How to Use Jumper Cables

jumping car battery — If you drive a car, you need to know how to use a pair of jumper cables. DonNichols/Getty Images

What could be worse than walking out to your car after a long day of work only to have a dead battery. Maybe you left an interior light on or your battery has simply reached the end of its life. Either way, car batteries don't typically give you signs of trouble until it's too late, and then your car just won't start.

So no matter how old (or new) your car is, owning a working set of jumper cables — and knowing how to use them — is a must. First you need the right pair of cables. When you really need them, any set is better than none at all, but if you're buying new jumper cables, Popular Mechanics suggests ones that are 4 to 6 gauge and at least 20 feet (6 meters) long.

Those extra-long cables will come in handy and make connecting batteries easier if you can't put two cars directly next to each other. Also look for jumpers with thicker cables and heavy clamps. They'll be more durable and last longer.

Of course, when it comes time to jump your battery, it's very important to handle the cables properly. Always keep the red and black clamps from touching, and be sure the cables stay apart when you're getting set up — especially once you have the clamps connected to a live battery. If the clamps touch when they're "hot," it could short out one or both cars and will definitely create some sparks.

Since using jumper cables incorrectly can be dangerous, follow every step precisely. Most jumper cables have an instruction card or label so you can make sure you're connecting them the right way.

How to Connect Jumper Cables

Begin by parking the vehicle with the good battery next to the car with the dead battery.
Open the hoods of both cars and locate their batteries. If the batteries are covered by plastic hoods, remove the hoods so the battery posts are exposed.
Locate the positive (+) and negative (-) posts. The positive post may be red, but it's not always, so it's best to look for the plus or minus sign to determine the post's polarity. (Before you attach the clamps, be sure to remove any dirt from the posts.)
Next, get out your jumper cables and connect one of the red clamps to the positive (+) post of the dead battery. Make sure that the clamp is firmly connected. Clamp the matching end of the same cable to the positive (+) post on the working battery.
Connect one of the black clamps to the negative (-) post of the good battery. Make sure the clamp is firmly connected. But instead of connecting the other end of the cable to the negative post of the dead battery, clamp it to a bare, metal surface on the engine of the dead car — like a bolt or a screw. This will provide grounding for the jump start.

Jump Start the Battery

Start the functioning car's engine first and let it idle for a few minutes.
Start the dead car's engine and let it idle for a few minutes.
If the dead car starts without any issues, carefully disconnect the black clamp from its battery first. Then disconnect the black clamp from the other car.
Finally, remove the red clamp from the good car's battery and then from the once-dead battery.

It's best to keep your car running for a good while to help recharge the battery, but you should also consider having its voltage checked to be sure it doesn't need replacing.

Jumper Cables FAQ

Can I get jumper cables at gas stations?

While small cables may be available for purchase at gas stations, they're often not the best solution. This is especially true if you have an SUV or truck. It's best to use the thickest cables available, so they're able to conduct the energy required to jumpstart your car.

What are jumper cables used for?

Also known as jump leads or booster cables, jumper cables are insulated wires. They come in pairs and have alligator clips that are used to connect a car battery to another energy source. These sources can either be other vehicles, or batteries that have the same voltage as the car that needs to jumpstart.

Why do I need heavy-duty jumper cables?

Heavy-duty jumper cables are thicker and have a higher gauge, allowing more electricity to pass through quickly to jumpstart your vehicle. This ensures that you can jumpstart your car in the worst conditions, including cold weather.

Do I need to turn off my car before disconnecting the jumper cables?

Yes! Don't connect the cables when your vehicle is running. Both the car and auxiliary source must be turned off prior to connection or disconnection. Otherwise it can cause vehicular damage and potentially shock you.

How do I connect jumper cables?

First, make sure both vehicles are both turned off. Then, connect the red clamp to the positive terminal of the dead battery (often marked with a "+" symbol). Next, connect the other end of the same red cable to the positive terminal of the working battery (or the source that is being used to jump-start the car). Attach the black connector to the negative terminal of the "donor" power source. Then attach the black connector at the other end of the cables to a metal part of the vehicle with the dead battery. When the cables are connected, start the working car. Let it run for a minute or two, then try to start the other vehicle.