Cold-Weather Car Starting Tips

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Figure 1: Temperature Derating.
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Auto tips: Never add acid to your battery; use only soft or distilled water.
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Figure 2: Specific Gravity Correction Factor.
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Figure 3: Freezing Point of Lead-Acid Battery
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Chart: Trickle Charger Parts Layout.

Car starting tips to help MOTHER EARTH NEWS readers cope with starting your car in winter weather. 

Cold-Weather Car Starting Tips

Cold is a stopper. In some pans of the country, the harshest day of the year is practically a traditional holiday-one that’s most widely observed by cars. The drivers, unfortunately, still bundle up to give ‘er a try. Not until the grinding moan of the starter has faded to a series of pathetic clicks will we admit that no creature with ordinary sense (a car, for example) would go out on a day like this for any reason other than to fetch more firewood.

We at MOTHER don’t mind people taking an occasional unofficial holiday, but the premise bothers us. Why should the car decide when we take the day off?

What the Cold Does 

Cold weather makes the engine hard to start for two main reasons. First, oil thickens when it’s cold, which increases friction and makes it harder for the staner motor to spin the engine. Cold also slows the chemical reaction in the car’s battery, reducing its power output. Battery output is usually rated at 77 degrees Fahrenheit, below which starting power drops drastically. As shown in Figure I, (See the car diagrams in the image gallery) at 5 degrees Fahrenheit a fully charged lead-acid battery has only half its rated amp-hour capacity.
You can ensure that your car will start in the worst weather by keeping the engine or the battery, or both, warm. But before we discuss some of the ways – short of building a heated garage – to keep the underhood cozy, let’s review basic maintenance.

Prewinter Tune-up 

Send your car into winter with its edges finely honed by giving it a tune-up in the late fall or early winter. When the engine is struggling against thick oil and a weak battery, it needs ignition and fuel systems that are in top shape.

Always switch to a lighter-weight oil in the winter. In general, a IOW30, 40, or 50 multigrade oil is a good choice, but follow the manufacturer’s recommendations. Even if you use a wide-range, multigrade oil year-round, change it before cold weather sets in.

Viscosity (thickness) increase is one of the ways that oil deteriorates with use.

It’s vital to keep the battery’s terminals clean and securely fastened to prevent resistance. It’s also helpful to wash the battery’s case (particularly the top) with a baking soda-water solution whenever any deposits appear. These accumulations can drain your battery by conducting current directly from the positive terminal to the negative one or to the chassis. And be sure to inspect the connection of the negative wire to the car’s engine block or frame. This junction is just as likely to loosen or get dirty as the one on the battery itself.

Check the electrolyte level and the specific gravity of each cell in the battery at least once a month. If electrolyte is needed, add soft water. You can get a battery hydrometer for testing specific gravity at any auto parts store for less than $5. Test the battery after it’s been sitting overnight without being charged and before you add water or try to start the engine. Use the data in Figure 2 to correct the specific gravity reading for the electrolyte temperature, which in this case you can assume to be the same as the air temperature.

Specific gravity is the best indicator of battery charge, which is of vital concern in winter. A flat battery won’t start your car and may freeze up in the cold. As Figure 3 shows, even a partially discharged battery-say one operating at only 50% charge-may have enough oomph to start your car, but its electrolyte could freeze solid if its temperature drops below 10 degrees Fahrenheit! Try to keep the battery charged enough so that the corrected specific gravity of all the cells is above 1.250. The variation between cells should be less than 0.015.

You may find that the charge level in your battery drops below 1.250 in the winter, even though you drive the car each day and the charging system is working fine. After a difficult cold-morning start, a car’s alternator throws a high rate of charge at the battery to bring it back to full capacity as quickly as possible. When the battery is attacked by high amperage, it takes only a “surface” charge-one that dissipates a few hours after you shut off the engine. (This problem is more common with older cars that have mechanical voltage regulators.) The only sure way to keep a battery fully charged (particularly in winter) is to periodically attach a low-rate charger such as the trickle charger described at the end of this article.

Extraordinary Measures 

In climates where the temperature drops below O degrees Fahrenheit, it’s a good idea to apply heat to the engine compartment at night. We’ve heard of people who build a fire and park the car over the coals (DON’T!) and others who leave the engine running all night. A safer solution, if you get home late in the evening and rise early, might be to throw a blanket over the engine. This could hold enough warmth to get you going in the morning.

More likely, though, you’ll need to run an extension cord and attach an electric heater of some sort. There are three main types of engine heaters: the oil heater, the block heater, and the circulator. The most common oil heater is a replacement dipstick with a plug, but there are also elements that attach to the car’s oil pan. Dipstick heaters that lack thermostats have been accused of overheating oil, and oil that gets too hot loses some of its lubricity and becomes thicker. Oil-pan heaters avert this problem and are simple to install because they are external.
The engine-block heater avoids overheating oil by warming the antifreeze/coolant instead. It consists of an electrical element inserted in a freeze plug opening or other removable block fitting. Convection currents in the engine’s water jacket then indirectly warm the oil and bearings.

A circulator is plumbed into the hoses of the engine’s cooling system. An external element heats the coolant, and a pump forces the warm liquid through the water jacket. Because it heats all the fluid in the cooling system, the circulator probably does the best job of warming the entire engine. This may not make the engine any easier to start, but it will reduce wear in those crucial moments when the parts are warming up to operating temperature.

Auto parts stores may stock (or at least can order) all of these devices, and a number of European manufacturers (Volkswagen, for example) offer them as options. NAPA Balkamp parts dealers sell a dozen versions, including propane-fired models. Sears offers a circulator and a magnetic external heater in its winter catalog. Prices range from less than $10 into the hundreds. Heaters consume as much as a few hundred watts of power per hour but still use less energy than an engine that runs all night.

Another way to make your car easier to start in the morning is to keep its battery warm. People have been known to unhook the terminals and take their batteries inside at night. When they reinstall a room-temperature battery on a subzero morning, it has three times as much cranking capacity as a cold one. That ritual gets a little wearing when repeated day after day, so most folks eventually look for a more direct method to warm a battery.

A battery blanket—an insulated, thermostat-controlled heating element that wraps around the unit’s case—can be found in colder pans of the country, but you won’t find one listed in the catalogs of major parts distributors. Still, a simple heating pad—like the one you bought for that achy wood-splitter’s back—makes a good substitute.

Get a Charge in Life 

You can also keep a battery warm by charging it. TJ Byers’ 3-amp trickle charger will keep your battery up to its full-capacity temperature while ensuring that any losses from surface charges will be replaced. The charger’s output is low enough so that you can run it overnight without damaging a cell, yet it will fully charge just about any battery within a day.

Because it has so many other uses around the house and shop, a battery charger is a particularly good way to handle cold-weather starting.
Of course, you might not take any of the prevention measures we’ve described. If you’d rather take the day off when your car does, we won’t tell.

Build a Trickle Charger

Whether you use it to keep your car battery from freezing overnight or just to maintain a charge in a little-used boat battery, a trickle charger is mighty handy to have around. It’s also simple in design and makes a perfect afternoon project for even an inexperienced tinkerer. Why not build one yourself?

How It Works 

A trickle charger consists of nothing more than a 12-volt transformer and a DC rectifier. The transformer is a standard step-down device commonly found in solid-state appliances, such as clock radios. It reduces the 117-volt AC line voltage to 12-volt AC.
The 12-volt AC output converts to 12-volt DC (the form of electricity produced by batteries) via a bridge rectifier. That DC voltage, placed across a lead-acid battery, puts charge into the cells. Because the charger applies such a small current (about 3 amps), it’s virtually impossible to overcharge or damage cells. Thus the charger may be left connected to a battery for extended periods—even overnight.


The least costly way to build a trickle charger is to salvage a 12-volt transformer from an old appliance and add a bridge rectifier. The bridge rectifier is an inexpensive item you can get from Radio Shack or other electronic parts outlets. Add a homemade or salvaged cabinet, a fuse, and a line cord from an old appliance, and you can have a battery charger for under $5. About all you’ll have to buy besides the rectifier is the fuse (or a circuit breaker), which prevents the unit from catching fire in the event of a failure.

If you’d rather save time than money, you can buy it all at Radio Shack. While you’re there, though, you might want to pick up a few fancy accessories. A more sophisticated version, like the one shown in the photo, has the transformer and rectifier housed in a custom metal box and includes a voltmeter.

Position and mount the transformer, circuit breaker, power cords, and bridge rectifier in the case. The illustration above shows how the parts are connected. Apply a thin layer of silicone grease between the bottom metal tab of the rectifier and the metal chassis before mounting it with a bolt through its center hole. Don’t overtighten the bolt, or you’ll crack the plastic case.

Beginning with the transformer’s primary (black) leads, connect one wire to the AC line cord and the other to the tab on the circuit breaker. Solder the other leg of the line cord to the remaining circuit breaker tab. Use a wire nut or some electrical tape to protect the exposed transformer-to-line-cord splice, and fit a strain reliever to the cord where it passes through the case.

The transformer secondary has three wires: two red and one black. Remove the black wire by cutting it close to the body of the transformer, and tape over the end to prevent shorting. Now solder each red wire to one of the two bridge rectifier terminals identified by a small sine (-) symbol. They’re located catty-cornered from each other; don’t mistake them for the minus (-) symbol.

The minus (-) terminal connects to the minus (-) side of the voltmeter and to one of the leads going to the battery. The battery lead has a black battery clip on its end. The remaining bridge rectifier terminal—identified by a plus (+) sign and located at the truncated comer of the package—connects to the second lead going to the battery and has a red battery clip on its end.

The plus ( + ) bridge rectifier terminal also connects to the voltmeter through a resistor that comes with the meter. Solder it between the rectifier’s plus (+) output and the meter’s plus ( + ) input. You could follow the assembly sequence above but leave out the voltmeter, which would save almost $8.

Using the Battery Charger 

Check the performance of your battery charger by plugging it in and noting the reading on the voltmeter. It should be 12 volts. (Caution: NEVER spark the battery clips together to test for the presence of voltage. This will bum out the bridge rectifier. )
The voltmeter does more than tell you whether the charger is working, though. When the voltmeter is connected to a battery, the reading changes with the state of charge. The following tables summarize battery conditions at room temperature (68 degrees Fahrenheit). You may wish to make a copy of these tables and paste them to the cover of your battery charger for easy reference.

PARTS LIST   Description 
Radio Shack No.  
273-1511 12-Volt Transformer
270-1754 0-15 Voltmeter
276-1180 6-Amp Bridge Rectifier
270-1310 Circuit Breaker
270-343 Battery Clips
278-1255 AC Line Cord
278-1636 Strain Relief
270-252 Metal Cabinet