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.
I think you might be onto something. Probably the reason why they went for a built-in battery is space limitations
Did anyone see the MASSIVE rumble/haptic motors in the grip area? Yeah. This is the answer. They packed a lot of stuff in there.