I bought the extended Gamepad battery. I see Nintendo planned a spot for it all along. : r/wiiu
Wii U gamepad battery upgrade/replacement : r/diyelectronics
Comparing all three batteries for the Wii U Gamepad - YouTube
WUP-012 1500mAh 3.7 v Battery for Nintendo Wii U Game Pad Controller | eBay
Replacement Battery for Nintendo Wii U Gamepad - HHD10221 at Batteries Plus
1pc 3.7V 3600mAh Rechargeable Li ion Battery Pack for Nintendo Wii U Gamepad Battery for Wii U Wiiu GamePad Controller Joystick|Batteries| - AliExpress
Nintendo Support: How to Remove, Reseat, or Replace the Battery in a Wii U GamePad
Nintendo Wii U GamePad Battery - iFixit Store
KMD 3000mAh Rechargeable Battery For Nintendo Wii U Pro Controller GamePad | Walmart Canada