.
But even so, this is where they always seem to run - about .04 volts difference from one another.
The first six years of their life they get worked pretty hard in the boat all summer. The boat has a built-in Minnkota dual channel battery charger so they always get recharged right away when the fishing is done. The Minnkota unit absorbs them at 14.4 volts but it has no temperature compensation on it. For summer time temps, that's probably a good voltage for them.
When I put them on my bank in years past, we had a 12 volt system back then. I used to absorb at 14.5 and equalize at 15.0 so they endured 5 years of that during the winter, many times being worked harder than in the boat.
If I end up getting 8-10 years out of these cheap $65 batteries then I'm going to conclude that the early demise of most deep cycle batteries is caused by inactivity and/or being undercharged. The last thing that's happening to these poor old things is being underchanged harnessed up with my Surrettes, at the voltages I run those at. I absorb the Surrettes at 30.0 and equalize them at 32.0 (temperature compensated).
So lets consider it The Experiment. LOL! As of right now, these old batteries have endured unbelievable pounding in the boat making fast runs on rough water, get pulled down to 50% or below on average 3 times a week during fishing season for six years, have been worked on my battery bank every winter since new, and they still appear to me to be pretty dang healthy.
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Chris