I bought the extended Gamepad battery. I see Nintendo planned a spot for it all along. : r/wiiu
Amazon.com: iFixit Battery Compatible with Nintendo Wii U Gamepad : Video Games
Nintendo Wii U GamePad WUP-003 Replacement Battery: BatteryClerk.com Game
Wii U GamePad battery life gets a boost from new Nyko accessories - Polygon
Amazon.com: UCEC Wii U Gamepad Battery, Wii U Accessories 3600 mAh Wii U Battery Replacement Rechargeable Battery Pack Compatible for Nintendo Wii U Gamepad : Video Games
Extended Battery Pack For Wii U GamePad - Tomee - Stone Age Gamer
Extended Battery Pack for Wii U GamePad – Retro Raven Games
Somehow managed to fit a 10,000mAh battery inside of my Wii U game pad! Battery life from around 2 hours to (so far) more than 13 hours at full brightness. See captions,
Nintendo Wii U Wii U GamePad WUP-010 1500mAh Replacement Battery: BatteryClerk.com Game
Source Factory 3.7V 1500mAh Rechargeable Battery Pack Replacement For Nintendo Wii U Gamepad on m.alibaba.com
Replacing Wii U Gamepad Battery (Enhanced Capacity) Not Charging Fix - YouTube
KMD 3000mAh Rechargeable Battery for Nintendo Wii U Pro Controller GamePad - Walmart.com
1 PC Replacement Battery For Nintendo Wii U Gamepad Controller WUP-012 1500mAh - AliExpress
Nintendo Wii U Gamepad replacement Battery - Extended Life 3600mAh aftermarket | eBay
New Original OEM Nintendo WUP-013 Wii U Gamepad Controller Battery 2550mAh | eBay