Hi,
This is the setup I usually use when recording.
RED-E PowerBank 4000mAh with 8 Pin Slider + GoPro Battery BacPac + internal battery. Lasts +/- 4-6 hours depending on the recording quality.
Alternatively I would use a EC Tech 22000mAh battery pack (from Amazon) for a full days recording. A bit bulky but does the job + I can charge my mobile if I need to with second usb port.