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.
The idea of using disposable AA batteries seems nice... Until the day you go to open the compartment and find they've leaked and corroded the contacts (or worse) in the controller. Regular lithium are ok, they do last a good long while, but not exactly the most eco-conscious choice either. Rechargeable AA take forever to recharge. Like seriously, we are talking all night for the higher capacity ones.
So have some extra ones ready to go. They're cheap.
I'm not sure what kind of black magic they employ, but I can charge three sets of 4 enloop pros in a day with the official charger, more if they weren't completely dead. I'd been using an older charger before and it would take 10+ hours for a single set with that thing.
Even cheap Ladda from IKEA recharge in mere hours now.