There’s a thread for this as well 
: Working external battery pack?
The short answer is that it needs to support USB-PD output with 15V and at least 3.0A, preferably 4.0A but there seems to be very few battery packs capable of 4.0A.
There’s a thread for this as well : Working external battery pack?
The short answer is that it needs to support USB-PD output with 15V and at least 3.0A, preferably 4.0A but there seems to be very few battery packs capable of 4.0A.