Electrical things can be fairly complicated. So what I do and suggest to others is to
figure things out with your car(s) BEFORE anything is wrong. For the E53, that would mean:
See if the orange LED by the shifter goes out after 16 minutes when the car goes to sleep. Some swear it must. I know mine does not, and my car does go to sleep (from monitoring voltage on a sleep-controlled circuit), and I have no drain issues. This test requires you to look through the driver's window before unlocking the door one morning.
If that was not too hard
, then ...
Turn the key to ON, if you have the high cluster, see what Test9 says. This number is typically 0.3-0.5 Volts less than at the battery / jump port / alternator. I believe it is lower since it is a regulated voltage used by the more advanced electronics, so is lower than the raw, unregulated system voltage.
Put a cig-lighter voltage gauge in the outlet, see what voltage that says. (for bonus points, you can calibrate that gauge vs. your voltmeter (or two, if there is any doubt in that accuracy).
Use a voltmeter to measure voltage at the engine bay jump port. And at the battery some time when it is convenient. Those two should be identical when the engine is not running.
^^^ those measurements tell you something about your battery voltage and how your car's electrical system responds to that voltage.
Then you can run the engine and check all those voltages again. That will let you see what changes when your perfectly good alternator is running.
You can then stop the engine and repeat the ignition ON, engine stopped tests from before, to see what a freshly partially charged battery measures, and how quickly it drops to those original numbers.
If you do all that when the car has no problems, you will know that baseline for comparison when you have doubts one day, and those numbers will be far more relevant (your car, your known good battery, your known good alternator, your climate, your driving routine) than general numbers you see on the internet.