The Warranty booklet said if driving less than 10 miles per trip and no highway miles, you may have this problem... But if you are not...
Felt your pain, you are under warranty right!? may be a power leakage test to see if there are electronic modules drawing power after the car goes to sleep. If leakage, Dealer should fix it under Warranty.
My 2006 e90 still has the Original battery. 7 1/2 years battery life.
Daily commute 24 miles, non Idrive. Use a trickle charger(12V/1.5A) once a month or whenever out for a long trip.