Okay, the results are in!

First off, here's my car in its usual parking spot:

It's 11:00 AM. I park facing east, and as you can see, the trunk will be baking for the rest of the day.

Here's inside the trunk (That cooler sat in the garage for a long time. I should have dusted it off before I took the pic! ) :

L-R: GHB and wide-brimmed hat, Cooler, Toolset and junk bag, unused floor mats, and reusable grocery bags.

And here are the results:



As others suggested, a cooler doesn't prevent stuff from getting warm, it just smooths out the curve. What I didn't realize was that meant it would also delay things cooling off!

I've had four .7L bottles of store-bought water in that cooler for two weeks (along with 2 1L bottles in the GHB). I cracked one open just now, and other than being tepid, it tastes fine. I intend to continue that over longer and longer periods, to get a feel for how long I can leave water in my trunk.

Fantastic information.

Good info. What thermometers did you use to measure the various high/low temps?

Sitting on the junk bag you can see my little indoor/outdoor thermometer, and the wire leading to the remote sensor inside the cooler. It's just a cheapie that I've had for several years, but when I've compared it to my Radio Shack Wireless indoor/outdoor at home, it's always been within a few tenths.

"Trunk" is the "Indoor" reading, from the unit itself.

"Cooler" is the "Outdoor" reading, from the remote sensor.

"Outside" I got from Weather.com, "High" is at work, and "Low" is at home. I live in an area of San Diego called Normal Heights, about 10 miles from the coast, and work in El Cajon, about twice as far inland, on the other side of a low ridgeline.

Even for El Cajon, this week was hot. Twice we broke the daily record, and we came within one degree of the record for the entire month.

(I just noticed a typo. Friday was 106, also. All gooder, now. )

Those are actually very good results Compugeek.
The cooler seems to be keeping what is inside it somewhere between 26 and 40 degrees cooler than the high temperature in the trunk of your car.

Would I be right to assume the cooler was fairly empty, and that if it was full the thermal mass might keep the stuff inside even cooler?

Or would that be incorrect and it would not matter much?

That's a cheap little six-pack cooler. The 4 .7L bottles laying on their sides all but filled it. I couldn't put a fifth one on top of them and still close it.

I may pick up one of those styrofoam "disposable" coolers in the future, and see how it does. For now, I'll just be checking periodically to see how the heat affects the water.

I figure warm, slightly plasticky-tasting water is still better than NO water. Especially in SoCal.

Thank you, 'Geek, for taking the time to do this. It certainly does answer some questions. And I hadn't considered the slowed cool-down time, either. Funny what you learn when you actually do it!

Sue

Excellent data, Compugeek. You are really living up to your name (and I mean that in a very good way). I had been considering using a cooler like that to store emergency food in my truck down here in Houston, but it seems it would only extend the lifespan of the food a little bit.

So now I'm picturing a modified cooler with an automated vent fan that comes on two hours after dark to flush the hot air out of the cooler...

-Blast, dreaming

Could something like this 12V cooler be powered by something like this solar battery tender?

Place the cooler in the trunk, and run the wiring to the PV in the rear window of the car?

Kinda a stretch I know, but an idea...

I only need the cooler to get me through hot areas and when I stop it's someplace with ice to rechill the interior.

Compugeek gave me a great idea with the indoor/outdoor thermometer though and for that I thank him. I ordered one of the wireless units and will instrument the back/storage area of the truck and may put a second sensor inside the cooler. First I get to experiment with temp differentials over time, but after that it can be used to monitor storage temps driving across the desert on trips. If cooler temps get too high it's time for more ice.

Unlike Compugeek's car, I don't have that big piece of glass acting like a greenhouse over the bed of my truck -- never liked hatchbacks and SUV's are definitely no better. To play on Desperado's solar panel idea -- maybe a larger flat panel could be used to block the light coming through those big pieces of glass and provide power to a cooler -- that might be a great idea.

I have a white pick-up with a white fiberglass bed-cover which should do a fair job of keeping the storage area to a max of ambient -- or so the theory goes. The black asphalt road may interfere, but to what extent? Now I can find out.