I understand that ambient temperature is a very important factor in Peltier cooling.
Am I right in assuming that my portable cooler can only cool things down to a fixed amount below ambient temperature, then? Say, if it’s 90 degrees on the outside, it can go down to 60. Does that mean that if it’s 50 degrees outside, stuff will freeze in the cooler?
Yep. There’s a lower and upper limit of course, but for the typical human range of temperatures, Peltier devices will maintain a fixed temperature difference across them (for a given rate of heat pumping–which is also dependent only on the temperature difference).
I believe it’s possible to stack or cascade Peltier modules, so as to achieve a greater end-to-end temperature difference, although I expect it’s a game of diminishing returns.
The heat flow of a peltier unit is dependent on a difference in physical properties between two materials times the current. So there is no loss by adding in more pairs in series between to plates nor by stacking hot/cold sides.
The problem is that the units themselves put heat into the system–and they aren’t very efficient to boot. So to maintain the same maximum difference, you need more units on the hot end to pump all the extra heat produced by downstream units. In the past I’ve used a 4-2-1 setup, assuming that each unit is about 50% efficient (the exact number depends on the temperature difference and other factors). And even then I ran into diminishing returns–I couldn’t get below about -20 C (starting from ambient).
This my be stating the obvious, but your cooler undoubtedly has a thermostat that cycles the peltier element as needed to maintain a certain temperature. So even if it is capable of freezing stuff in lower ambient temperatures, it won’t.
My cooler doesn’t have a thermostat. It just runs.
That’s a good point, but this is more a practical vs. theoretical problem.
it’s a very important factor in pretty much any type of cooling 
Yeah, same here. I don’t know about the cooling element itself, but the fan is constantly on as long as the unit is plugged in.
Thanks y’all for the replies, that was very instructive.
How do you know it doesn’t have a thermostat? If you base this on whether the fan runs, as I gather Fish Cheer has, consider that the fan being on or off isn’t necessarily the same thing. I think the cooler would probably be more efficient and require smaller Peltier modules (which are expensive) if it was designed to cool constantly, and run its fan constantly, but adjust the electrical power to the heat pump according to how cold its interior is. That would be a thermostat, just not the kind that cycles the entire system on and off to modulate its effect. Similarly, cruise control on a car does not alternately drive at maximum speed and stand still to achieve an average; it incrementally adjusts the power to the wheels.
Well, why not try running the thing outside (assuming you don’t live in Hawaii or something) with a thermometer inside the cooler and see how cold it gets versus doing the same inside? I would think it would have to have some sort of temperature control, but maybe it’s simply close enough for most ambient temperatures.
I based it on my recollection that the instruction manual stated it cools to 40 degrees below the ambient temperature. This is not consistent with there being a thermostat, but it is consistent with it just running continuously.
Rather than trying to find mine, I found a manual (PDF) online. It seems to cover many models including mine, but it says:
So GreasyJack’s statement is wrong.
Well, whaddya know. I would think it would greatly reduce the usefulness of a portable refrigerator if it turns into a freezer if the outside temperature is in 70’s.
Ah. Well, that’s pretty good, then. I yield. Thanks for the surprising information!