BMW Repair: 98 328, Electrical / Charging System, voltage regulators, battery cables

Question
Hello Tino, I'll be working on my friends 98 328 in a day or two.  He stated he's car cut off while driving, the charged battery and he can get it to run but disconnects the battery and car cuts off.... typical alt failure symptoms... yet he says alternator is new, new starter, and that even before the car cut-off any time he would disconnect the battery while engine running the engine would cut-off (w/ new alt. which doesn't sound right to me).   I am a experienced mech.  that's why I'm asking Q's... I've never dealt w/ a BMW and know there's heavy electronics in them.  So based on what little I have here (this is all I have until I get it to my shop and run some actual checks).... what suggestions do you have?  I'm assuming they have internal voltage regulators?  I keep getting hung on the fact that he states that w/ the recently purchased alt.  he could remove the battery cables and the engine would cut off, this indicates something is going on... but still car had been runing fine for several months, but know it cut off, he charged his bat, got it running to my shop, and I'll be looking at it in a day or two.  Any do's and don'ts and suggestions will be greatly appreciated.  Thanks.

Answer
Hi Kevin,
The first thing you should not do on any BMW is not to connect any battery with the key on. The current surge could fry modules which are never cheap. Also, do not connect any control modules with the key on.
The DME(engine control module) has separate inputs from the battery and the  generator/alternator. Thus, even if the alternator is working, if the DME senses no power from the
battery then it will cease to function.
I see no logic in disconnecting the battery while the engine is running just to test the alternator. This is a carry-over of long-ago car power systems. You yourself know how to test a charging system without disconnecting the battery.
I suspect that your friend's car has a bad battery or at the very least it is being discharged by a parasitic load. Try testing the battery and also doing a current draw test
(with the car shut down, manually lock the trunklid lock with a screwdriver, test for current draw but allow for a 16 minute span for the computer to put the car to sleep, if it does not sleep or if the current draw is high, then there is
a power drainer somewhere). Normal current draw should be between 35 to 45 MA, spikes up to 60 even 70 are permissible so long as they stay as momentary spikes.
You know how frustrating it is to check a bad battery. Even with the use of sophisticated testing equipment, some defects can not be verified. So you have to use common sense
and your years of experience to do a judgment call.
I hope I have helped. Your fellow mechanic,
Tino