Yes, the Wii U has a battery, but it isn’t very powerful.
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For example, The Legend of Zelda for the Nintendo Entertainment System made history when it released with a battery inside its cartridge so players could save their game. A lot of those batteries still work today, more than 25 years later.
But they do die. If it dies on a Wii U, Nintendo included a hatch that users can screw off to quickly access the battery. The full instructions are in the image to the right.
Replacements are not expensive. Amazon has a pair of Energizers for $1.99.
Again, according to the manual, this backup battery is only powering the clock, so it should last a long time without requiring attention.
Veteran gamers may remember that the Sega Saturn had a little trouble with its battery backup since it was drawing power into a memory unit as well as the clock. This drained the cells faster than just powering a clock would.
The Wii U uses Flash memory, which doesn’t require constant power.
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Source: NeoGAF