Maine;
You say that an AGM bank requires a "minimum" charge output of 40% battery capacity. Why? What does the battery bank care how fast it is restored? Certainly is great that the AGM can take a monster charge, but not a necessity. Getting to that kind of charge capacity for a 400Ah battery would require two big shore chargers (and the the relevant installation hassles). Or are you referring to a boat on moor relying on an alternator?
Odyssey batteries want to see a minimum 40% of Ah capacity for
optimal cycle life but 30-35% will be better than 20% etc.... Lifeline wants to see a minimum of 20% of Ah capacity but more is better. Neither company is willing quantify what the impact is but it can be comparable to DOD in effect on cycle life..
Odyssey Battery Technical Manual:
"Note the charger current in the bulk charge mode must be 0.4C or more."
The engineers at Odyssey/EnerSys don't mince words and actually use the word "must"..
The higher the charge current the better cycle life you get with AGM. Dave V., the head engineer at Lifeline, presented this data back in the 90's after conducting some studies on military batteries. Both Lifeline and Odyssey have minimum charge current guidance because they have studied it at length. I have had a number of conversations with Kalyan J. of EnerSys and Dave V. of Lifeline and both are fully in the "
the more current the better" camp....
I am doing some testing for a new type of AGM that will be out pretty soon and Kurt, the head engineer, says the same thing. The PSOC testing I am conducting,
for real world type of marine use, is being conducted at .46C. They initially wanted me to do these PSOC tests at 1C but that is simply not realistic and we want to have realistic data not lab type data. Even .46C is a stretch for most boats.
The sad reality is that many folks completely ignore the technical manuals, and instructions for charging, then get upset when a $1500.00 bank of batteries is dead in three years.
The basic take away of the study is from the abstract:
"The results show that cycle life is strongly affected by the rate of charge, as well as the depth of discharge."
Can you charge at lower rates? Yes, of course, but it
will dig into cycle life. AGM batteries are a puzzle and all the pieces really need to work in concert to yield the best cycle life to dollar ratio...