Looking for an external battery/power source for a laptop with a considerable run time....Does anyone know of one? For example, when you go to the hospital and they have that registration cart with a computer on it, but it is all wireless. I'm assuming there has to be some sort of battery power source that allows it all to run. I'm looking for something like that, but not having very good luck with my google-fu and have never had to shop for such an item before. Any help would be appreciated.