Check out the Optimate 4 Dual Program. It has a mode specifically for charging a BMW over the CANBUS. And a mode for charging regular batteries.
If you go with a Battery Tender, you might want to go with the Battery Tender Junior. It has a lower maximum current than the regular BT. The regular BT, can, under certain conditions, supply too much charging current and F up a gel battery.
Iron Butt # 4999, (former) WERA/CCS EX # 666, AMA Life Member, Drummer, Bassist