There is a battery lead under the hood in the engine compartment. I just hook up a "Super Smart Battery Tender" (brand name--they sell for about $50) trickle charger to the battery lead and the ground wire to a bolt on a shock tower or other grounding bolt. I have a '95 840ci. It works every time for extended periods. Perhaps the 850 has a different electrical system, but we all have the dual battery setup.