What makes these cars a pain is that battery control module. On a normal car you can tell that the alternator is working just by measuring voltage at idle with everything switched off and then lights heater defroster fan on. A huge drop gives it away. But in your case, the balancer was on the edge of failing, surprised it didn't just flat out fail! That's why on e90post the trend has been to replace it preventatively, because it also drives the water pump, but still better than the tdis with water pumps driven by the timing belt!