I can also attest to Optima's decline over the last 3-5 years. They used to be a really good battery to have. I think they got bought about 5 years ago, if memory serves, and their quality has drastically declined under the new owners. I did this research a long time ago though so I can't remember any specifics... but I think most agree now that Optima is just a name now...

I will be looking into an AGM battery replacement for the X. Nothing wrong with the current batter that's in it (from 09) yet, but I want an AGM in it after this one goes. Will be reviewing this thread in it's entirety then.

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2005 X5 4.4i Build 04/05 Maintenance/Build Log
Nav, Pano, Sport (Purchased 06/14 w/ 109,000 miles) (Sold 8/15 w/121,000 miles)

2006 X5 4.8is Build 11/05 Maintenance/Build Log
Nav, DSP, Pano, Running Boards, OEM Tow Hitch, Cold Weather Pckg (Purchased 08/15 w/ 90,500 miles)

2010 X5 35d Build 02/10
Nav, HiFi, 6 DVD, Sports Pckg, Cold Weather Pckg, HUD, CAS, Running Boards, Leather Dash, PDC, Pano (Purchased 03/17 w/ 136,120 miles)