How much capacity loss per year with an average (Lifeline advertising as being the "best" notwithstanding) AGM battery can one expect if they've treated it "perfectly"?
Our style of camping is a long way from full-timing. This means our AGM coach batteries get brought up to, and held at, 100% SOC most of the time here at home between camping trips. Between camping trips our stock converter takes a "long time" to bring them to 100%, but we have a long time. When camping it's for only 2-3 days at any one campsite, so the AGM batteries get plenty of 3-6 hour-long exposures to the Ford alternator plus some generator/converter time at the campsite. This means they are getting brought up to around 100% for each camp site change.
So I'm guilty of being one of the folks who say "They have lasted 8 years come this November". Am I kidding myself that they are still any good? I guess I'll have to break down and load em' up with a 20 amp draw-down rate and see if they still make it to five hours with their at-rest terminal voltage dropping down no lower than around 12.1 volts (it's a 200 AH AGM battery bank).