I used my phone, which lasts about 7 hours without the screen on. I use it to see how far until the next turn and then check it again after that step to see the next.

I bought a double sized battery (Galaxy phone) and that lasts another 6 hours. And I bought an external battery for it. Last weekend I did a 400k, 17.5 hours total time and the phone was almost full at the end (External battery was down to 10%) The external is a 6000mA Anker battery pack