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Hello and welcome to JustAnswer BMW,
My name is XXXXX XXXXX it will be my pleasure to assist you today.
What you are experiencing is called parasitic draw. This is actually somewhat common.
There is some component on your vehicle that is draining power overnight while the car is supposed to be off.
If you have any kind of aftermarket equipment installed (GPS, ipod dock, etc), this is the first suspect.
IF there is no aftermarket equipment installed, the blower motor resistor is by far the most likely culprit. After that, the electric engine fan is also somewhat common. Beyond these items though, it could be ANY component on the car.
In order to determine the cause of this, you will need to unhook the negative battery cable and hook up an amp meter and measure draw after the car is locked and has sat untouched for 20min (this is when it is supposed to be in "sleep mode"). If there is more than about 50ma of draw at this point, you will need to pull fuses one by one until the draw stops. Whichever fuse stops the draw is the problem circuit.
If you are not familiar with using this equipment or performing this type of test, you should have it carried out professionally.
I hope that is helpful, please let me know if you have further questions, or if I can clarify anything for you.
It has been my pleasure to help today and I wish you the best of luck with your vehicle.
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