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'm with you. I loved how the AAs slotted into the handles of the original steam controller. I bought some Eneloop rechargeable AAs for it that still work great. I use them in my toothbrush, which I also bought because the rechargable lithium version died. The AA battery version of the toothbrush was like 1/5 the price and will probably last forever