I went and had a complete charging system test done on my Jeep. It's all working as it should. The tester showed that my battery was fully charged but only had 80% of its rated capacity.

At what percentage of capacity loss should a battery be replaced?

This is not a technical answer, but I shop for a replacement battery before I need one. I think anything over 5-6 years is asking for trouble, besides, if you stretch the old battery too far it will put extra strain on your charging system.

I kinda go by time. After 5 years, I keep an eye out for drag...
The factory battery in my old 93 Toyota 4WD 22RE lasted more than 10 years.

If you see signs of trouble, I would replace it to avoid getting stuck.
Especially if it was wifey's car.

Agree 100 percent ^^^^^^

Especially the lady's car part has well.

It's 3 years old and in my Jeep.

Well I've got a 12 year old AGM battery. I check the voltage, it's been about the same as last year. Could have gotten a Champion battery on Black Friday at Pepboys, I guess I'll wait another year or so...

I went 3 yrs on a battery similar to yours....total of 6 yrs. At the 3 yr point it had fluid level well below the cell tops and lots of sulfation. Was lucky to bring it back with a lot of charging. 6 yrs total was fine with me. I replaced it once it acted up again. I'm sure the capacity was never near 100% after is run down so low. You need enough capacity to start the car in cold weather. California is pretty temperate. But then again, batteries tend not to last near as long in hot desert climates.

Optima Red Top battery in my 1997 Suburban 2500. Manufactured in 2004, still running strong.

That vehicle has that battery in the trunk too....

Extremely helpful to battery lasting a long, long time.
Vs having the battery in the motor compartment with all that heat exposure.

I believe that if it drops below 70% of rated capacity then it's time to replace the battery... Maybe as low as 60 percent...

My 2011 F150 still has the factory battery and is going strong. Just tested it this weekend, read 12.4v and 800 cranking amps when it was tested.

Wish I could find a aftermarket battery as reliable as that.

I replaced our car's battery every 4 years.Costco battery most of the time.

I go 4-5 years on a battery and replace it. I've never had one fail, never needed a set of jumper cables, and it has never left either me or my wife stranded.

I replace the battery when I notice it getting sluggish.

At 80% you are fine. You are not in Maine where the temps get really low and battery capacity is reduced.

At 60% I would start to think. At 50% I would definitely get a new one.