Best way to extend Wii U Gamepad's battery life

So my friend and I are planning to marathon the Hyrule warriors campaign. However, I realize the gamepad's battery life may not last the marathon.

Does anyone have any recommended accessories and/or batteries to help extend the battery life.

Wow, that sounds horrible, is it a punishment from one of your parents?

Get a new battery

There's not too much you can do. Play it while plugged in? That would be the easiest answer. Otherwise you can do as @Jaysonguy says and buy the 8 hour battery from japan but it won't come soon enough to really help.

get the official nintendo extended battery. lasts about 8 hours.

they stopped selling it in the usa. i dont know why. but you can get it at yesasia for 30$ http://www.yesasia.com/us/wii-u-gamepad-battery-pack-japan-version/1033491481-0-0-0-en/info.html

i bought it from there and works perfect in my US gamepad.

^This ..new battery :P

^This ..new battery :P

Play while plugged, get an extended battery or use the pro controller(if you own one). Personally I would just plug it in when it needs charging. :>