The battery is external, i.e. already next to you anyway, so plugging in power is rather simple. You could also have multiple batteries with you, though I don't know the price of an additional battery.
It's $200 for an extra battery, all of the accessories have a steep early adopter tax. Besides, has anyone confirmed whether or not the AVP has an internal backup battery so you can hotswap the external battery without rebooting it?
In that case if you want to use it for many hours without access to wall power you're better off buying a generic USB power bank and plugging that into the Apple battery. Very elegant.