Hi @glitchyavacado
All batteries including rechargeable ones have a lifespan.
If you’ve gone 5 years without replacement you’ve done very well.
Here’s a link to the ifixit [guide|12824] guide.
ifixit also sells batteries for the gamepad, both new and used, here’s a link. [product|IF207-041-2]
Alternatively search online for ''Wii U Gamepad battery'' to find suppliers that may suit you better''.''