So, after new AGM battery went in... and paid $100 for "battery registration", it died again, with all systems shutting down in rapid progression. I left it for an hour, restarted again and (without using any electric equipment) got it to my independent mechanic. Within 5 min his voltage tester confirmed what I had argued with Beverly Hills BMW over the first time... bad alternator. I was having voltage swings from 9-15 volts... so the battery was only charging occasionally. Now... this seems like a pretty easy test which really infuriated me, thankfully i didnt spend $560 on a factory battery so I was only out $280 for the aftermarket one & registration. However... how do service techs not catch such an easy to diagnose problem and just push me out the door with a prescription for a battery?!

CPO warranty had a new alternator in the next day for $50, also the service dept at Century West BMW in north Hollywood is much better than my experience at BH BMW.