How is the alternator used by a BMW?

[Brian M Kennedy <kennedy@xxxxxx>]

In a number of recent posts, the alternator operation has been mentioned, 
discussed, or questioned.  In those posts, they seem to imply an on-or-off sort
of behavior:  when battery voltage drops to a certain level, the alternator 
kicks in and recharges it to a certain level, and then kicks off again.

I didn't think that was how it worked at all.  What I thought was this:

The engine always spins the alternator; the alternator is always tied into the 
electrical system (basically in parallel across the battery).  The alternator 
generally maintains a constant voltage of 12-14 volts, which is higher than the 
battery voltage.  As long as that is so, the battery plays no significant role.
As the electrical load increases, the alternator must generate more current to 
maintain 12-14 volts.  In generating more current, the magnetic forces in the a
lternator become much greater, causing the engine to have to work harder to turn 
it.  When there is no current draw, the alternator is real easy for the engine 
to turn; when there is a lot of current draw, the alternator takes several 
horsepower to turn.  If the electrical load ever exceeded the current-generating 
ability of the alternator, then the voltage would start dropping until it reached 
the battery's voltage (usually 12.5-12.7V).  At that point the battery starts 
adding current beyond what the alternator could supply.  Of course, if you keep 
that up, the battery will gradually run down.  When the load is later reduced, 
the current draw will reduce to less than the alternator capacity.  At that point,
the battery will also be a load on the alternator and will suck up what current it 
can draw until it is charged again.  While it is charging, the alternator (and 
thus, engine) is having to work harder to supply that extra current.  As it 
charges, the current the battery draws reduces until it finally approaches 
zero (or close).

Am I wrong?
Can someone explain precisely how the alternator is operated by the car?
Is there a voltage "thermostat" that kicks on and off the alternator?


Also, many modern cars automatically cut off the A/C (or reduce its action and l
oad) under heavy acceleration.  Do any of the cars do the same thing with the 
alternator?  (In particular, do BMWs cut off the load on the alternator at WOT.)  
Yes, I know there are aftermarket companies that sell devices that do that for 
most cars -- I was just wondering if BMWs already did that -- seems like a notable
performance win for some cheap-o wiring.


Thanks for any enlightenment,

Brian

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