You can play while its charging…

It’s a li-on battery. Just connect it to the charging puck. The internal battery lasts 35 hours.

You can remove the 9 screws in the back of the controller and fully remove and replace the battery. Going by valves track record they will have replacement parts on ifixit. Also one of the engineers confirmed while speaking with tested that you can easily access the battery compartment and interior of the device and replace it.