BMW sells a solar battery charger. You just place it on your dash and plug it into your power port. If you will be parking your car outdoors, it might be worth a try.
If you google "BMW solar charger" you will find some examples.
BMW part number: 82 30 0 143 365
2016 X5 35d | Mineral White | Terra
2013 535i - ED, Oct 2012 | Cashmere Silver | Oyster/Black Nappa Anthracite | ZCW | Sirius | Air Blue 80
- 2008 X3 | Monaco | Sand/Black Light Poplar | ZAP | ZPP | ZCW | Xenon | Rear Fog
1988 Grand Wagoneer | Burgundy | Ghost Flames | 360 ci | Holley carb
BMW CCA #422951