You can't "repair" a battery once the hydride starts shorting out cells.It was a good time for that battery to give up the ghost, I had noticed a few months ago it seemed like it was a little sluggish when cranking the engine, even with the gear reduction starter. The date on the side of the battery was 10/15.
Seems like a fair run. The last 2 AGM batteries I put in my truck only lasted 4 years. I was reading up on the cold weather durability of AGM batteries, unfortunately like everything else, there is so much sales pitches and he said she said type of talk about them it's hard to come up with something conclusive.