This is what I use http://www.tekkeon.com/site/products-mypowergo.php even comes with a connector that works with with my 770 which i believe is same as the N800. I use 2500 mha AA batteries with it and was able to easily get 10 plus hours out of my 770 watching movies. And even then the AA has 50% of life left while the 770 battery was still fully charged. And it was a nice price, under $20.

But it should be noted that it doesn't actually charge my 770, I don't think there is enough current to do so, but works great as a supplement power supply.