if you don't know the age of your battery, you might also dig into to see a date code. It's good info to have. On an X5 I bought a year ago, I just had an AGM battery dated 12/2016 fail on me. I replaced with w/a non-AGM H8 from Wal-Mart for $129.

I have been getting 6-8 yrs out of my batteries in my E39. I live in New England with annual temp ranges from zero to 100F.