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in order to open this lock without battery power, you need to turn the key as far as possible, past the usual point where you turn it. if you have never done this before it can be very tough since the mechanism could be bound up.
this is how BMW designed the car for entry during a power failure - this is the only way that they have allowed for.
if there is a problem with your mechanical lock, which is preventing it from opening, then there is no other way into the car
in that case I can advise two possible courses of action which do not involve damaging the car
1) attempt to raise car and access either the starter motor or alternator to apply jumpers from another car/battery to provide power to the car this way
2) call road side assistance and have them gain entry using a door wedge or other type of professional entry tool
again, turning the key as far as possible SHOULD open the door mechanically. you can try wiggling it, pounding with a fist near the key hole as your turn, or possibly spraying a little lube into the lock. If this will not work for you then you are out of simple options, as this is the only way to enter without power.
Sorry to have possibly bad news, please do not shoot the messenger, I can only give the honest answer, which I have done. Please remember that a low raing PUNISHES me for my honesty.
Please let me know if you have further questions. Please do not forget to click a positive rating as this is the only way I receive credit for my answer, thank you. Best wishes with your repairs.