Do you have a source for that, I'd love to read it? I have numerous AGM's sitting in my test room all still hoovering above 13V and many of them have been off charge for 4+ weeks. I have found the self discharge of AGM slower than flooded but I have also not seen flooded batteries self discharge at anywhere near the claimed rate the AGM makers suggest they do...
What you are seeing is surface charge on the plates. A small resistive load for a short period will peel that surface charge (SoC) off and give you a better reading of state-of-charge, however, battery voltage is a poor indicator of SoC. Sulfation of the plates due to inactivity creates a high resistance at the surface of the plates, thereby the high voltage. How much of a load to apply is moot, as long as you do the same thing every time. The loss from the load can be calculated, but should really be minor.
Using deep cycle batteries for this application of starting a sailboat engine has way more benefits than drawbacks. No, its not the best, but it won't hurt the batteries. I prefer flooded (wet) batteries to AGM due to cost, but cost comes at a price, you must water them and equalize them occasionally. AGM and wet batteries charge at different voltages and not applying the correct voltage will ruin your batteries.
Don't over-think it. Charge them, discharge them. Keep them charged when not using them. Deep cycle batteries like to be used.
On a side note-someone was talking about breaking in batteries, they were entirely correct. Lead Peroxide turns to lead sulfate crystals on discharge. These crystals are embedded in the plates. How deep you discharge determines how embedded the lead sulfate crystals reach. This exposes more surface area to the electrolyte and more amp-hour capacity. So an initial deep cycle or two is good, as long as they are fully recharged between cycles. Everything always comes at a price. A lead-acid battery is a consumable item, always trying to reach back to its equilibrium (discharged). Cycling the battery causing sloughing of the plates, can't be avoided, which reduces life. There is a balance, less discharge equals longer life but means bigger battery bank. Deeper discharge (using the battery the way it was meant to be used) means shorter life due to loss of lead.
Never leave your battery in a discharged state, as those lead-sulfate crystals get hard, and they are hard to return to sulfuric acid solution... thereby creating a "sulfated battery" with high internal resistance, that shows voltage because of surface charge but can't deliver any current.
The biggest offender of sulfation is not leaving a trickle charge on your batteries (but be sure to water them if they are wet). Morningstar (sunsaver) has an excellent PWM controller that allows charging two separate banks, select battery chemistry (AGM or wet), allowing 90% to one set (house) and 10% to the starting battery (usually less capacity).
Thomas Edison had a quote that went something like "...batteries bring out the inability of sales people to tell the truth..." That is me paraphrasing.....
As for references, this is the "bible" on VRLA batteries. I don't have the second edition, just the first. However, it is VERY heavy into VRLA electrochemistry.
Battery Technology Handbook (Electrical and Computer Engineering) Hardcover – August 29, 2003, by H.A. Kiehne (Author)
ISBN-13: 978-0824742492 ISBN-10: 0824742494 Edition: 2nd
This would be what I would consider the bible on Lead-Acid. While lead acid technologies have evolved, if you read this, you will understand 95% of all lead-acid wet battery technology. The newer stuff is like adding carbon to make batteries into "Ultra-capacitors", etc - those are more exotic....
Storage Batteries: A General Treatise on the Physics and Chemistry of Secondary Batteries and Their Engineering Applications Hardcover – December, 1955, by George W. Vinal (Author)
