The failure point on a long summer drive is almost never the first three hours. It is hour five, somewhere past the last decent rest stop, when you reach into the cooler and pull out a can that is the same temperature as the door pocket. By then the ice has turned to water, the water has warmed to room temperature, and the whole box is just a bucket of lukewarm soup with drinks floating in it.
Most of that heat did not come from the air. It came through the floor of the cooler, from the transmission tunnel and exhaust routing underneath the rear seat, and through the top from direct sun coming in the rear window. A cooler sitting on carpet behind the driver’s seat is sitting on one of the warmest surfaces in the car. Lifting it onto a folded towel or a cheap foam mat buys real time, and it costs nothing.
Same goes for anything else riding back there. Lithium packs lose cycle life sitting in a hot trunk, and they want exactly what the cooler wants, which is shade and some distance from the floor. Not ice. Condensation on a battery is its own problem.
Ice, insulation, and the third option
Block ice outlasts cubed ice by a wide margin because it has far less surface area for the same mass. If you can freeze water in a loaf pan or two the night before, do that instead of buying a bag of cubes. Then fill the dead air. An empty third of a cooler is a third of a cooler working against you, since air transfers heat faster than packed contents do. Crumpled newspaper, a rolled beach towel, extra bottles you froze solid, anything that displaces air helps.
Pre-chilling matters more than people expect. A cooler that has been in a hot garage starts the trip by pulling heat out of its own walls into your ice. Bring it inside the night before, or sacrifice a few cubes to cool the shell for twenty minutes before you load it.
Then there is the category that skips ice entirely. Compressor-driven 12V car refrigerators have gotten considerably cheaper and quieter over the last few years, and they hold a set temperature regardless of how long you have been driving. The tradeoff is draw on the battery and a fixed footprint you cannot collapse when it is empty. Capacity, compressor type, and whether the unit can run off a portable power station once the engine is off are the specs that actually separate them, and a model-by-model rundown of those differences sits at https://www.allnewsandissues.com/car-refrigerator-recommend-2026/. Korean retailers stock a far broader spread of these units than most outdoor stores here do, from small single-zone boxes up to dual-zone drawers, so comparison writing on the segment accumulated there while the selection in the US was still thin.
Whichever route you take, open the lid less. Every opening dumps the cold air out the top and replaces it with hot air. Keep the drinks you want in the next hour in the door pocket or a small second bag, and leave the main box shut.