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.
The Steam Controller battery is user replaceable. It’s not AA “pop it out and throw a new one in from the charger” like a Logitech F710, but you can open the back cover and replace the battery.
Valve have not provided any details around sourcing the batteries. All they’ve said is that the controller is able to be serviced by opening via the screws and clips - ie it’s not glued closed like many user-hostile companies, eg Google Stadia.
The PS3 Dualschock 3 controllers are also exactly as user-serviceable - clips and screws, no glue. So I agree with OP. Batteries that are built for user replacement to a standard format are far superior.
They’ve provided batteries for the Steam Deck, and the specifications of the battery will be public and available for outside manufacturers. I presume they’ll provide batteries through iFixit after release as well.