I know of nothing battery-powered that will provide real gig power.
For an outside gig for that many people, where they might be having any conversation at all, you simply cannot find anything. You would need a couple of hundreds watts minimum to even make an impression, and that number of watts for any period of time would require multiple car batteries worth of power.
You can find some things which will provide at least some support for a quiet audience:
http://www.rentcom.com/catalog/Battery_PA_Systems.html
I don't know where you can rent this one:
http://www.pyleaudio.com/itemdetail.asp ... PWMA950CSD
I wouldn't count on either unit giving you battery power for any significant period at a high volume -- rechargeable batteries have a long history of disappointing their users.
I suggest you look into getting some power, either with a generator or arranging a different location. Or simply go battery-powered and accept its limitations, which are basically that everyone has to gather around and shut up.