My voltage meter is always higher when my car is first started. After it is run for an hour or so it is usually 1 to 1.5 volts lower than when first started.

 

Something else you have to watch is when the car is idleing its not moving. Moving airflow over the alternator is its only cooling. The diodes and rectifiers get very hot during operation. If you are working it for 2 hours then just turning it off, I forsee cracked components in your future.