I recently went looking to see if there was a practical way to expose a USB powerstation to Linux as a battery under /sys/class/power_supply, the way internal laptop batteries are. Unfortunately, that didn’t appear to be the case. There are UPSes that NUT can monitor, but not a route to treat them the way Linux does laptop batteries. Kind of annoying, since for a luggable computer, it’d be really neat to have an external, expandable battery.