I marked this as 'no Guzzi content', but enough people use AGM batteries that I thought there may be some value here.
The short version:
I have an AGM battery on a solar charger. The battery generally sits fully charged, and the charger just generally tops it off every day at a 13.6V float charge for a couple hours. It has been like that for a couple years. A few months back the battery just crapped out. In the end, playing with the battery, and putting it on a high amperage power supply at 14.7V DAYS and DAYS, appears to have restored it.
The long version:
I used to tow a camper behind my EV. I kept an Odyssey AGM battery in the camper, that charged from the EV. I would use it to charge devices at night. The battery matched the one in the EV, so if traveling and the EV battery failed, I had a backup.
When not camping, I put that battery on my back porch, to power a laptop and other devices. I have a 30 watt solar panel to keep it charged ((barely over 2 amps available on a good day). And since my hobby is gadgets, I designed a PWM charge controller using an Arduino.
If the charge controller measured less than about 12.5V in the morning, it would go into the full bulk and saturation charge at 14.7V, then drop to 13.6V for a float charge when the current dropped. All as defined by Odyssey. But, in the morning when the sun came up, if it detected that the battery was already over 12.5V, and it ended the previous day on a float charge, it would just go straight to the 13.6V float charge.
All was well for a few years, then suddenly the battery voltage dropped below 12V, and every day even after a charge it would just drop below 12V quickly. I started by popping off the caps and putting a couple of drops of water in each cell, thinking the had dried out. You aren't ever supposed to do that, but I was experimenting. I used my lab supply to charge it at 14.7V for a while and it appeared to come back to life. A load tester showed it could provide good cranking amps. So back to the back porch and the solar charger.
That worked great.....for a few days....
So back to the drawing board. I'm now pretty certain that the battery lost charge on one cell. It simply became imbalanced. So my solar setup was fully charging 5 of the 6 cells, and that is why I had close to 10.5V, instead of the normal 12.7V, at rest. So I connected my lab supply at 14.7V and I left it there. For days and days and days. It has been working perfectly now for over a month since doing that. It provides a huge CCA according to my cheap load tester. My solar charger is still programmed the same, as in, if the battery voltage is good in the morning, it goes straight to a 13.6V float charge. So if/when it fails again, I will change the charge profile to go to 14.7 no matter what, every day. And I'll see if that restores it.
Maybe 100% crock of crap, but I'm thinking that if you are charging an AGM battery long term like that, you need to occasionally hold a high charge voltage for a while to balance the cells. I know old flooded wet cells are supposed to occasionally have an 'equalization' charge to balance them and stir the acid. But you are NOT supposed to do that to AGM batteries as it vents them and dries them out.
