It's a poor design issue.
It's absolutely crazy that we just "take it" when the SA explains that this problem is due to our short trips. When was the last time that ANY car company sold a car that required long trips to keep the battery alive? I've had to buy and install two chargers to keep the batteries in my two BMWs from deteriorating and needing premature replacement. This together with the run flats are the worst so called "features" of our cars. It's a shame BMW, it's a shame
Maybe it's too late for your battery but make sure you use a charger for the next one and charging it fully once a week should preserve it and protect it from further deterioration. I bought these:
http://www.amazon.com/CTEK-Multi-700...9875296&sr=8-1 and I have them installed next to each car's parking spot so they are easy to connect. Only the positive terminal is visible but you can use any of the other nearby bolts to connect the negative. The Ctek charger comes with everything you need to connect it to the terminal under the hood - there are cheaper versions of this Ctek charger available.