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Welcome to justanswer. What you are experiencing is not that rare actually (BMW has a service bulletin on this, posted below for you).
Your dealer should be able to find the problem, it is really not acceptable that they haven't yet found it.
Sometimes, when different control units go bad, they start keeping the car "awake". A closed circuit current draw is the only way to identify the problem in cases like this. This is not a difficult test, but it is time consuming. You should have this test done by your dealer or an automotive electrician.
Before performing a full test, I can give you a few likely places to start:
The two most common problems that cause a draw on this vehicle is a faulty blower regulator or faulty auxiliary cooling fan. These are the first things I suggest disconnecting, and then see if there is a still a draw on the car. If it isn't either of these, a full-on current draw test will have to be preformed, as there are no other likely culprits.
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SI B 61 08 00
General Electrical Systems
This Service Information bulletins supersedes S.I. B61 08 00 dated June 2007.
[NEW] designates changes to this revision
Closed-Circuit Current Measurement
Increased closed-circuit currents may occur permanently or intermittently, and cause the battery to discharge prematurely. The increase in closed circuit current may be caused by a faulty control unit, or by the installation of a non-approved accessory.
In a situation where a vehicle has broken down due to a discharged battery, for diagnostic purposes it is important not to disconnect the battery. This is because a control unit will be reset if the battery is disconnected. Following a reset, the faulty control unit may start functioning correctly again, making accurate diagnosis impossible.
To correctly measure closed-circuit current, the 50-amp clip-on probe can be used in conjunction with the DISPlus to properly diagnosis closed circuit current problems over an extended period of time. Refer to Service Information B04 07 01 for information about the 50-amp clip-on probe use with the DISPlus. A new 50-amp clip-on probe (refer to SI B04 16 04) can be used with the GT1 in conjunction with the MIB (Measurement Interface Box). Refer to SI B08 02 04 for information about the MIB.
1. NEW It is very important that any "Power management" faults stored are diagnosed and corrected and the "Energy Diagnosis" test plan carried out before the following procedure is performed.
2. Check and test the battery using the BMW Battery Tester. Refer to SI B 04 25 02 for information about the BMW Battery. If necessary, recharge or replace the battery.
3. If the battery is installed in the trunk, open the trunk and turn the lock to the locked position using a screwdriver or similar (simulates the trunk lid being closed). The hood must be closed. If the battery is installed in the engine compartment, open the hood and pull the front lid contact switch fully up and lock in this position (workshop position, simulates the front lid being closed). The trunk must be closed.
4. With the exception of the trunk / hood above, all other doors / lids must be closed.
5. In order to simulate normal closed-circuit conditions:
- Turn ignition on and activate all electrical consumers, including any accessories. Turn ignition off. In some cases a drive cycle may need to be carried out in order to duplicate a closed circuit current problem.
- Open and close the driver's door (simulates somebody getting out).
- Lock the car, arming DWA if this is installed.
6. In general, closed-circuit current consistently over 50 mA must be investigated. Depending on the vehicle's equipment, closed-circuit current by vehicle model is approximately as follows:
E31 50 milliamps after 16 minutes
E32 50 milliamps after 16 minutes
E34 40 milliamps after 16 minutes
E36, Z3 30 milliamps after 16 minutes
E38 50 milliamps after 16 minutes
E39 40 milliamps after 16 minutes
E46 40 milliamps after 16 minutes
E60, E61, E63, E64 40 milliamps after 60-70 minutes
E65, E66 40 milliamps after 60-70 minutes
E53 40 milliamps after 16 minutes
E70 40 milliamps after 60-70 minutes with TCU (30 minutes without TCU)
E83 40 milliamps after 16 minutes
NEW E90, E91, E92 40 milliamps after 60-70 minutes with TCU (30 minutes without TCU)
E85 40 milliamps after 16 minutes
E52 50 milliamps after 16 minutes
HI, STEVE I HAVE SPOKEN WITH BMW AND THEY ADVISED ME THAT I NEED AN ALTERNATOR . I DON'T UNDERSTAND WHY THIS PROBLEM WAS NOT DETECTED WHEN I REPLACED THE BATTERY 3 DAYS AGO. I REALLY DON"T FEEL LIKE THEY KNOW WHAT THEY ARE DOING. I GAVE THEM YOUR RECOMMENDATION I DON'T KNOW WHAT TO DO.
If the alternator indeed tested bad then it will need to be replaced. However, they should be able to identify a current draw if there is one, and it sounds like you may have one, so I would make sure they do the appropriate test.