I have both an Engel 45 quart and a Whynter/Edgestar 86 quart. I can plug both of them into the cig lighter and run them as long as the vehicle is running. Not running you will get an hour both or maybe 2 if just the small one, and still start the car. The nice part is you can also transport non frozen stuff.
Dry ice is the way to go if it is only frozen stuff unless you cant obtain it easily.
Here is a review I did years ago on power draw from these coolers.
https://www.amazon.com/EdgeStar-Qt-Portable-Fridge-Freezer/dp/B0012EKHJM/ref=cm_aya_orig_subj
