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 have mixed feelings about this. On the one hand I like the idea of AA because if the controller dies mid-session you can just swap them and keep playing, on the other this is easily solvable by having a dock like the 8BitDo Ultimate, which makes it so that the controller is always fully charged when you pick it up, so the only advantage that the AA had disappears, and it's even more comfortable to have the controller always charged than having to get up in the middle of the play session to find new batteries. And the Steam controller has a charging puck, so it should never have the issue where AA are better. So my feeling that it would be better is not justified.
The other supposed advantage is longevity, since all batteries eventually die off, if it's an external battery you just buy new ones and are done. Being internal makes it more of a hassle. But Valve has been very open with the repaiedness of their devices, so I expect this to not be a big issue, as long as the batteries are still being manufactured by the time the one in the controller dies off (which should take a lot more time to happen than regular AA).
Literally every wireless controller I have ever owned that used rechargable batteries could be plugged in while I was using it if it started to die. I would bet that 99% of wireless controller users have a power outlet at least somewhere near where they sit to game.