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.
You don't have to worry if there will be a battery available in 20 years because AAs are standardized and will always be made. Or at least far more likely than whatever proprietary crap they're putting in these controllers. Devices that take AAs will last until the hardware wears out. You don't have to worry about the battery being the weak link. Which has been the case for every rechargeable device I've ever owned. The battery is always the first to go.