Most of the batteries that I have seen have less power, so stay away from those. There have been reports of internal batteries exploding if they are not Nokia batteries. I use the Proporta USB mobile device charger. It gives about 300% extra battery life at 3400mAh. Combined, that's 4900mAh which will last quite a long time. It is external, but it is much smaller than the internet tablet itself. I am yet to have a problem with it.

http://proporta.com