In general there is not enough power from a battery. Just pick up a real 7 inch buffer and you immediately know why before you ever plug it in. Can't store that much energy in a battery, it would be depleted almost immediately.
That said, what you are really doing is not polishing, but waxing. Fundamentally different and does not REQUIRE that much energy (though nice if you have it). So, you likely can find "something" cordless that will likely achieve what you want. Just don't expect to do any actually polishing with it. Wax application/removal will be about the limit.