I've noticed this on our 2007 Equinox too, but this is actually normal operation and nothing to be worried about. I was first very concerned since I saw the voltage meter go down to 12.6 volts while the engine is running. I noticed this occurs more during long trips. The dealership reassured me that this is normal operation due to the Electric Power Management feature. I've attached an excerpt from the service manual which explains this feature.
Sunday, May 20th, 2012 AT 8:59 PM