Battery replacement is fairly simple if you have a basic knowledge of electronics repair. I've done it a few times, and need to do it again on my spare key now actually. If you don't have experience, then as upallnight so "eloquently" put it... just go order a new key and send your current one off for repair.

Part number for the battery is: Panasonic VL2020

Edit: And if you aren't comfortable dealing with the above batteries incorrect pin placement. This one has the correct 90 degree pin placement for the diamond style key circuit boards.

https://www.amazon.com/HQRP-Battery-...mw+key+battery

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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)