Sounds like you have a draw on car, something is staying on in your car after you shut off the key. You can sit in your car and listen after you shut it off for noises like the blower motor staying on. If you don't hear anything you will need to connect amperage meter in line with the negative ground post with the car off and watch the draw it should be about .040 amps any more and it will drain your battery after you connect the meter and watch the draw start removing fuses to see which one eliminates the draw and trace that back to components on the circuit and unplug them to find the problem.
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if this doesn't work, what would be my next step?
If you dont have a draw. It can only be a faulty battery even though it is new it may not be any good every now and then there is a "dud".
Technician
BMW Master Certifed level 1 Technican