Battery Question Asus N81vg

Hello, I have an Asus N81vg, and I am not too happy with the battery life; it's just enough to get me through my two classes, but next semester, they'll last an hour longer which it won't be able to handle.

I was looking at mobile-power-packs but they seem to be extremely pricy and only around 2 hours out of one of them. If anyone knows a good one, please tell me.

Also, the battery in the laptop is an A32-A8, +11.1V-4800mAh. It is a 6-cell, and I'm wondering if there is a 12-cell equivalent or one with more charge capacity.

Thank you!

By mobile power pack I meant something along the lines of http://www.engadget.com/2009/08/28 [...] 000-lapto/

However, the battery does seem to come cheaper as far as some sites I've seen. Thanks for your insight on Asus' usually making only one type of battery for each notebook.