Good day!
In your case I suggest you to recharge the battery first and do load test test, if it's failed, replace it with the BMW specific battery, if battery is good, check your charging system, the voltage should atlest 13 or not morethan 15 volts when engine at idle mode (with or without load). If it's Failed then check Alternator Power Supply and replace part if neccessary. If it's good then do Ignition Off Current Draw Test.to do this, you have to open both front Doors, Hood and trunk. close all door striker by using screw driver, make sure no electrect components are On, Start and off the engine just to weak up all control units. Remove key and lock the doors using remote control. disconnect battry terminal connect your ammeter (at lest 10A) to battery lead and battery terminal, wait for at lest 16 min ( 70 min. if the car equipt telephone) to process the vehicle going into sleep mode then you can measure current draw while the ignition is off or vehilce is in sleep mode. The current rate should settle at max 40 milliamperes (E39), 50 milliampers(E60). if the current value exceeded then remove fuse one by one and do not return it untill the current drops to specific value. In this way you will determine which component is consuming high current draw and replace part if neccessary.
If you car is equipt of telephone, there is another way to test it. Disconnect the Telephone or ULF control unit and observe it if the battery will drain agin because a faulty telephone or ULF control unit is a common cause of battery keeps flat for this car model. I hoe you can find the problem of your car
Good luck!