Just get yourself a trickle charger.
BMW will categorize you as being a battery abuser and won't cover the failure.
Your warranty / maintenance docs say you need to drive 10 miles or so each day.
2014 328i Mojave Metallic
2009 328i Black Sapphire Metallic
2007 328i Black Sapphire Metallic
2007 Honda S2000 Berlina Black
(Hey Jim... am I diversified?)