It looks like Nintendo has come up with solution for your Wii U gamepad power issues.
Nintendo has released an official "high capacity" battery for the Wii U gamepad in Europe, according to UK and German Amazon listings. The battery, first announced for the Japanese market in May, costs £25 or 35 Euro.
The extended battery holds 2,500mAh, allowing for five-eight hours of play time. The standard battery only hold 1,500mAh, and works for three-five hours.
Nintendo has not indicated when or if the extended battery will hit North American store shelves. If the suspense is simply too much to bear, there are currently third-party battery packs available.