Based on the description on their site, the controller includes a built-in battery: "8.39 Wh Li-ion battery, 35+ hours of gameplay… "
That was disappointing for me. Specially condidering the Steam Frame’s controllers make use of AA batteries: “One replaceable AA battery per controller, 40hr battery life”
AA Batteries might not be as convenient to use, but being able to replace them is a great advantage. All my Xbox360 controllers still work fine, but none of my PS3’ Dualshock 3s.
The official docking station could be used to recharge (rechargables) AA batteries so the functionality could remain the same.
Literally every wireless controller I have ever owned that used rechargable batteries could be plugged in while I was using it if it started to die. I would bet that 99% of wireless controller users have a power outlet at least somewhere near where they sit to game.