Regulation of Idle Speed based on Voltage/Electrical Load

[mboucher70 hotmail.com]

Was curious about how well the car battery and alternator voltage adjusts to varying loads.  I attached a digital voltmeter directly to the battery and drove like that for a few days.  Battery is in good condition and began fully charged (12.9 V).  

What I found wasn’t surprising...the Alternator does its job keeping the voltage at a comfortable charging voltage (13.4 +/- ) under most loads and engine speeds.

The sole exception is when there’s a heavy load at idle.  For example you’re stuck in traffic in bad weather, engine is at idle speed, and you’ve got lights, heater/blower motor, fan, defroster, etc, etc) all running.  Under condition like this, the voltage at the battery would fall to about 11.8 volts.  At this point the engine is idling at its usual rpm, around 750.

What I was wondering: shouldn’t the computer up-regulate the idle speed dependent on battery voltage?  I know that the ICU and ECU get input from multiple sensors. I know that the idle adjusts up a bit when the A/C is turned on.  But should it also adjust based on electrical load/voltage?

It’s a 1990 Audi 100, NF engine