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.
Pardon me, I realize I was a bit vague in my statement. I think what I was referring to was specifically the Xbox 360 wireless controller.
If I remember correctly, AA batteries didn't quite fit directly in the controller. Rather, they fit inside a small detachable casing designed specifically for their cylindrical shape, which was then clipped onto the underside of the controller. If you then purchased a rechargeable battery pack, it replaced that attachment entirely, clipping directly onto the controller in its place.
I had totally forgotten about that, but you’re right.