You are getting Increased Battery Discharge warnings? The high discharge persists after you have shut off the car. Overnight it draws your marginal battery down to enable protective functions.
Buy a smart-battery-CHARGER and charge IAW your owners manual for 24 hours or until the smart-CHARGER indicates FAIL. See if the problems don't go away.
If they don't, then bite the new battery bullet. Only BMW dealerships can properly test the battery and they will have their pound of flesh from nearest the heart. Car parts places just want to sell, never believe a salesman.
DO NOT go to a bmw dealer for a battery test or a new battery...you will be screwed.
As suggested above- buy a smart charger, see if that can recover the battery. But chances are you will need a new one (a smart cahrger for the NEW battery, say once a week, is not a bad idea...especially if your driving patterns are infrequent or short trips)
The proper size AGM battery, from anyone. Drop it in. Then find someone (andone with automotive software capabioities) will need to 'register' the battery (or 'reset' the car's charging system for the new battery)
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