How do you figure? By my calculations I could get a 15 hour day on a full charge (assuming battery usage is linear... that's why I'm going to do further experimentation).

I'm actually contemplating getting this for primary navigation (the same one listed on the thread I linked) and then using my phone if the battery dies on this:
https://www.amazon.com/dp/B00SIE2YQ8...I1AIW0PCTNT6PV