This refrigerator/freezer recommended by a friend worked well at a re-enactment camping event 4th of July weekend.
I ran it at home to see if it worked, ran it in the car using the included plug for the cigarette lighter power thing, plugged it into the Jackery battery at the event Friday.
I unplugged it when I packed up on Sunday. It was still holding at 37F, and the Jackery battery said it still had 52% left. So both of them worked extremely well. Do read the instructions - there are different settings for plugging it into a regular power outlet, or for using a battery pack.
Everything stayed amazingly cold.