Grant Bennet-Alder wrote:
The other important parameter is that it must support USB Power Delivery, and Voltages far above the 5 Volts required to charge a Phone -- likely 20 Volts.
a 5 Volts only power bank will NOT WORK.
It will work, just not all that effectively. In a pinch it might be possible to charge a Mac battery while it's sleeping or to slow down the battery discharge.
I've got a Morphie USB-C 8000 mAh Powerstation with a 5V/3A USB-C output. I don't expect that it would be adequate for anything more than getting a few more hours of run time out of a Mac notebook, but if that's all that's available it's OK.