I've been using a CTEK MUS-4.3 battery tender (purchased from Amazon) over the past three winters which appeared to be the same one BMW marketed under their own name at that time. I also bought the CTEK Comfort Connect Extension Cable so the wiring could be fished down next to the manifold opening rather than closing the hood on the wiring.
Current CTEK models -
https://smartercharger.com/pages/cha...election-chart
BMW's current offering -
https://www.shopbmwusa.com/product/1...LLIGATOR-CLIPS