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.
My main concern about that is that using AA and AAA form factors promotes the use of disposable dry cell batteries, even if nickel or lithium cells are just as widely available. And, realistically, not even the closest Li-ion form factor is fully compatible with AA, and Ni-MH sucks balls.
The controller is also filled to the gills with hardware. Doesn't look like there's enough volume left for AA or AAA receptacles without giving it an underbelly to rival the Xbox Duke.
On a slight side note, the Duke is the only controller I have ever liked, they just don't make controllers for giant hands at all anymore. Lol