a lithium ion battery pack for a laptop might be a good idea. if you can get an external charger for one.
lithiums hold a very steady voltage full charge to no. they also all have a builtin charge controller and will simply shut down when low on power. (most provide a signal to the laptop for charge level)
i think messing around with a regulated nicad pack or nimh would be worth it if you were building the whole thing from scratch. lithium bat packs can be gotten on ebay for a pretty decent price though.
