That would be epic, but at that point, why not wire it into your car. You'd have to change your idea of what portable is, but it would still be completely wireless...

a laptop uses about 20w. and they have power saving systems. they scale back the processor power when its not being taxed to save even more power. etc. My friend has a "laptop" thats about the same as a 360, and it has a 8aH battery in it, and it goes flat in about 25 minutes because it uses so much power. laptops only last any period of time on battery because they are tuned to be power-efficient. the 360 really isnt.

You guys sure know how to make a person feel welcome..

If your looking at batteries then you will be forking out at least 700us for one that is decent.
Why not simply jtag the console and use a 500gb hdd?
I know that a guy was screwing around with a terabyte and he got it to work.
Take your pick

oh okay if there was just a way to give the xbox power save abilitys, i guess u would have too start from scratch too do that and the hell with that you'd be there for 5 years trying too figure that one out

Remember mooks, the 360 was never intended to be made into a portable, just really really small. It would be like trying to take a desktop PC and turn it into a laptop. While technically possible, it would require a lot of work and money just to get it to the point where it could last an hour. These devices were never intended to be portable.... at least with the PC it would be possible to use software to lower it's power consumption. The 360 never has a reason to try to lower it.

FWIW I think it would be a cool and terrifying mod to see two of these batts wired in series plugged into a custom 360 laptop to serve as the "power brick" that allows it to run for a few hours while not being connected to a wall.. if only there was someone with $10,000 lying around.