[Kage]

one of the mentors from our team brought a portable laptop charger, and it worked phenomenally!
it was kinda like what you are suggesting, but in a light, conviniently small package. it weighed no more then 3 pounds, and was about the size of a letter envelope, and about a half an inch thick. it powered the laptop for the entire day whenever the laptop was not in the pits, and it still had charge to spare at the end of the day! (it is also re-chargable itself, so its uses are unlimited)
our classmate battery never dropped below green!
it definitely saved our butts in the finals.
I was happy we had it when I walked around and saw red battery indicators on some of the other teams' screens...

If I knew the model number or anything, I would tell you, but I don't.
Maybe Ill ask our mentor at the next meeting, and post it here. the thing was a lifesaver!

EDIT: AND!
we talked to the head ref, and he said that it was legal to keep it plugged in durring a match, so the laptop did not go into power-conserve mode.
It seemed to speed up our camera-image speed a bit.