How to Jump-Start a Car Battery Safely

What You’ll Need

A set of insulated jumper cables in good condition, with clearly marked red and black clamps.
A working vehicle with a battery voltage and system compatible with the disabled vehicle. Most passenger cars use 12-volt systems, but verify both owners’ manuals.
Safety glasses, if available, and gloves. Remove metal jewelry, and keep clothing, hair, and hands clear of belts, fans, and other moving parts.
The owners’ manuals for both vehicles. Some cars have designated jump-start posts or require a particular procedure; follow the manufacturer’s instructions if they differ from the general steps below.

A jump-start uses power from a second vehicle to help start a car whose battery is discharged. It does not repair a failing battery or fix an electrical or charging-system problem. If either vehicle has a hybrid or electric powertrain, use only the jump-start points and procedure specified in its manual; do not use its high-voltage battery as a source.

Step-by-Step

Check for hazards. Do not attempt a jump-start if the battery is cracked, swollen, leaking, unusually hot, or frozen, or if you smell sulfur or rotten eggs. Keep flames, sparks, and cigarettes away: batteries can release flammable gas. Arrange roadside assistance or professional help instead.
Position the vehicles. Park on firm, level ground, close enough for the cables to reach without stretching, but do not let the vehicles touch. Put both in Park (or in gear for a manual transmission), apply the parking brakes, switch off the ignitions, and turn off lights, climate controls, and accessories. Keep the vehicles away from traffic if possible.
Identify the correct terminals. Find the battery or the designated jump-start posts in each vehicle’s manual. Positive is marked “+” and is commonly under a red cover; negative is marked “−.” Do not guess based on cable color or location. Keep clamps clear of one another and of metal parts.
Attach the first red clamp. Connect one red clamp to the positive (+) terminal or designated positive post on the disabled vehicle.
Attach the other red clamp. Connect the second red clamp to the positive (+) terminal or designated positive post on the working vehicle.
Connect the black clamp to the donor. Attach one black clamp to the negative (−) terminal or designated negative post on the working vehicle.
Ground the final black clamp. Attach the remaining black clamp to an unpainted, stationary metal engine or chassis ground on the disabled vehicle, away from its battery, fuel lines, and moving parts. Use the ground point specified in the manual. This helps keep any connection spark away from the battery. Do not attach it to the dead battery’s negative post unless the manufacturer specifically directs you to.
Start the working vehicle. Start it and let it idle for a few minutes, following any timing guidance in the manuals. Do not rev the engine unless the manufacturer’s instructions say to. If a cable becomes hot, smokes, or sparks continuously, stop and disconnect only if safe; otherwise step away and get help.
Try the disabled vehicle. Start it using its normal procedure. Crank only for a few seconds at a time, then pause before trying again. If it does not start after a few attempts, stop rather than repeatedly cranking or leaving the cables connected indefinitely.
Remove cables in reverse order. With the engines running, remove the black clamp from the grounded point on the disabled vehicle, then the black clamp from the donor. Remove the red clamp from the donor, then the red clamp from the formerly disabled vehicle. Keep every clamp from touching another clamp or metal as you work.
Let the car run and get the battery checked. Follow the owner’s manual about how long to run or drive the vehicle; a short idle may not recharge a deeply discharged battery. Avoid switching the engine off until you are somewhere safe, because the car may not restart. Arrange a battery and charging-system test soon, especially if the battery goes flat again.

Common Mistakes

Reversing polarity: Connecting positive to negative can damage vehicle electronics and create sparks. Check the + and − marks at both ends before each connection.
Using the wrong ground: The final black clamp should go to the vehicle’s designated ground point or suitable unpainted metal, not a random painted or moving part. Follow the manual.
Letting clamps touch: Clamps connected to a battery can cause a short circuit if they meet or touch metal in the wrong place. Hold each clamp by its insulated handle and keep the cables separated.
Jumping a damaged or frozen battery: Charging or jump-starting a compromised battery can be dangerous. Do not proceed when it is cracked, swollen, leaking, frozen, or giving off an unusual odor.
Assuming a successful start means the problem is fixed: The alternator may not be charging properly, or the battery may no longer hold a charge. A repeat failure needs diagnosis.
Ignoring vehicle-specific instructions: Battery locations and approved connection points vary. A manual’s designated posts and sequence take priority over general advice.

Tips

Read the instructions for both vehicles before connecting anything. If either manual prohibits a conventional jump-start, do not improvise.
Check that cable insulation is intact and clamps are clean and secure. Do not use damaged cables or cables that cannot reach without tension.
Keep the cable path clear of fans, belts, and pulleys. Some cooling fans can start unexpectedly, even when an engine is not running.
If the car starts, leave accessories off and follow the manual’s guidance for recharging. Driving may help recharge a battery, but it is not a substitute for testing a battery that repeatedly loses charge.
A portable jump starter is an alternative to a donor vehicle, but its own instructions and the vehicle manual must be followed. Never connect it to a battery that appears damaged or frozen.

Troubleshooting

The car does not crank: Switch everything off and check that the clamps are firmly attached to the correct terminals and ground point. Confirm that the donor vehicle and cable setup are permitted by both manuals. If there is still no response, stop and call for assistance; the problem may not be the battery.

You hear clicking, but the engine will not start: The battery may be deeply discharged, the connections may be poor, or another component may be faulty. Check the connections once, then try only as allowed by the manuals. Do not keep cranking for long periods.

The engine cranks but will not start: A jump-start may not solve the issue; fuel, ignition, or another system could be involved. Stop repeated attempts and seek roadside or repair assistance.

Clamps spark, get hot, or smoke: Stop immediately. Do not touch hot clamps or continue the procedure. If safe, switch off the vehicles and step away; get professional help if there is persistent sparking, smoke, or a battery odor.

The car starts but stalls or will not restart: The battery may not be holding a charge, or the charging system may not be working. Avoid relying on another jump-start; arrange a battery and charging-system inspection.

Frequently Asked Questions

How long should I leave the jumper cables connected?

Let the donor vehicle idle for a few minutes before trying to start the disabled car, unless either manual gives different instructions. If it does not start after a few short attempts, stop and get help rather than leaving the cables connected for a long time.

Can I jump-start a car in the rain?

Follow both owners’ manuals and do not work in standing water or unsafe conditions. Keep connections dry where possible, avoid touching exposed metal parts, and call for roadside assistance if the area or weather makes the procedure unsafe.

Can a smaller car jump-start a larger car?

Do not assume it can. Check the manuals for both vehicles; battery voltage, engine size, and manufacturer restrictions matter. If the setup is not explicitly suitable, use a compatible jump starter or call for help.

Why did the battery go dead?

Leaving lights on is one possibility, but an aging battery, loose or corroded connections, or a charging-system fault can also cause a dead battery. If it happens again, have the battery and charging system tested.

Can I remove the cables while the cars are running?

Generally, remove them in reverse connection order while both vehicles are running, unless a manual specifies otherwise. Keep the clamps apart throughout removal and do not let them touch vehicle metal.

What if my vehicle has no obvious battery terminals?

Look in the owner’s manual for remote jump-start posts and the approved grounding point. Do not connect clamps to unidentified terminals or high-voltage components; contact roadside assistance if the correct points are unclear.

Further reading: National Highway Traffic Safety Administration. Details can change, so check the official page for anything that matters to you. Spotted a mistake? Tell us. How we write an answer.