I have a USB battery which has a USB-C port on it.
When I connect it to my MacBook Pro (regardless of whether the MacBook Pro is running off its battery or via the mains), the Mac doesn't charge the battery, but instead uses the battery to power the Mac.
If the Mac is not on its own power supply, it will use the USB battery for charging (very slowly, but useful in a pinch).
In every case, the Mac tries to make power flow from the battery to the computer.
How do I reverse this? How do I make power flow from the Mac to the battery?
Apparently this is selectable, and other OSs provide an option.