Unfortunately, it appears that this is the only sure answer for those having the problem:

Amazon.com: Clore Automotive ES5000 12 Volt Portable Battery Booster Pack: Automotive

BMW will probably get this figured-out in 3-4 years or so. That's about how long it took them to fix a similar problem on their motorcycles in the early 1990's.

For a while the battery problems on their motorcycles were so bad they were automatically giving a free battery charger with every new motorcycle sold in the U.S. OF course, BMW denied there was any problem, and it was the owner's fault for not riding the bike enough. But, oddly enough, after a few years they stopped with the free battery chargers and there were no more complaints about dead batteries. I guess all of a sudden the US motorcycle riders started riding their bikes more often and on longer trips!