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.
Those exist, but they're really expensive and not very good. Lithium needs circuitry to get down to the 1.5V of a nickel battery, but that circuitry takes up a good chunk of the available space in an AA.
Why AA Batteries Still Suck - Hank Green
NiMH are 1.2 V. You’re probably thinking of alkaline.