The cooler worked very well when plugged into a standard 120V AC outlet. It worked less well, but still acceptable, when operating off of the car battery as follows: the recommendation to run the unit with the battery saver setting on High may be for when the vehicle is not running. I was driving with it set to High and the temperature was close to 30 degrees when I got to my destination, even though it was set to -4 degrees (the max low) when I left. On the next drive two days later I set the battery saver to Medium and the temperature again to -4, and this time when I arrived at my destination (about 380 miles) the temperature in the unit was at 6 degrees and the contents remained nicely frozen. But if you're running off the car battery when the engine is not running, you may want to set the saver to High, but maybe don't expect the temp to stay in the low single digits.