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there are a few different things which can trigger this warning
the first is a simple weak battery. if your battery is still original, it may be in need of replacement. When the battery is replaced, the car's computer needs to be updated so that the charging system properly monitors and charges the new battery. This needs to be done by a main dealer or BMW specialist garage with the necessary equipment for this computer update.
another possible cause of this warning is a problem with battery monitoring system.
finally, a faulty electrical component that is placing a high drain on the car when it is off is also a possibility. This requires specialist testing to determine if it is in fact the cause.
so, at this time, I see two possible paths forward.
1) replace the battery and have the computer updated to accept it, and hope it is the cause, which it could be due to its age
2) invest in a proper fault scan and test plan to determine the exact cause of the warning
if you opt for 1, it would probably be worthwhile having the faults scanned at the same time as the computer is updates to get an indication of whether or not there may be a drain issue (it should not cost much to have this done when already paying for the battery registration update)
Sorry if you were hoping for a different answer, please don't shoot the messenger - I can only give the honest answer, which I have. Please remember that a low rating PUNISHES me for my honest and prompt response, and I receive nothing for my time and answer until a positive rating is clicked.
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