Something to consider as well is to have them verify that the Battery you have is coded correctly to the car. When I went in and checked mine after the battery died, I discovered they had it coded incorrectly. They had it coded at 110Ah Lead Acid, instead of a 90Ah AGM. The 6 series is supposed to charge the battery in an "optimum" method, and you are supposed to tell the car when you chnaged the battery as well. My favorite comment by someone here about the "smart" system, was something along the lines of, "if it so smart, why can't it figure out when the battery was changed" GRIN. Come to think of it I had my system reset and software updated at BMW not too long ago. I will have to verify that they have my battery coded correctly too.
I use the C-tek Battery Tender, based on the information I found here on this forum. I have been putting her on the tender once a week as well. She typically responds well to that. I have had a wealth of other problems, of which I'm hoping I will have resolved this weekend. Fingers crossed, yes i am resorting to crossing my fingers, LOL.