Very good question. It must have the ability to maintain the charge, not just charge continuously. Also, must have the ability to charge AGM type batteries.
Here is a solid recommendation for a "smart" battery charger.
CTEK (56-864) MUS4.3 12 Volt Fully Automatic 8 Step Battery Charger
So, this will get you in the right direction. The cool thing with our BMW's is that you can charge from under the hood...and not the truck (where the battery actually lives)...
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