I have a 2006 Forester. Haven't been driving it much. Tried to drive it a few weeks ago and the 640 CCA lead acid battery was dead. Jumped it and drove a bit. Checked and learned that cranking and charging tests were fine. The battery test indicated that it was good but needed to be recharged. I ran it on a charger for about 8 hours over two days but only increased from 473 - 522. This morning after sitting around for 2 days dropped to 379. Driving it around the block a couple of times it was back up to 453. And rechecking after being parked a few hours was about the same. Charging it was confusing. 10A charger only charged at about 1.4A, voltage was about 14.5. If it isn’t the charger which wonder if there is some plate sulfate issues? I can remove the battery and charge but that is a bit of a pain as the battery is about 4.5 years old. Not driving it much as probably harmed it. Thinking about a AGM as I think that sulfate issues are less common if not driving the car much. Any ideas as to what to play with or does it seem like just getting a new battery is in order. If I change it, what are the best ways to avoid losing settings?

If it's 5 years old, it's probably just time for a new battery, as long as it's reading >14V with the engine running, showing the alternator is good.  You can try using a non-automatic charger - one with no blinky lights or any intelligence - to un-sulfate it, by disconnecting the battery from the car and leaving it on the charger for several days.  You want the car disconnected because you're trying to intentionally overcharge the battery, and don't want the car seeing high voltage.  An automatic charger won't work.  Has to be one that's just a transformer, rectifier, and meter.