FSU is the most common parasitic draw fault on e53. It can fail in a mode where it functions normally but draws power when off.

That said get a reading on current draw for the starter. When they start to fail they can draw up to double the current.

Next: you can have a battery fail in a weird way that it can deliver enough current to start the car but not have any capacity. The symptom exactly matches your description.

In my case 30 minutes of charging an iphone was enough I couldn't start the car and the final straw was that I ran the hazard flashers for about 8 minutes.

The car started just fine even multiple starts in a row including at 6°F, it also passed a cold crank test at batteries plus.

The battery should be able to run the flashers for hours and still start the car.

What tester did you use? Something like the 100A resistive tester? Like I mentioned my failed battery passed that test just fine. Put a modest load like 4A on it and watch voltage. 90ah battery should last 90/4=22.5 hr. If the battery loses voltage quickly it has suffered the same fate as mine.

__________________
2011 E70 • N55 (me)
2012 E70 • N63 (wife)