How to Use Jumper Cables
Connect jumper cables in the safe order, ground the last clamp away from the battery, start the dead car, remove the cables in reverse, and drive 30 minutes to recharge.
You will end up with the dead car running, the cables off in the right order, and the battery recharging on a 30-minute drive. With a set of jumper cables and a second car, the jump takes about 15 minutes.
The connection order is the part to get right. A reversed clamp can blow fuses and damage control modules in either car, and making the last connection at the dead battery puts a spark next to the hydrogen gas a battery gives off. Steps 3 to 6 take the four clamps one at a time, and step 8 covers what to do if the car still will not start.
A set of jumper cables (4-gauge or 6-gauge and 10 to 20 feet long for a passenger car, the sizes AAA suggests) A second vehicle with a charged 12-volt battery Safety glasses and gloves A dry cloth or old toothbrush for cleaning corroded terminals
Park the cars close together and turn everything off
Put on safety glasses and gloves before you open either hood. Battery fluid contains sulfuric acid, and Mazda's owner's manual says to wear eye protection and protective gloves when you work near a battery. AAA gives the same advice for jumper cables. Pull the working car close enough that the cables reach both batteries without stretching, nose to nose or side by side, and make sure the two cars are not touching. If you cannot find a battery under the hood, check the owner's manual. Some cars keep it in the trunk or under a seat, and the manual shows where to connect the cables. Shift both cars into park (or first gear for a manual) and engage the parking brake. Turn off both engines, headlights, radios, and climate controls. Pop both hoods and prop them open. Before you touch a cable, look at both batteries. If either case is cracked or leaking, do not jump it. AAA says not to try. AAA also says a swollen battery should be replaced, because a jump may not work. Call roadside assistance instead.
Heads up: Never jump a frozen battery. Mazda's owner's manual warns that a frozen battery can rupture or explode when it is jump-started. If the car sat dead through a freezing night and the battery case looks swollen or frozen, call roadside assistance instead of jumping it.
Identify the positive and negative terminals on both batteries
Every car battery has two metal posts on the top or side. One is positive and one is negative. A reversed clamp can damage both vehicles, so confirm which post is which on both batteries before any clamp goes on. Go by the mark stamped on the battery. The positive terminal is marked with a + sign or the letters POS. The negative terminal is marked with a − sign or NEG. Where there is a red cap or cable, it is on the positive side, and black is on the negative, but the stamped mark decides. If corrosion covers the markings, brush off the white or green buildup with a dry cloth or old toothbrush until you can read them. Clean posts also let the clamps make solid contact, and a loose or corroded connection keeps current from flowing.
Connect the red clamp to the dead battery's positive terminal
Uncoil the jumper cables and look them over. AAA says to make sure the cables are not frayed and the clamps are free of rust before you use them. Then keep the four clamps apart. From the moment a clamp is on a battery, do not let the red and black clamps touch each other, and do not let a loose red clamp touch bare metal on either car. Either one is a short circuit. Open one red (positive) clamp, place it squarely over the positive (+) terminal of the dead battery, and let it grip. Wiggle it to confirm it is secure and making metal-to-metal contact. Set the other three clamps down on a non-metal surface like rubber, plastic, or cardboard.
Good to know: Remember the connection order as "red, red, black, ground." Positive terminals first, negative side second, and the very last clamp never touches the dead battery.
Connect the other red clamp to the good battery's positive terminal
Walk the cable over to the working car. Take the red clamp on the other end and attach it to the positive (+) terminal of the good battery. As before, squeeze the clamp open, press it onto the terminal, and check that it grips. You now have a red cable connecting the two positive terminals. Both black clamps are still unconnected.
Connect one black clamp to the good battery's negative terminal
Pick up one of the black (negative) clamps. Attach it to the negative (−) terminal of the working car's battery. Press it on firmly until it grips. Three of the four clamps are now connected, and the last one does not go on a battery.
Connect the last black clamp to bare metal on the dead car
Do not lean over either battery while you make this connection, which is AAA's advice. The last clamp completes the circuit, and it is the one that can spark. The final black clamp does not go on the dead battery's negative terminal. Attach it to a ground point on the dead car, away from the battery. Use the point your owner's manual shows. Honda's manual names an engine mounting bolt and says not to connect the cable to any other part, and Mazda's marks the location in a figure. If your manual shows no point, use a solid, unpainted metal bolt or bracket on the engine, as far from the battery as the cable allows. Keep the clamp and both cables clear of the cooling fan and the belts. Mazda's manual warns that a cable near moving parts can get caught when the engine starts. Batteries give off hydrogen gas, and a spark next to the battery can ignite it. Grounding the last clamp away from the battery keeps the spark away from that gas.
Heads up: Check all four clamps before you start an engine. Mazda's manual says to make sure the cables are connected securely enough that engine vibration cannot shake them loose. If the last clamp will not grip, or the metal under it is painted or greasy, reseat it on bare metal. Move it to another bolt or bracket on the engine only if your manual shows no single ground point.
Start the working car and charge the dead battery
Start the engine of the working car. This sends charge through the cables into the dead battery. AAA advises letting it run for 5 to 10 minutes. Use the full 10 minutes if the dead battery was completely flat (no dome light, no dashboard lights at all) or in cold weather, when AAA says a longer charging interval helps. Raise the donor engine to a fast idle and hold it there with light, steady pressure on the accelerator. An alternator produces more current the faster its rotor spins, and Honda's and Mazda's owner's manuals both tell the assisting driver to raise the engine speed. Do not floor it, and do not blip the throttle up and down.
Try starting the dead car
Turn the key (or press the start button) in the dead car. If it starts, let the working car drop back to a normal idle and go to step 9. If the engine cranks slowly or does not catch, the battery needs more charge. Check that every clamp has a solid grip, which is the first thing AAA says to do, then wait another 3 to 5 minutes with the working car running and try again. If it still does not start after two or three attempts, stop cranking. AAA lists the starter, the alternator, a blown fuse, a bad battery cell, or another electrical fault as what is left at that point. Call roadside assistance or arrange a tow to a shop.
Disconnect the cables in reverse order
Once the dead car is running, take the cables off in the exact opposite order you connected them. Keep each clamp away from the other clamps and from bare metal as it comes off, and keep the cables clear of the fan and belts, which are now moving. The order is: 1. Remove the black clamp from the ground point on the car that was dead. 2. Remove the black clamp from the good battery's negative terminal. 3. Remove the red clamp from the good battery's positive terminal. 4. Remove the red clamp from the previously dead battery's positive terminal. Set the clamps down on a dry, non-conductive surface until you can coil the cables back up. Leave the revived car running.
Good to know: A simple way to remember the disconnect order: last on, first off. The grounding clamp was the last one you attached, so it comes off first.
Drive the revived car for at least 30 minutes
A jump start gets the engine running, but it does not recharge the battery. The alternator does that, and it needs time. AAA says to let the car run for at least 30 minutes. Drive it for that long without switching it off. Highway or steady-speed driving does more than stop-start traffic, because the alternator charges faster at driving speed than at idle. Turn off heated seats, the stereo, and air conditioning for the drive, so less of the alternator's output goes elsewhere. After the drive, have the battery tested at a repair shop or auto parts store, and ask before you go whether the test is free. A battery that needed a jump has to show it still holds a charge before you rely on it again. Then wipe down the jumper cables with a dry rag, coil them loosely, and put them back in the trunk.
If the car stalls on the drive or will not restart afterward, the battery is not holding a charge or the alternator is not charging it. Jump it again and take it straight to a repair shop to have the battery and charging system tested. If you are standing next to a car that will not start and you are not sure the battery is the problem, our guide on how to jump start a car covers how to tell, and what to do when there is no second car around. FAQ 1 below covers when a set of cables needs replacing.
Questions we get asked about this
Answers from experience, not a textbook.
Yes. AAA says clamps can corrode, insulation can crack, and the wires inside can fray, and that each of those cuts performance and can be unsafe. Look the set over before a long trip. Replace it if the clamps no longer grip firmly, the insulation is brittle, or any wire is exposed. To make a set last, coil it loosely without tight kinks and store it dry, in a bag or in the cargo area.
Only one with a 12-volt battery. AAA says to make sure the two batteries have similar voltages, because a large mismatch can damage a vehicle's electrical system. Mazda's owner's manual is specific. It says to use only a 12-volt booster, and that a 24-volt supply, such as two 12-volt batteries in series, can damage the starter, ignition system, and other electrical parts beyond repair. If the donor is anything other than an ordinary car or light truck, confirm it is a 12-volt system before you connect. If either vehicle is a hybrid or electric, read its owner's manual first and never connect cables to the high-voltage system. FAQ 5 in our jump-start guide covers hybrids and EVs.
Yes, with extra care. AAA's advice for wet weather is to keep all the connections dry and avoid standing in deep water. Wipe the terminals and clamps before you connect, and do not use cables with frayed or cracked insulation. If conditions are hazardous, AAA says to call roadside assistance instead of risking slips and sparks.
For a passenger car, AAA suggests a 4-gauge or 6-gauge set that is 10 to 20 feet long. A lower gauge number means thicker wire. Four-gauge moves current more efficiently, and 6-gauge is lighter and cheaper. For trucks, SUVs, and diesel engines, or if you expect to help other drivers, AAA says to choose 2-gauge. It advises against thin 8-gauge and 10-gauge cables, which are slow to deliver current. A longer set helps when the cars cannot park nose to nose.
It can if you reverse the cables. Clamping red to a negative terminal can blow fuses and damage control modules in either car. Check the + and − marks before each clamp goes on, follow the order in steps 3 to 6, and let the working car return to a normal idle before you disconnect anything. Use only a 12-volt donor (FAQ 2).
If you turn the key and hear rapid clicking, or the engine cranks slowly, the battery is likely flat, and dim or dead dashboard lights point the same way. If the lights and radio work normally but nothing happens when you turn the key, suspect the starter motor, a blown fuse, or a loose connection. If the engine cranks at normal speed but does not start, the problem is probably not the battery.
Car Owner Essentials
How to Check Tire Pressure
How to Change a Tire
How to Change Your Oil
How to Check Your Oil
How to Jump Start a Car
How to Parallel Park
How to Use Jumper Cables
← you’re here