I'm curious what the battery voltage and Ah is on the USE unit.
I'm wondering if there is a way to make your own battery and shove in there as a spare. While a self contained unit is clean looking one of the major downfalls I can see if they propritary battery inside and changing it later when it dies out. You'll be paying a lot if the battery is messed up later. With a external battery you can buy another battery from places like BatterySpace and you're good to go again on the cheap.