You could upgrade to the mugen 3600mah battery

http://shop.eten.hu/mugen-power-3600...lue-p-475.html

I've got one of the 1800mah mugen batteries and it works as good if not better than the original nokia battery. It might add a little bit of size to the N810, but if battery life is what you're after, battery life is what you'll get.