Dim Lights

So I drove the dart last night with my radar detector plugged into the cigaret lighter so I could see my voltage as I drove. After the car warmed up I averaged about 15.5 while driving and 10.5 while sitting at a light in gear. I had the heat on high and don't have a radio in the car. I'm tempted to say this is a voltage regulator problem but want to hear what you guys think before I throw a part at the problem. I do not know the age of the battery but it does look newish and I try to keep a battery tender on it.