In winter, driving with a broken defroster can be a nightmare, especially if the cause is breakdown or bypass heating element . The last thing anyone wants before a long drive is to stand in sub-zero weather, painstakingly scraping across the thick layer of ice on the windshield, only to climb inside and find the fogged up window.

In such cases, the only solution is to install a defroster, whether it be replacing a leaking heater core, rear defroster heater grille, or the entire window. Repairs like this can be costly, and suffering through a long cold winter can look less and less burdensome when those dollar signs add up.

Problem with 12V defrosters

There are two main problems with heaters and 12V defrosters, both of which are related to the way these devices work. The first is related to the power supply, which will usually be the cigarette lighter socket.

Because cigarette lighter sockets and 12V auxiliary sockets tend to fuse at 15A or less, you can’t run a very powerful heater from them. And because 12V defrosters depend on heat to operate, you won’t see the same results you got from your HVAC system before the heater failed.

Another issue has to do with relative humidity and the fact that dry air absorbs more moisture than moist air. That’s why you can clean the windshield of a car that has a broken heater while the air conditioner is running. The air conditioner dehumidifies the air that passes through it, allowing it to absorb moisture from the windshield.

Since 12V defrosters do not have this capability, the air these defrosters deliver to the windshield has the same humidity as the rest of the air in your car. The only difference is that it is warm and not freezing.

Less effective than a hair dryer?

To understand why 12V defrosters aren’t as good at clearing dust, it’s helpful to compare one to a device most people are familiar with: a hair dryer.

While there are many 12V defrost devices out there, a typical model designed to plug into a cigarette lighter only puts out about 200W or so. For comparison: the hair dryer is designed for power from 800 to 1000 watts.

Does that mean you’re better off using an extension cord and defrost your windshield with a hair dryer? Technically sure, but if you’re using a cord anyway, you also have the option of using a normal heater to heat your car.

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