Stationary Mode (this is one of the three different modes)
The Power Dock will still work when the battery is disconnected and the Power Dock is receiving power just from the Micro USB cable.
Checking the Battery Level
The power-dock2 application allows you to turn on the Battery Level indicator LEDs as well as output the current voltage of the battery on the screen.
You can monitor the voltage on the command line while it is in Battery mode or Stationary mode.
I think this remote sensing is one of the few unique features in the World of the Power Dock 2 ;-)