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.
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.
The thing with "easily serviceable" is that no matter the actual difficulty, once you need to involve a screwdriver to take apart a controller (anything other than a door you can easily open with your hands) you've lost like 85% of people. Most average people are not comfortable taking apart electronics to do ANYTHING and will just buy a new one and toss the old one in the trash when it stops holding a charge. But AA batteries? everyone is comfortable with replacing rechargeable AAs.
I've had a couple of devices over the years that require one to unscrew a screw to open a cover to replace batteries. It's not that common, but I've certainly had them floating around.
In fact...I think that my analog multimeter does that, with a 9V battery.
goes to look
Yeah, Phillip's head screw. Though you only really need power on that thing for continuity testing, and some people might never even need to power it.
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.