To update the map as you move requires constantly downloading data for the area around the vehicle. I wonder if switching to 3G only would improve the situation. The data consumption + the GPS + Bluetooth (if you are sending the audio to the vehicle's radio like i do) will use a good amount of power.
This is one of the things I will experiment with when I finally get the phone (today). I typically stream some music to my car radio via Pandora at the same time and run GPS with the Google Nav (the coordination of the audio is quite good) - I have not had a problem on a Motorola Triumph even when it is off the charger - but I wouldn't want to go for an extended period that way.
I'm thinking its the 4G consuming the most.
I plan on using my OEM Motorola charger. It's a good charger.
I will report my experience.