Connect the red cable to the dead battery's positive terminal first

The positive terminal on a car battery is marked with a + symbol and is usually a larger post than the negative terminal. On the dead battery (the one in the car that won't start), attach the red clamp firmly to this positive post. Make sure the clamp makes solid metal-to-metal contact — a loose connection will not transfer power.

The red cable's other end goes to the positive terminal of the working battery in the other vehicle. Again, clamp it firmly to the + post. At this point, you have completed the positive circuit.

Connect the black cable to the working battery's negative terminal

The negative terminal is marked with a − symbol and is the smaller post. Take the black cable and attach one clamp to the negative terminal of the working battery — the one that is running and providing power.

Do not attach the black cable to the negative terminal of the dead battery. This is the step that prevents sparks and potential battery damage. Instead, you will ground the second black clamp to an unpainted metal surface on the dead car's engine block.

Ground the black cable to the dead car's engine, not the battery

The final connection is the most important one to get right. Find a bare metal bolt, bracket, or unpainted surface on the engine block of the car with the dead battery — anywhere that is metal and not painted. The engine block itself works well. Clamp the free end of the black cable to this spot.

Grounding to the engine instead of the negative terminal prevents a spark directly at the battery, which could ignite hydrogen gas that batteries release. This is why the order matters: red to dead, red to working, black to working, black to engine.

Key Takeaways

  • Red cable connects positive to positive: dead battery first, then working battery.
  • Black cable connects the working battery's negative terminal to an unpainted metal surface on the dead car's engine block, never to the dead battery's negative terminal.
  • Make sure every clamp is clamped tightly to bare metal so current flows without resistance.
  • The order of connection prevents sparks at the battery and reduces the risk of damage to either vehicle's electrical system.
  • Remove cables in reverse order: black from engine, black from working battery, red from working battery, red from dead battery.

Why the order and placement matter

Connecting in the wrong order or to the wrong terminals can cause sparks, damage the alternator in the working car, or even cause the dead battery to explode. The reason is that a car battery produces hydrogen gas as a byproduct of its chemical reaction. A spark near the battery terminals can ignite this gas.

By grounding the final black cable to the engine block instead of the battery, you move the spark risk away from the battery itself. The engine block is a large metal mass that can safely dissipate the electrical charge without igniting anything.

What to do after the cables are connected

Once all four clamps are in place and tight, start the working vehicle and let it run for two to three minutes. This gives the dead battery a chance to receive a charge. Then try to start the dead car. If it starts, let both vehicles run for another minute or two before removing the cables.

If the dead car does not start after one or two attempts, stop and wait five minutes before trying again. Repeated attempts can overheat the starter motor. If it still will not start after a few tries, the battery may be too damaged to jump-start, and you may need a tow truck or a replacement battery.

Removing the cables in the correct order

Disconnecting in the wrong order can create a dangerous spark. Remove the cables in reverse order: black clamp from the engine block first, then black clamp from the working battery, then red clamp from the working battery, and finally red clamp from the dead battery.

After you remove the cables, keep the previously dead car running for at least 20 minutes so the alternator can recharge the battery. If the car dies again shortly after, the battery is likely worn out and will need to be replaced.

Common mistakes to avoid

The most common error is connecting the black cable to the negative terminal of the dead battery instead of grounding it to the engine. This creates a spark risk and defeats the safety purpose of the grounding step. Another mistake is clamping the cables loosely — a loose connection will not transfer enough power to start the car and can cause the clamps to overheat.

Do not touch the metal clamps together while the cables are connected to the batteries, and do not let the clamps touch any other metal on either car except at the intended connection points. Doing so can create an unintended short circuit and damage the electrical systems in both vehicles.

Frequently Asked Questions

What if I cannot find an unpainted metal surface on the engine block?

Look for a bolt on the engine block, a bracket, or the metal frame near the engine. You can also use the negative terminal of the dead battery if you absolutely cannot find bare metal elsewhere, though grounding to the engine is safer. Scrape away a small area of paint with a screwdriver if needed to expose bare metal underneath.

Does it matter which car's battery is the "working" one?

The working battery should be in a car that is running or can start reliably. It does not matter which vehicle is which, as long as one battery is healthy enough to provide power. A battery in a car that is off but in good condition will also work.

Can I jump-start a car with a dead battery if the cables are old or damaged?

No. Damaged insulation on the cables can cause electrical shock or fire. Frayed or corroded clamps will not make good contact. Always inspect the cables and clamps before use. If they are cracked, burned, or heavily corroded, do not use them.

How long should I wait before trying to start the dead car?

Let the working car run for two to three minutes after connecting all cables. This gives the dead battery time to receive some charge. If the dead car does not start on the first or second try, wait five minutes before trying again to avoid overheating the starter motor.

What if the battery terminals are corroded with white or blue-green buildup?

Corrosion reduces electrical contact. If possible, scrape away the buildup with a wire brush or old toothbrush before clamping the cables. If you cannot clean it, clamp as tightly as possible to break through the corrosion layer. Corrosion is often a sign the battery is old and may need replacement soon.