I use Co-Pilot Live for navigation on long trips because it uses locally stored maps. I found the data stream required to run Google Navigation to be extreme enough to discharge quicker than I could charge. I still prefer Google Navigation for short in town navigation.
Also, get your phone out of the sun and away from any heat blowing out your vents. I do most of my driving in AZ and found that a windshield mount exposed my phone to too much sun which made the phone run hot and discharge quicker. After discovering this I switched to a vent mount cradle which worked great during the summer when I was blowing cold ac out my vents, but once winter hit I started running heat out my vents and had the hot phone problem again. Now I use a ProClip cradle that keeps my phone out of the windshield and away from my heater vents. Keeping your Nexus cool will work wonders when running navigation.
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