Get a couple of the Anker or Shargeek (north of 20k mAH) packs and you should be covered.

What's oftentimes overlooked is the settings on the phone;

Turn down display brightness and lessen screen time-out settings.
And of course, turn off cellular data, wifi and bluetooth so it's not searching for a signal.

[Linked Image from i.imgur.com]

WWP53D