Both the all important main GamePad and the Pro offshoot will feature rechargeable batteries that can be replaced by the user.
For now though, replacement batteries have to be ordered directly from Nintendo. This may change with either Nintendo brand replacements or licensed third party batteries being offered in stores.
Nintendo stated that the Wii U GamePad's standard 3.5 hour battery life may degrade to 70% after 500 charges. The main Wii U unit also contains a battery, which if it fails will cause the system's clock to do clever things.