I need a external battery for the samsung galaxy tab 7"

I just got a galaxy tab 7" and want to get an external battery for it. I was looking at some but they didnt have the galaxy tab as compadable. Cant you just plug the usb charger into the usb port and have it work? I was looking at this one. http://www.amazon.com/PowerGen-External-Capacity-sensat...

Will that one work? I would also like to be able to charge my phone at the same time. Its a htc inspire. Dont really need 2 usb ports though. I would like to buy it of amazon and would like to have it have prime or free supper saving shipping. Dont want o pay over $50.
Thanks