Just had the same problem with my 2005 X5 3.0i and I went through the steps to diagnose it like others have posted. My 6 year old battery had low-ish voltage, and as this is my pregnant wife's car, I opted to start there and replace it. No success!

Next, I looked to the alternator and found the power steering reservoir leaking pretty significantly from the swollen rubber lines and dripping down into the alternator. Though the voltage while running was 13.5-13.9 (within spec) I know I've seen alternators fail intermittently in the past so I opted to just replace it. Again, it's my pregnant wife's commuter so better safe than sorry. As it turns out, this also did not solve the flickering battery light issue.

Finally, I searched on the forums and found several other stories that go: "new battery, new alternator, battery light still flickering". The resolution was a new ignition switch!

We last replaced our ignition switch in November 2015 around 120k with a genuine BMW part so I assumed that couldn't fail again so soon. I decided to throw in a $59 Febi switch (still made in Germany) and that has totally resolved the issue.

Here's a little video about my experience this week:

https://youtu.be/Rdo-IeK9HzY