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Before reading my answer below, please note that I do not receive any credit or compensation whatsoever for my time and effort until the green ACCEPT button is pressed. Thank you.

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. I would try locking the car rather than leaving it unlocked, see if anything changes.

If not, 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, it may be related to a malfunction of the comfort access system and also the blower motor is a likely culprit. Also, 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.

BMW dealers also have special equipment they can use to help identify the cause of this, so this is a second option, besides the above test.

Assuming the car is still under warranty, you will want to have it addressed by BMW.

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.

If satisfied with my assistance, please press the green "ACCEPT" button so that I may be credited for helping today. I am not credited or compensated whatsoever until you press the green ACCEPT button.

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thank you in advance.

Steve