I have used a 12 V cooler for a couple of weeks. I put it behind the rear seat as you can hear the fan when you are stopped. I find it very handy to have cool water to drink when I am out and about. When the car is off the cooler does not operate so I really cannot say it will drain the battery much.
Nice to know, folks...but nobody has been able to answer my question yet. How long will the cooler keep something cold without power (ie; like if I go watch a movie and get the biiiig popcorn bucket, will my frappachino still be cold when I return?).
I have had a few 12 volt coolers over the years. Most of them draw 4 to 5 amps when running. The better ones have a sensor that will shut the cooler off before it drains the battery, but most do not. The good ones have a thermostat to maintain a set temperature, while the others just run constantly. The better ones have two fans, one outside to exhaust the heat and one inside to move the cold air around while the others only have the outside fan. The better coolers also have more insulation which allows them to cool the contents to a lower temperature and to keep the contents cool longer.
When not plugged in, they keep things cool for about the same amount of time that any cooler would. Most can cool the contents down to about 40 degrees F. so, depending upon the outside temperature, they should keep the contents cool for at least a few hours when not plugged in.