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.
If it's not a standard battery design, then being able to replace it doesn't matter as much. It would mean the company can charge excessive fees because their 'custom' battery is the only one that's compatible with their product.
It very likely might be a standard size, you can buy prismatic and cylindrical cells in tons of sizes.