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there are several common things that can cause a battery to go flat like this. There is no way to point to one of them simply based on what you've described.
the most likely is simply that the battery is bad and needs to be replaced. FYI, when the battery is replaced on this model, then the computer needs to be updated using a BMW programming computer so that the new battery is charged and monitored correctly. If you have had a new battery fitted and not had this computer update preformed, you could run into additional issues as a result.
the next is a faulty alternator which is not properly charing the battery while the engine is running.
finally, if the battery is good and being charged properly, then a parasitic draw can drain the battery while the vehicle is parked. there is specialized testing which can be carried out to identify the cause of the drain, as it can be ANY electrical component on the car.
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 vehicle.