External Battery Recommendations?

While you can do many things with a tablet these days, you can't do everything, and so I am thinking of moving up to a notebook. However one of my big needs is to be able to check email and work all day long with little if any access to a power outlet. So I am thinking the answer is to get an external battery to add several hours to whatever the notebook might handle natively. Any suggestions for a good external battery that can be used for this?

 

The only external battery I've ever used:

http://www.amazon.com/Lenmar-PowerP...HOEO/ref=sr_1_1?ie=UTF8&qid=1326016905&sr=8-1

It was for a small Acer, I bought just recently. I got 3-4 hours out of each charge. I liked it and it helped when in a pinch.

 

now you may think this is totally stupid and not practical. (hell i think it) but what i did with a projector was to buy a car power kit (those dc to ac things) and hooked it up to a deep cycle marine battery. I dunno how much longer life itll give a laptop or how much longer life it gave my projector but it didnt die on me for 8 hrs. Being batteries you could get more then one and piggy back em to get longer life. The only problem here is that its not cheap but you might already have this stuff.

 

You may want to consider a 9 cell slice battery ThinkPad Battery 27++ (9 cell slice) - Overview but only for:
ThinkPad T410, T410i, T420, T510, T510i, T520, W510, W520

 

getting an additional battery for a specific model is even more practical!