I bow to your experience with AGM batteries and chargers on a boat. The technology (AGM) in my industry does not have the life expected, even with carefully controlled environmental and charging conditions. (Although, none of the VRLA battery technology today has the life expectancy of good wet cells. I have seen studies that “20 year” batteries give 5 years in a controlled environment if you’re lucky.)
My figure of 1.75V/cell (10.5V on a 12Vstring) is for a battery under constant load, defined typically as a minimum of 1A for 100AH of battery. I use a low voltage disconnect to make sure my batteries do not fall below this level (actually I have the disconnect level set for 1.8V/cell or 10.8V to give me reserve time to crank up the engine to charge the batteries).
If you remove the load at the 10.5V level, batteries in good condition will drift up to 12.24V.
I agree with your advice to Michael, everyone needs to understand the operating conditions of the electrical system in order to diagnose any potential problems. Correct me if I’m wrong, but doesn’t the Link 2000 have all of the information needed to diagnose the system, average current draw, peak current draw, capacity, etc? I’ve not used one and am considering putting the Link 1000 in my present boat.
Michael, more information of this type will be needed in order to give you a meaningful answer. A data dump from your instrumentation will definitely help in addition to the answers of the questions from Camaraderie.