I was getting the same warning intermittently over the past few months. I also had strange DTCs for various support and ancilliary electrical systems stored.

Battery never failed to start the engine but winter was coming so I took steps to replace it. When I took it in for the airbag recall, I asked them to inspect the vehicle condition, perform a power analysis and report.

They kept it over night and provided a loaner vehicle. The report said for a seven year, 130k mile vehicle that it was in very good condition with no electrical issues and that the installed BMW battery was an AGM and appeared to be original to the vehicle: 26 11 was stamped on the post, IIRC. They recommended replacement. There was no cost for any of this.

I replaced the BMW battery myself with an Odyssey PC1350 and registered it with Carly. I did not have to change the capacity from the original 90 Ah battery.

I was surprised to get almost seven years of in-vehicle use out of an automotive battery. Wish the BMW battery in my M5 lasted a third that long. I've owned the X5 for four years. I keep it on a battery tender if I'm not driving it for an extended period of time but there's been a few times in my ownership where it has sat in long term airport parking for one to three weeks with no issues.

I'm still settling in to my new place but I have put the old battery on my battery conditioner and once I'm set up, I'll perform a capacity test to see how it performs.

Note: I've since installed an Interstate MTP flooded-cell battery in the M5 and it's been over five years.

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8/2011 X5 xDrive35d Sport, Black Sapphire Metallic, Black Nevada Leather, Fine Burr Walnut Trim
2/2001 M5, Jet Black, Exclusive Complete Black Walk Nappa Point Heritage Leather, Black Cubic Trim