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.
One of the nice things with Xbox 360 controllers were the rechargeable battery packs. By default, the controllers used 2 AAs, but you could instead use a battery pack. Just remove the AA battery cover, pop in the rechargeable battery, and that's it. You could then connect a cable to the controller to recharge the battery. And, if the battery happened to be empty (or dead because of old age), you could just replace it with 2 AAs, and continue playing. Some of 8bitdo's controllers uses (or used) the same design, but they come with a rechargeable battery pack in the box.