Update: my Switch kept dying at 40% so I opened it up with a $9 kit from Harbor Freight
Was playing Zelda on the couch in Garland last Tuesday when the battery hit 40% and the screen just went black. I watched a 12 minute video, popped the back off with the tiny screwdriver that came in the kit, and found the battery connector was barely seated from the factory. Pushed it back in snug and it's been holding a full charge for 5 days now. Anyone know if that loose connector is common on the older models or did I just get lucky?