Here's what you need to pick up: 9V battery snap connectors: $1.99 (#270-325) Crimp-on connectors, ie: butt connectors *snicker*: $1.99 (#64-3073) 9V alkaline battery: $3.49 (#23-875) Wii sensor ...

Mercifully, Nintendo has answered your prayers: the Wii U GamePad has a battery which can be replaced by you, the end user. The only tool you need is a screwdriver.