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.
Also, AAs become literal toxic waste after like 20-40 hours of gameplay, and need to be properly disposed of (that is assuming that you live in a developed country and there are recycling options available at all). Single-use battery suck and should fuck right off.
Use rechargeable AA batteries
There are like 3 or 4 different types. NiCd and NiMH have slightly different voltages and vastly different voltage curves, and it's a gamble whether your device will work with either of them and how long they will last. Li-Ion (with a voltage regulator and charge controller) are quite expensive (compared to a pouch battery of the same capacity) and you won't be able to buy them in the nearest grocery shop. Also, it's not safe for the controller to even attempt to charge any of them, so you will need a separate charger, and you'll have to take the batteries out of the controller, put them in a charger, and then put them back every time they go flat. At that point it's just so much easier and more convenient to have a pouch-style battery that the controller charges by itself, and you can very easily replace every 5 years or so by just removing a couple screws and slapping a new AliExpress special in there. The key here is to make batteries easily replaceable, of course, ideally without any tool, but a standard philips screwdriver is acceptable too.
Almost no one is using NiCd anymore. It's not hard to design something to run properly from alkaline, NiMH & Li-Ion cells. We have efficient switch mode power supplies that can step the battery voltage up or down to whatever the device needs to run.