Do 12V Defrosters Work, or Is There a Better Option?

What do you do when your factory defroster fails you?. Martin Hospach / Getty Images

Question: Do 12V Defrosters Work?

My defroster is on the fritz, and I hate wiping condensation off my windows in the fall and winter. It doesn’t seem to matter what I use, because I always end up with streaks that I never see until the sun or somebody’s high beams suddenly turn my windshield into an opaque mess that you’d need x-ray vision to see through. Do 12V defrosters work, or am I going to have to actually fix the broken defroster?

Answer: 12V Defrosters Are Not Created Equal

Driving in the winter with a busted defroster can be a nightmare, especially if the defroster is “busted” because your heater core is plugged or bypassed, leaving you with no heat. The last thing anyone wants to do before a long commute in sub zero weather is stand out in that sub zero weather, scraping diligently at a thick layer of ice on the windshield, only to climb inside and find that the window is foggier than London before all the pollution controls kicked in.

While a broken rear defroster isn’t quite as big a deal, it’s still useful to be able to see out your rear window glass just in case you feel like changing lanes or merging without causing an accident.

So what do you do, other than just grin and bear it through chattering teeth, if your defroster is broken? Well, the only real solution is to just fix the defroster, whether that means replacing a leaky heater core, a rear defroster heater grid, or even the whole back glass.

Of course, repairs like those can be some pretty expensive operations, and suffering through a long, cold, opaque winter can look less and less onerous as those dollar signs stack up.

Electric defrosters are an option, but you might not be happy with the results.

The Problem with 12V Defrosters

There are two main issues with 12V heaters and defrosters, and they’re both related to the way that these devices work.

The first has to do with the power supply, which is usually going to be your cigarette lighter socket.

Since cigarette lighter sockets and 12V accessory outlets are usually fused at 15A or less, you just can’t run a very powerful heater from them. And since 12V defrosters rely on heat to work, you’re just not going to see the same results that you used to get from your HVAC system before your heater core broke.

The other issue is related to relative humidity and the fact that dry air is able to absorb more moisture than wet air. This is why you can defog a windshield with a car that has a broken heater core as long as the A/C works. The air conditioning dehumidifies the air that passes through it, which allows it to absorb moisture from the windshield.

Since 12V defrosters don’t have that ability, the air that they blow on your windshield has the same humidity as the rest of the air in your car. The only difference is that it's lukewarm instead of freezing.

Defrosting Vs. Defogging Vs. Demisting

Another problem with 12V defrosters is that the word "defroster" means different things to different people. The terms defrost, defog, and demist are often used interchangeably, although they can refer to a handful of very different activities.

If your windshield is literally "frosted," meaning it is covered in ice on either the interior or exterior surface, then a 12V defroster can actually do some good. While it still won't work as well as a functional HVAC system, even a modicum of head pumped out by an anemic 12V defroster can help loosen up ice and make it easier to chip away.

If there isn't any ice involved, and you're really trying to defog or demist your windshield, a 12V defroster is much less likely to yield anything like a satisfying result.

One of the nice things about a functional HVAC system is that turning on your defroster, with both the A/C and heat on, provides a couple benefits.

The dry air absorbs moisture from the windshield, which effectively defogs it, and once the engine is warmed up enough, the heater will warm up the interior of your car and make it a whole lot easier to remove any accumulated ice or snow from the windshield.

Less Effective Than a Hair Dryer?

In order to visualize why 12V defrosters aren't that great at defogging, it may be useful to compare one to a device most people are familiar with, like a hair dryer.

While there are a lot of different 12V defrosters out there, a typical model that’s designed to plug into a cigarette lighter will only put out about 200 watts or so. By comparison, your hair dryer is probably rated at 800-1,000 watts or so.

Does that mean that you’re better off running an extension cord out and defrosting your windshield with a hair dryer? Technically, sure, but if you’re running a cord out anyway, you also have the option of using a regular space heater to warm up your car.

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