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My 2009 Ford Mondeo Clipper has a very nice "Max Heating" setting, where it both defrosts front and back windows, and sets the airco to max temperature, max fan speed. When I find my car windows frozen, I always first put the car in neutral, start it, rev up the engine to 2000 rpm a couple of times, and enable this "Max Heating" function. The car heats up, defrosts front and back windows while I scratch the side windows. I scratch back and front windows last, and it goes much easier this way.

A friend of mine told me I should turn off my headlights while letting the car defrost like this, to save the battery so it can heat faster/better. Does this really help? I thought heating was done using gasoline? (I see my fuel usage increase from 0,6 to 1,1 liters/100km running stationary)

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If you are trying to defrost/deice your windows then your headlights should be turned on. If the window glass is frozen and obscured then your headlight lenses are also frozen and obscured. If they are on they will warm up and melt the ice off so the headlights work properly. –  Freiheit Jan 25 '13 at 15:14
    
How do you measure your fuel usage? When at idle, the only correct measure would be volume of fuel per unit of time (L/hour), because your car is going nowhere, ergo it cannot travel those 100 km. –  theUg Jan 25 '13 at 18:46
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4 Answers 4

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Turning off your headlights won't make any difference to your battery, but at idle, you may find your "Max Heating" setting works better without them on. The reason for this is that your alternator can only put out so much current at any given RPM.

So, using completely made up numbers, let's say your Alternator can push out 20 Amps of current at 800 Revs Per Minute (~Idle speed). If your "Max Heating" setting uses all 20Amps to run your fans, aircon and heated windscreens that's fine. But, if you turn on your lights and they use 5 Amps then now your "Max Heating" setting is only using 15.

In my 2003 Mondeo, the fans run noticeably slower at idle than they do when I begin to rev.

But to clarify, if the setting is doing what you need already, there is no harm in having your lights on.

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When the car is idling waiting to defrost, I tend to start some electric consumers in order to increase the load on the engine, so it warms faster.

Also, on cars with automatic idle control, there is no need to rev it up at startup. The engine ECU regulates the engine speed.

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My car doesn't have this "Max Heating" setting, so my experience may not help you, but I have two reasons why I always turn the lights on while defrosting the car:

  • the extra load on the engine actually provides more heat so the interior heats up faster, because the engine revs harder to keep the battery output voltage at the same level
  • the lights being on helps to melt the ice on the headlights, as I don't want to have to scrape ice off a plastic surface
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Window heater wires are electrical (rear window and front wipers area if present). Warm air from the heater is from engine coolant fluid, however there's electrical fan that blows the air through heater core.

Usually cars have several idling speeds: high RPM (1500-2000) till the engine heats up to working temperature and low RPM (700-900) after heating. When you start a car it works on high RPMs and there's plenty of current to charge the battery and power the heaters and lights.

I suggest to check on Dans answer and test if there's a difference in fans speed (noise) with and without headlights on. If the difference is audibly noticeable then it means the car doesnt have enough current to run everything at once. If you turn on lights that means heater wires and fans will be getting less power and would not work on their full.

P.S. In my area I turn headlights "On" to heat them while I clean the rest of the car, so that I can scrap the ice from them with ease :)

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