I'm sure the community would love to hear what you learn. As you noticed, running GPS plus on-screen navigation plus the other phone features can draw more power than your car will pump out. You might try using the automatic screen dimming function built in to the Navigation app, which can help conserve power by dimming the screen when there's a few miles between navigation points. You can access this feature by launching a navigation session by opening the menu in the navigation screen's lower-right corner, then selecting More > Settings > Screen dimming.