Power Banks - for GoPro
The average GoPro battery lasts for about 2 hours, give or take. This fluctuates depending on a number of factors such as Resolution, frame rate, using protune and ambient temperature.
Depending on the activity you are engaged in, having spare batteries to change over may be sufficient when your battery goes flat.
However, in cases where you don't want to disturb the camera or have it power down, an external power source is required.
This is particularly helpful when doing long Timelapse sequences. One of the best methods for a static timelapse is to use a Power bank where it can be located close to the camera and plug in using a short USB cable.
Power banks vary by maximum Battery life and are measured in MillAmp Hours (mAh).