Iv got a 2008 ram 4.7 V8 lone star. My problem began in November. Since then iv replaced the battery, alternator, and serp belt. I'm still having this issue. When I'm stopped at a light with my headlamps on my gauge will drop below halfway. Then if I sit long enough I can feel the truck idle up and the guage will return back to a little past half, or if I place it park itl return above half as well. Also when the headlamps are on it makes a soft whistle noise under the hood and when I turn them off it will stop.

 

The charge rate for the alternator is controlled by the ecu, which also looks at the temperature of the battery via a temp sensor. Check to see if any dtc's have been set using a scanner or the odometer. Also, check as many chassis grounds as you can find for a good connection as they are critical in these computer controlled devices.
Its hard to say if your charge problems are related to a low idle speed, mine idles at 650, and it charges great, or the ecu is backing off.
If checking all the obvious stuff doesn't help, a trip to the scanner at the dealer may be the only option.

 

Ok, this has been going on for months and its driving me crazy. When I stop I can feel the truck idle down and the gauge will drop and if I place it in park or set long enough in drive its like something kicks on and itl go back a little above half way. That's when they put the bat, alt, and belt on and it didn't help. It all seem to have started when my blower resistor went bad or maybe that was a coincidence idk. And it only does it after the trucks been driven long enough to kind of warm up.

 

I hope it's not the powertrain control module that's expensive.