Dell external battery

My Dell Latitude XT3 works fine, but the battery runs out in a couple of hours. Is there a small external battery that I can buy that can make it run for 5 or 6 hours? Or are there any compatible after market batteries that fit and have more life than mine?

The standard main battery is a 6-cell 44 WHr, they also have a 9-cell 76 WHrmain and a 97 WHr battery slice that attaches to the bottom.

Thanks for the information. The computer belongs to my employer and since I was doing this for my convenience those prices are too high (for me) to pay. Anything less expensive third party out there?

Slice for $135 - no endorsement of the vendor.

You can also go for a generic external battery pack like this:
http://www.intocircuit.com/?page_id=2127#toggle-id-2

And charge your laptop using an adapter plug. If my math is right, that particular one has about 65Wh of capacity, although it’s hard to say exactly because the vendor doesn’t provide that figure.