I don't know why they didn't seem to explain this well enough as everyone is kinda missing the point. AVP is exactly like a MacBook or iPhone - except the battery and charge port happen to exist at the end of a cable.
Plug in power (whether it be a USB-C AC adapter, or a 30w USB C PD power bank) to the battery and it will charge and you can use the AVP at the same time, exactly like you'd do with a Mac or iPhone. No need to eliminate the battery from the equation, it's part of the device.
If you want more battery time, I'd rather look at a 20,000mah 30w USB C PD battery bank rather than a second $200 Apple battery - you'd have to fully power down the device to swap whereas if you just treat this like any other Apple device, you can supplement power anytime you'd like from either AC or a power bank.