Trick for starting car in the winter. Fact or fiction?

In class last week my EE professor said the following:


I understand his reasoning. But at the same time, turning on the headlights will also deplete some of the energy from the battery.

So does this work? I’m skeptical. If it is true, I figure there must be an optimum amount of time to leave the headlights on. Not sure what it would be.

According to a major battery remanufacturer, your instinct is right on:

It’s -30C right now, and I’ll be starting my car in a couple hours. I am not going to try this. As it happens, I have a block heater plugged in and I expect no difficulties.

Q.E.D. I think anyone would say that warming the battery by plugging it into a wall outlet would be better, but the OP assumes that we have the car w/o any external power.
What you want to do is start the car with as little power taken from the battery in total. What we have is the choice to crank the engine w/ a totally cold battery, or to preheat it by using the headlights (yes using battery power). It seems to make sense that at some temp’s it might be worth it to use some power to preheat the battery, this will deliver more current and voltage to the starter motor, which should help turn the motor faster and make it more likely to start.

Unless you know what the state of the battery’s charge and overall chemistry is, it’d be a crapshoot, IMO. Might get you started, might get you stuck. Additionally, I’m unconvinced that turning on the headlights for only a minute is going to be enough to warm the battery significantly. The internal resistance of a car battery is extremely low–on the order of a few tens of micro-ohms. Very cold weather will probably increase this; let’s be very generous and say .025 ohms, for the sake of argument. If your headlights draw, say, 10 amps, this means that 10[sup]2[/sup] * .025 or a whopping 2.5 watts are dissipated inside the battery. I bet I can generate more than that just breathing on the thing.

Only really cold days, many cold batteries have 1-2 ‘starts’ in them when you come out in the morning. I suggest you use that power very wisely, with all accessories off and leave the only chore for the battery to do is to crank the engine.

It is wise to use the proper oil for your climate and have it changed at ‘extreme’ intervals.

If you’re battery’s good, a few minutes of lights shouldn’t prevent your car from starting. I’m not saying it’ll help the battery “warm up,” though. You’re talking cold cranking amps (CCA), which is the ability of the battery to supply current in a short amount of time, not the total amount of amps that the battery contains until depleted. So it doesn’t help, but it shouldn’t hurt, either.

Trying to start the car in very cold (the current) weather, will not be improve by draining any power. A starter is also a large drain, just like your headlights. You will find that 30 seconds of trying to turn over a car and failing to start it doesn’t make it work better. Leaving it sit an hour may get you one revolution the after that. The chemical reactions work a lot slower in 10 F or 0 F weather. The only warm the battery solution that works is to remove it and leave it in your house. Also important is to have 5W rated oil so it’s thinner than grease in the morning. Remember heat increases resistance in wires. Ask the professor if you can jump start a car better in extremely cold weather. His logic will say yes, experience say no.

But heat also increases the chemical reaction inside the battery giving more available power. I’m not saying that the headlight on side is correct, just that there are reasons on both sides and as Q.E.D. put it it’s a crapshoot.

My point in saying ask the professor, is if the professor thinks the battery does what he claims, then see what he thinks the slight difference in resistance because of the cold wire makes in theory. In other word’s neither one makes any appreciable difference for a person trying to start that cold car. Theory is all nice and fine, but is it practical in the application stated. No. The car will not start up better on a borderline start because you used the head lights for a minute. In fact what happens is the slow chemical reaction gives a brief surge of power that drops greatly after initial discharge. Do you want that to occur through the starter or headlights? The outdoors during cold weather will quickly suck away any little heat the reaction in the battery outputs, which is a minor side point.

This on the other hand will help, because it is can keep the battery warm.

I’ve heard this too, before. But is there any reason not to try starting the car instead of turning on the lights? If running the lights will help heat up the battery, so will trying to crank the engine, and maybe it’ll start.

If this were true, those of us living in cold weather would practice it.
We don’t.

It isn’t.

I left my headlights on all day long a few years ago when it was -40C, and when I went to go home the engine didn’t crank once! :mad:

Yes, it allows the heat generated in the plates to get into the electrolytic fluid, allowing the time for the heat to travel and to have the fluid at a constant temperature, instead of a temperature gradient from the plates to the cold ‘middle’ section.

But if QED’s calculation of 2.5W is even close to correct, that’s just pissing in the ocean at -30. There’s more wind chill than that unless it’s dead still - you’d be looking at 1000 watts/sq metre or more in even a light breeze.

Yes it would seem so if his calculations are correct, which we do not know if they are. I also wonder if there is some gelling of the electrolyte that a small current may dissipate, or even perhaps some convection currents.

Another question comes up, is the wattage used in a device a good measure of the heat given off in the battery, or is there heat given off just in the chemical reaction itself or even heat absorbed?

Back in the days before multi-grade oil, or oil dilution systems on aircraft, in the Alaskan bush, the worm oil was drained after the flight and the battery removed and put in the warm tent, cabin, igloo, what ever. If room allowed, people were allowed in.

pre flight was to build a fire under the engine with a blanket or tarp over it to trap heat. the battery inserted, blanket removed and engine started. when / if if started the wrm oil wwas poured back in while the engine idled. They never put the oil in until they were sure the engine would run because at - 30 to -50 if the fire under the engine was not possible or too little, the oil would get to think very quick in a cold engine and then the would never get the engine started.

When very cold, warm battery and warm oil are the name of the game. I have even used a hair dryer to to warm the carburetor
so as to get good atomization for starting…

With todays fuels, oils and injections, we don’t have to get too wild with things until you get into the double digit minus columns. A lot also depends on how much if can affect your life span. When it is very very important to survival, you tend to take the extra steps… Or you should…


We have block heaters, though!

Heh. There’s nothing worse than that feeling when you turn the key, and absolutely NOTHING happens. Not even the clicks.

How does wind chill affect a battery inside an engine compartment?