BMW Repair: 2005 530i battery drain, charging system, life thanks

Question
My "new to me" 530i seems to have a battery issue.  It will start fine normally, but if I leave the trunk lid open for 30 mins, it barely cranks.  Same if I have the doors open for a cleaning, it will barely crank over.  If I don't touch it between drives it will labor, but start acceptably.  Any ideas?  40,000 miles, 3 years old, seems a little short for battery life.

Thanks,
Sam

Answer
I still would have the battery checked to make sure it is up to par.Although it isn't that old it may still have a problem.And 3 years does seem short but it also depends on the use it has had in the past.Have the battery checked and also your charging system as well.