Talking large banks of expensive true-deep-cycle cells, not the stuff you see in mass retail channels. Maybe the 6v golf cart batts from Sam's, at the "value" low end.
I think end-of-life ratings are based on the battery losing 20% AH capacity, and 500 50% discharges would be optimistic in real life, even for top-shelf true marine grade AGM like Odyssey or Lifeline.
Premise: getting such Lead chemistry banks up to ** true 100% full *** SoC as often as possible, is necessary for them to last even close to their rated lifespans. This cannot practically be done from fossil fuel engine-based charging, except for commercial trucking or ships motoring along most of every day.
Particularly true for AGM, but FLA as well.
In regular usage, I've seen recommendations like "at least once a week" to "every 3-4 partial charge cycles".
Assume daily cycling use, SoC always kept > 50%, no solar nor shore power available for weeks at a time, and regular but short-session high voltage (2-3 hrs, 200A @ 14.4V+, assume that's optimal) bulk charging a few times a week from engine generation.
The goal is to minimize starting up the engine(s) for the sole purpose of charging the banks, and certainly not running them for many hours just for low-amp floating purposes.
My question is: How close would an alternative source ** at 13.6V input current ** get to that "100%" level, assuming huge AH capacity available and hooked up say for 12-hour sessions?
Would the answer "X" be significantly different for FLA vs AGM chemistry?
Say X = "98-99.5% SoC", would it still be necessary to regularly supplement these 13.6 floats with long sessions of higher-voltage floating?
If so, what sort of pattern would be the minimum to ensure longevity from this factor. For example assuming daily cycling from "X" down to 60% SoC, maybe 50% once a week, then daily bulk(CC) + absorption (CV) recharge for 2-3 hours at 14.4V from fossil fuel.
Then, every day additional 8 hours of float at 13.6V, all with no loads.
How often, if at all, would I need to do additional many-hours floating at the higher voltage?
If this extra step will only get me from say 400 cycle lifetime to 420, it's likely not cost-effective to buy an expensive DC-DC charger to boost the long-float voltage up to 14.4V.
But if it's a difference of 100 cycles, it would be worth it.