There are some power banks that will not work with the EtherScope nXG.  The PowerArc ArcPack PA1801 is a recommended power brick because:

1. It is 54Wh capable for 100% charging the 46Wh EtherScope (some efficiency lost) so well matched in terms of capacity.
2. This is also a USB-C brick which matches well the the EtherScope power adapter.
3. It is also well made (aluminum case) and uses premium Samsung cells.
4. The real bonus is it comes with a high quality USB-C to USB-C cable that works well with ES. 

While there are likely other ones that will work, the above has been tested to work with the EtherScope nXG.