I just came across this. Make sure you have latest version of maps and after you start navigation I got a pop up asking me If I would like to dim screen in between turns. Not sure if this is a new option that you can access through settings or if it just pops up if you have a considerable distance between turns. I had a too mile trip and it worked great even played Pandora the whole time. Now my battery never drained but did increase very slowly. Hope this helps you.
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