Battery died (8 years old)
Very easy with the aid of the ifixit guide (approx. 30 minutes)
Take your time removing the lightly glued top & bottom to avoid cracking these plastic pieces or damaging the aluminum iPod case (I carefully went around the perimeter multiple times with a utility knife).
Make a note of how the battery wires are routed and take care not to pinch/cut them when sliding the "guts" back into the iPod case.