It hits you like an electrical shock just as the airliner door slams and the flight attendant oh-so-politely instructs you to turn off your cellphone. Less than an hour ago, you parked your car in the middle of a 10-acre airport parking lot, expecting the vehicle to take you home when you return in five days. Except you realize that you forgot to turn off the headlights after your predawn departure from home. Great. Now when you get home you’ll have to call the airport-approved towing service and drop close to a hundred bucks for a jump.

This project was originally published in the September 2000 issue of Popular Mechanics. You can find more great projects at Popular Mechanics DIY Central.

Step 1: Neglect Is Abuse

Lead-acid automotive batteries are actually remarkable at delivering extremely high current for the few seconds it takes to start your engine, even in extremes of heat and cold. What they are not good at is being able to recover after delivering more modest amounts of current for a long time. Irreversible chemical changes take place. Specifically, sulphate needles that bridge the separator between the positive and negative plates form when a battery is deeply discharged. These needles not only internally short the plates, causing a high self-discharge rate, they also coat the plates and interfere with the normal lead/sulfuric acid reaction that makes electricity to spin your starter motor and run your fuel injection and ignition long enough to coax your engine into life. Before you do anything else, check to see that the dead battery isn’t frozen. While a fully charged battery is almost freeze-proof, a highly discharged battery can freeze when temps hit the low 20s. Remove one or two of the filler caps and look for ice crystals in the electrolyte. Don’t try to charge a frozen battery. It won’t work, and will damage the battery further.

Step 2: Safety First

Your first task is to get jumper cables. Preferably nice fat ones with a quality, heavy-duty set of clamps. This is more than just tool envy—there’s a lot of current passing along those wires, more than an arc welder, at least for a few seconds. Resistive losses in the cable can reduce the voltage available to your stalled car’s electrical system to the point at which it will still be difficult to start, even with a healthy donor battery and alternator adding their all to the mix. It’s important to use fat cables and to have good electrical connections to reduce these losses to a minimum.

A set of 6-ft. cables won’t do you much good if you’re parked nose-in to a parking space—unless you feel up to pushing a 3-ton SUV back a truck length to make the engine compartment available. Twelve or 15 ft. is better, which makes using heavy-duty cables more important because resistance losses are proportional to the length of the cable. Keep your cables clean and dry to prevent corrosion from becoming a high-resistance factor.

Step 3: Doing the Deed

It’s nitty-gritty time. You’ve got cables, you’ve got a healthy donor car available. Open the hoods and position the donor car nearby, so that your cables will reach.

Some cars, like the one illustrated on the previous page, have a remote positive terminal someplace in the engine compartment. The battery is mounted in an inaccessible area of the engine compartment or in the trunk. As for where to attach the jumper cable, this junction will be clearly marked and covered in a red plastic sheath. If in doubt, consult your owner’s manual.

Wear eye protection, even if it’s only a pair of sunglasses. Once in a blue moon, a battery will explode when you try to jump it. Explode? Yes, explode. It’s caused by hydrogen gas, which is normally vented by a battery that’s being charged or discharged at a high rate, say, when you’re trying to start a car, the battery runs down and you need a jumpstart. Hydrogen is explosive, and a spark from making a connection can ignite it. It won’t be a big explosion, but it can certainly blow the top of the plastic battery case off and spray acid into your eyes. Connect the red clamp on one end of the jumpers to the positive terminal on the dead car. Verify the polarity of the terminals by the plus symbol molded into the battery case. Don’t just use the red terminal—someone may have installed an incorrect, red-colored terminal onto the negative pole of the battery. Do the same on the positive terminal of the donor car. Start the donor car and let it idle. Lights, heaters, stereos and rear-window defrosters—all electrical drains—should be off. If possible, cover the dead battery with a shop towel or a sheet of cardboard. Any acid that manages to bubble out of the vents will wind up on the cloth instead of on your clothes or the paint on the fender.

Connect to the negative terminal of the donor car’s battery with the black clamp. Verify the polarity. Now connect the remaining black clamp to the dead car’s engine block, an accessory mounting bracket or a protruding ear on a manifold. Use the battery’s negative terminal as a last resort. This procedure will generate any sparks far from any hydrogen gas venting from the battery and reduce the risk of explosion. Now wait. This will let the dead battery recharge slightly. It will charge more when the dead car starts, but it will help the donor car’s battery start your engine a little if you give it a quick shot of charge. If the dead car’s battery had enough charge left to make the solenoid click and run the interior and instrument lights, then a minute or so is enough. If the battery was dead, dead, dead, give it 5 minutes or so.

Step 4: Crank, Zoom

Crank, Zoom Now you can actually try to start your dead car. When the car starts, let it run at fast idle for another few minutes still connected to the donor car to continue charging by both alternators. Idling provides only a modest charge rate, so after a few minutes the best thing is to drive the car at normal speeds for 30 to 60 minutes. Remove the jumper cables in the reverse order. Wash your hands if possible to prevent any battery acid from bleaching your clothes or getting into your eyes.

Step 5: Options

If your RV or second car is in storage for extended periods of time, the battery will self-discharge. The simplest solution is to use a trickle, or maintenance, charger. These chargers will automatically adjust their charge rate to a safe level, low enough to keep the battery at 100 percent charge without cooking it. They’re not recommended for charging dead batteries—just maintaining them.

If you’re not in a crashing hurry, you might find cigar-lighter cables will get you started. These gadgets just plug into the two vehicles’ lighter plugs, providing a modest level of charging, but not carrying enough current to start a car with a truly flat battery. But if the engine will almost start, a 10- or 20-minute charge will get you on your way, out of the weather and with clean hands. Remember that the lighter socket on the dead car must be electrically hot with the key off, or you’ll need to turn on the key in the dead car to complete the circuit.