Doing this from recall, but when you replace an old(er) battery, you need to tell the car's computer system that it there is now a "new" battery in the car (go to dealer). The reason behind this is that the computer is programmed to continuously update the output from the alternator over time as the battery ages. After about 4 years or so the alternator may now be putting out 20% to 30% more charge just to keep the weak battery at peak capacity.

If you don't reset the computer, the "new' battery will now be over charged because the alternator is still putting out the charge for an old(er) battery.

Overcharging can be disastrous to the sensitive electronics in the car over time. But I suspect that what might be happening in your case is that the system quit charging the battery and that you basically ran the car from a fully charged battery, until such time that the battery charge was used up again ...

Get the car reprogrammed at the dealer. Count on doing this every time you replace the battery (unfortunately).

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PAX5
BMW CCA #20645