This is my first winter with the Z, and consequently my first winter driving a car that has a voltage gauge that I actually look at I know that the owner's manual says that a reading anywhere between 11 and 15 Volts is normal, but outside of that range is cause for concern. I have been noticing that as the weather cools off here in Pennsylvania, the Voltage gauge reads what appears to be a bit higher than 15 Volts when I start the car for the first time, on a cold morning for example. Then as everything heats up a bit the needle drops down to around 14 Volts which is in the acceptable range per the owner's manual.

Anyway, I just did some research and found that as ambient temperature decreases, the internal resistance in the battery increases so that charging voltage increases to compensate for it.

Thought it was interesting and wanted to share, in case anyone else is experiencing this phenomenon and was curious about it!