Alright, now I'm super annoyed. I tried to find a manual or decent spec sheet on my X2 Power AGM, in order to find the 100% charge voltage, but only found
this. That said, according to this part of the manual, the 100% charge voltage should read above 12.6V:
View attachment 91033
Maybe this is truly only 80% charged at 12.6V? What gives?!
I should've stayed in EE instead of switching to finance. Math is hard.
Those X2 batteries are made by NorthStar (which is owned by EnerSys same as Odyssey and is the main competitor to Johnson Controls/Optima). They make a quality product so don't get too overly wrapped around the axle. Just know that in my experience EnerSys AGM batteries are just more finicky.
Anywho, 12.6V is IMO hard to call good or bad, it's kind of the meh, I dunno number. If a battery was brand new, temperature was a perfect 77°F and the battery had just sit for a day after a charge and measured 12.6V that would unquestionably indicate bad.
BTW the rest is important. You need to let a newly charged battery sit for about a day to let what's called the surface charge to dissipate to get an accurate measurement. It would require even more esoteric discussion than I've already dumped but suffice to say voltage isn't the best indicator for condition, state or charging. It's just one number out of context.
OTOH a few year old battery that's not been handled with white gloves just randomly measuring 12.6V is actually typical and doesn't tell you anything definitely. I wouldn't replace it anyway. Batteries age and won't measure an ideal 13.0V open circuit forever no matter how good you treat them.
In any case knowing what the battery voltage drops to while cranking (or alternatively with the fridge or radio on, basically under load) is the more important number. Comparing a battery OCV of 13.0V that drops to 10.0V cranking to another that sits OCV of 12.6V but only drops to 10.5V cranking the second is in better shape and just needed a charge.
Just my $0.02 but I'd get a good charger on it and see what happens. I think getting batteries on a quality shore charger regularly is the most important thing you can do. All the voltage bumping on alternators isn't going to substitute for that and comes with some risk of damaging the charging system or battery.
In fact I usually hesitate to really recommend tinkering with them unless you're willing to monitor your electrical system. Vehicle charging systems are never ideal and really only there to prevent from going flat. They can't fully charge and they do nothing about conditioning.