no thats not normal (should not go any lower than 13.5, you really want it in the 14.0-14.6ish) technically as long as it is over 12.0 you are fine but i know i dont want that if i can help it... mine was doing the same thing... took it to the dealer and they could/would not fix it. i told them i think i have a bad quiality ground and they gave me the run around (they did not feel like fixing it)here is what i have learned.... car COs pretty much skimp out on the ground wires on cars under $50k (rough estimate)... they go the cheapest route and just make sure the car can start and move.... if you want your voltage to be more constant, you are better off just installing a grounding kit.
the next time i went to the dealership, i rubbed it in their face when i showed them my improved voltage w/ the grounding kit. i told them since they did not want to help me... i helped myself and that is what you should do as well
Good luck
