That's normal, when there is low demand the PCM will cause alternator will only put out what it needs to maintain the battery at safe level. GM dropped the voltage down so that the battery will last longer instead of pumping it full of electrons all the time. Basically it's like a hose and a bucket with a hole in it. When the demand is low the flow from the hose is low as it is needed or when the PCM determines that there is a need (Reconditioning takes it to 15.1V) it will put out more to maintain the level. If you look at the above graph you can see that it starts at about 12.6 then increases to about 14.1 then it holds there for a little while then it drops back and the battery voltage drops down as time goes by and gets to about 12.6 then it jumps up again. This was on a steady state cruise at about 65 mph and is about 30 minutes of driving. I probably had a bad connection and that's why it's so jagged, usually it's a little more smooth. I had to replace the positive Terminal this weekend since it was broken and finally didn't want to start the car.
Years ago I was in a Impala rental car and had a long drive ahead of me and I had the display set to Volts and after a while I watched as the voltage dropped and dropped until it was at 12.5V it held that for about 20 minutes. As I was about to call the rental car company to see about switching out cars and the closest location and all that the voltage started to climb again and held about 14 for about 10 minutes then dropped down again. After that I noticed that most all of the GM cars I have checked do that since the mid 2000's.
Jeff
