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Mine (2.2L) also drops RPM with lights on but disconnecting IAC causes very high RPM, too high to detect a minor load variation I think.
The following is just my personal reasoning, subject to errors but anyway...
I believe, from my measurements, that the alternator on my 2.2L GMC Sonoma is putting 2 to 4 amps into the already charged battery at 14.6v, which of course is wasted as heat. This translates to about 44 watts which is 0.059 horsepower of wasted energy. If you consider that perhaps as much as half of that is lost as inefficiency of the alternator, before it got to the battery (more wasted heat), it then translates to a total loss of 0.059 X 2 = 0.118, or just over one tenth of a horsepower.
My thinking is that it would be better to design a regulator for the alternator that charges the battery at a lower voltage such as anything from 13.4 to 13.8 volts. My understanding from what I have read is that a 12 volt lead acid battery has to have a charge voltage of at least 13.4v or it will never reach a 100% charge which is required for maximum battery longevity as well as optimum battery performance.
Anyway, a lower charging voltage would keep from putting anything but a small trickle charge into the battery once it is charged, therefore no wasted horsepower. The only downside that I can see is that the battery will take longer to reach full charge after cranking the starter to start the motor. Nothing else should ever deplete the battery, other than the starter, except accessories being ON while the motor is OFF. I never have that situation though.
All this is just my thinking. I probably won't try to alter my charge voltage simply for fear of inadvertently screwing something up that will cost money to replace. I live on a very tight budget.
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