Thanks!

I was having difficulty because the usb-c battery was willing to go to 20v +5a (aka 100W) and so it was taking priority over the barrel plug's 19v.

I decided to switch to a different battery pack (specifically a UGREEN 145W Power Bank 25000mAh) which can provide the needed power and has pass-through power so it can charge while powering the Sigma.  So I just plug the battery pack into the wall and the sigma into the battery pack.  Works like a charm. :) 

Jay