For an external battery, you want to go with a PD (power delivery) battery. Like the one @joshuad11 posted above. That will provide the MacBook with 30W of power. For reference, the USB-C charger that comes with the MacBook is a 61W, so it will be a little slower charging at 30W.
My friend has the MacBook Pro 13-inch with Touch Bar. Sometimes she forgets her charging cable when she comes over to study, so she borrows my 50W Dell USB-C charger, and it charges just fine. We tried charging her MacBook with my Dell portable battery (5V@2.1A = 10.5W), and that also charged her MacBook, but it was super slow. So if you used an Anker battery that outputs 2.4A (5V@2.4A = 12W), then that should work as well. I often use that Dell external battery to charge my Dell Inspiron 15 using this method.
Side note: I love my Anker PowerCore 26800 battery. It is not the PD version, but it's half the price. Not ideal for MacBooks.
