I have a littlebox, with an unusual mod for my T12 powered mics, usually run on "low power". I run it off a 9.6v RC car battery. It's an 8 cell NiMH shrinkwrapped together. I don't remember the miAH, but it's a lot less than a DVD battery, maybe 2600 or something like that.
I charged up an RC battery, check with voltmeter, it says about 10.5v. Do a typical evening show... 3 hours or so. Checked it with a voltmeter, still right up there, did another evening show. And another show. After the 3rd night on the same charge, the battery is down to 9.3 or something like that. It probably would have gone another night, but I decided it was time to put it on the charger. I'm confident I can get "all day" with one battery in a festival setting.