"plates sulfate, capacity diminishes" Sulfation occurs in discharged batteries left over time, true.

If I leave my fully charged lead acid batteries in my boat in an average temperature of 41 degrees, after 18(a year and a half!) months, they will have self-discharged to 80%. So in fact on my 7 months storage, I should see about a 10% drop to 90% charged, which is what I saw in the newest battery. The oldest was down closer to 80%.

If you're in a hotter climate, lead acid batteries will self discharge at a faster rate. If I was stored in Florida, it might only take a year for my batteries to self discharge 20%.

I used to run a cord and charge mid winter. I'm concerned leaving electrical appliances energized in a stored-un attended boat, so I stopped doing that once I found out, I was wasting my time.

Rolls battery says it will take 250 days to self discharge a fully charged lead acid battery to a specific gravity of 1.190., which could freeze at -12F. I'm not worried about that.

For your AGM's. I haven't a clue. Charge them! They're expensive. But lead acid? You're wasting your time. I get 4 or 5 years-average, out of -$100 buck G27', no matter what I do.

My batteries rough service is in sailing season when they'll live for months slightly discharged(sulfation). I'm not good to them then, rarely bringing them up to full charge. But I've killed more batteries over the years, by overcharging.

I take the best care of my lead acids in winter. Charge 'em in the fall, and forgitaboutit.