I know this thread is a little old but the issue however is still ever present. I have the GoPro Hero 6 Black and exactly the same issue with the battery, where it will charge to 100% but instantly drop to 90% when unplugged and then rapidly fall by as much as 45% in less than 30 minutes.
The reason I am responding to the advice suggested by danielr15 . With all due respect, the suggestion to use voice commands is actually the opposite of what to do. The reason to not do this is rather simple when you understand how voice control uses power to remain active. The camera doesn't truly go into standby, it is actually in a sleep state and constantly listening for commands which in turn is constantly using power. Combine this with a few other draining features, avoid them as I show below and you will notice a dramatic drop in battery usage.
For anyone reading this who is having this issue can follow my proven to work method.
Step 1 = Disable voice control ()
Step 2 = Set screensaver to 1 min
Step 3 = Enable Quikcapture
Step 4 = Auto off to 5 minutes.
Step 5 = Reduce screen brightness as low as you can tolerate, dropping mine to 30%
You should then see a dramatic change to battery performance. Using a remote has no effect on battery for me, Quikcapture is all I need to operate mine. I started writing this 20 minutes ago when my battery was on 97% and screen purposely left on and it has dropped to 95% now.
Hopefully this will help someone who is struggling with the Hero 6 Black, I cannot say if it will work on the Hero 7 Black as I do not have one to test it on , maybe someone can try it on their Hero 7 and respond if it works for them also.
