Thermoelectric Coolers
Some modern coolers are thermoelectric, plugging in to a car's cigarette lighter socket. Rather than using a compressor and refrigerant such as a refrigerator or other heat pump, these use the Peltier effect along with an external fan to draw away the heat. By reversing the current, this concept can also heat the contents instead of cooling them, useful for keeping meals hot from a drive-through, or even to keep items from freezing in severely cold climates.
Thermoelectric coolers typically can drop the temperature by about 40°F or 22°C below ambient temperature, or can raise it by at least that much; this is really a function of the effectiveness of the box' thermal insulation. Some better units even have digital thermostat controls. They do draw a significant amount of power, however, and can drain a non running car's battery so much so that it cannot start. Most electric coolers have an undervoltage shutoff at around 10 or 10.5 volts to prevent this. Many come with power adapters, which use an electronic transformer to convert AC mains or line voltage down to 12 volts, with a lighter-like socket for the cooler's cord to plug in. Some also have a crossover-connection device to reverse the current for heating service.
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