12 volt coolers

First off, the idea of a normal bar fridge will not work at all. The vibrations will break cooling lines, and the refrigerant spraying around a wagon is not nice. If that does not happen, the oil is not likely to sit properly in the compressor, and that will fry pretty quickly. Better the 12 volt one. When it comes to how long the battery on the car will last, that will vary greatly, depending on the size of battery in the car, and the state of charge. Nora runs am 1,100 CCA battery, 1 gauge battery cables, and a 200 amp PowerMaster alternator. Most of the time, I can run half the world off her with no problems.

 

Battery protector

There are small devices available wherever electronics are sold, maybe Radio Shack or Wally World even, that you hook up to battery power. These sense low voltage in a battery and cut off whatever is attached. This would be good if worried about leaving the cooler plugged in for long times while parked.
Back in primitive times there was an ice house in every decent sized town where we bought a 25# block of ice for the ice chest. This would usually last a long weekend and sometimes a week if the cooler is not kept open too long.
I stress this because some people even leave their refridgerator door open long times during power outages.
Since ice houses have bveen closed down for a long time there were no places to get ice except at home where we made it ahead. Now there are ice machine vendors again. But for what they charge for ice over and over, an electric thingie is cheaper over time. Also no ice water to leak or dribble off your cans in your beautiful station wagon.
There's nothing like a cold soda or beer from a cooler. Especially from someone else's cooler.
Of course, when driving, a cooler with alcohol in it must be out of reach of the driver.