Problem with Battery draining on a 2009 BMW 535i

Sounds electronic to me. When they did the sync did they recheck the charging system to make sure the computer was activating the alternator and then controlling it with the proper volts and amps? Grounds are very critical on these vehicles so check them.

If you've been driving the car for several days and the check battery light just illuminated, I'd suspect that the alternator isn't charging properly; not enough to fully recharge the battery. Put a battery charger on it and see if this fixes the problem for a while. if so and the battery light returns, it's likely a defect in the alternator. Remember, new alternators are usually good; rebuilt or remanufactured alternators often are defective.

I was having a rough start issue with my 2008 BMW 528xi that was slowly getting worse over for 6 months or a year. I also noticed that my clock kept loosing time. I would loose 5 minutes every month or two. My repair shop replaced checked all the likely suspects and then replaced the battery cable from the trunk to the engine and my car was has worked perfect every since. Starts like it did when it had 40k miles and my clock no longer looses time. My BMW has 208,000 miles and now seems to be running much better since the repair.