Ugh, my brother had that same problem with his X5. He let it sit for 3 weeks in the winter in Chicago and everything died. We had to do the following:

1. Open drivers door using key
2. Crawl through to the back, and take apart the rear door to allow it to open manually
3. Once open, remove rear access panels and remove battery
4. Replace battery (his was dead), then put it all back together

I think the instructions to remove the rear door panel are in the instruction manual but I could be wrong. You can also try a battery tender under the hood I think using the jump points, but I'm not sure on that.

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Nathan
01 4.4i Pearl Beige Metallic