The HHR system seems to spend some time in the 12.5 to 12.8 range. I've been paying more attention to the volts recently, so I don't know if it's normal of a change. Perhaps it's like the Honda system, and I don't have to worry as much.
Hey check the voltage when you turn your headlights on and off and see what happens then. It should drop for a second the come back up to voltage maybe even a little higher when you turn on the lights. That will tell you if the alternator is working or not.