The battery may be just fine. I get that same message almost every day. My service adviser says I should go for longer drives so the battery has time to recharge, but that's ridiculous - my commute to work is 10 miles and I've never had a car that couldn't maintain adequate charge with that. A few times I have placed a trickle charger on the battery overnight to make sure it's fully charged, but two or three days later the message reappears, even though both the battery volatge and charging voltages are right where thety should be. At this point I simply ignore the message. The car has no issues cranking, even in very cold weather, so I figure the root cause is an over-conservative computer programmer. My advice is check the battery voltage - should be 12.6V with no load, shouldn't drop lower than about 10V when cranking, and about 13.5V or more when being charged by the alternator. If these readings are OK then you should be fine.
E93 335i convertible, F15 X5 xDrive35i
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