I don't know it has been mentioned. the nintendo US site doesn't seem to have Gamepad spec(or I just can't find it). The Japanese site has the specs for gamepad.
http://www.nintendo.co.jp/n10/e3_2012/wiiu/index.html
The interesting thing was that it takes 2 hours and 30 minute to charge the gamepad, and it runs 3-5 hours depending on the birghtness of the gamepad screen.
Edit: yes, the site does say you can play while you are charging.
http://www.nintendo.co.jp/n10/e3_2012/wiiu/index.html
The interesting thing was that it takes 2 hours and 30 minute to charge the gamepad, and it runs 3-5 hours depending on the birghtness of the gamepad screen.
Edit: yes, the site does say you can play while you are charging.