You definitely are doing diligence on your problem. Good job. There are plenty of posts here supporting how tough BMW is on a battery, and I have that experience too. Mine is a 06. In 09 I had to replace the battery. Soon after I had the alternator replaced at the stealer. Now the battery is showing signs of weakness again so I will be back at Pepboys for another battery soon.

When the alternator/regulator started failing, I had ugly gremlins popping up everywhere there was electrics. Really. It was bad, and I got stranded by failsafe twice. Since alternator replacement (about 100k miles ago) all has been well.

Some guys on here may have specific technical advice for you. I can only offer real life experience to the tune of about $1100 bucks. Good luck.