Anyone who has owned an Apple product will know that the company doesn't believe in user-serviceable power cells. Apple's reasoning is that by the time your iPod or iPhone's battery stops holding a charge, you'll be considering an replacement, or will have made the upgrade already. If you do insist on getting a new battery, you have to send your device back to Apple and pay a hefty fee.
However, with the Wii U GamePad reportedly having a stamina of around 3-5 hours and Nintendo's admission that after around 500 charges the battery will only hold approximately 70% of its original juice, you might be worrying about how you're going to bring new life to your GamePad in years to come. 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.
There's no indication of how much these fresh batteries will cost, but you'll be able to request them directly through Nintendo's own support site. The battery inside the Wii U Pro Controller is also user-serviceable, because Nintendo loves you.