857D wants to see 12v (well 13.8 in the specs) so 6v won't cut it. The 12v 7ah battery probably won't either because I doubt it could deliver the amperage the radio will suck out on transmit. If it did work at all on transmit, I would give it a couple minutes tops before falling below minimum volts necessary. I would be more scared of the thing melting having it sucked dry in minutes.
Go with freshly charged car battery if you are hauling it around or better yet connect up to battery of running car.