With the Wii U GamePad having only a 3-5 hour battery life, we see ourselves putting the tablet's built-in battery through quite a bit of strain. Unfortunately, rechargeable batteries have a finite amount of uses, which had us worried about how long our GamePads would actually last. By Nintendo's admission, usage time may be reduced to 70% after 500 charges (about 2000 hours of gameplay).
Thankfully, batteries for all Wii U accessories appear to be user-replaceable.
The Wii U GamePad has a battery cover on its back which must be removed by using a screwdriver. You'll be able to order a replacement battery on Nintendo's support website or by calling Customer Service. Because the console has yet to be released, there's no word yet on how much a replacement battery will cost.
Batteries in the Wii U Pro Controller can also be replaced, once again with parts ordered from Nintendo directly. However, with 80 hours of battery life, this is probably less of a concern. Also neat, but probably unlikely to happen: the Wii U battery can also be replaced, should your console's internal clock stop working.