Question for OT Street Musicians

I've got a buddy who busks in the quarter and is in need of a portable battery to power his guitar amp. While I'm sure we could somehow rig up a car battery or something similar I think he's looking for something a little bit nicer and compact. The last one he bought was a great battery but it just created a horrible feedback which made it useless for musical purposes. Does anybody have any good ideas on something that would work for him?