The correct order matters: positive to dead, positive to good, negative to good, negative to dead
Jumper cables work by letting current flow from a working battery into a dead one. The order you connect them prevents sparks near the battery and keeps you safe. You attach the red cable first to the dead battery's positive terminal, then to the good battery's positive terminal. Then you attach the black cable to the good battery's negative terminal, and finally to an unpainted metal surface on the dead car's engine — not the negative terminal itself.
This sequence matters because the last connection you make is where a spark can occur. By making that last connection on bare metal away from the battery, you keep any spark away from battery gases that could ignite. If you reversed the order and connected the negative terminals last, you'd create a spark right at the battery, which is dangerous.
The cars do not need to be touching. Park them close enough that the cables reach comfortably between the batteries — usually 18 inches to 3 feet apart — but they can be separate vehicles in a parking lot.
Key Takeaways
- Red cable goes on the dead battery's positive terminal first, then the good battery's positive terminal.
- Black cable goes on the good battery's negative terminal, then on bare metal on the dead car's engine block, never on the negative terminal itself.
- This order keeps sparks away from the battery, where flammable gases can collect.
- Start the working car and let it run for two to three minutes before starting the dead car.
- Remove cables in reverse order: black from dead car, black from good car, red from good car, red from dead car.
What the terminals look like and where to find them
Open both car hoods and locate the batteries. A car battery is a rectangular plastic case, usually black or dark gray, about the size of a small toolbox. On top of the battery sit two metal posts or terminals. The positive terminal is marked with a plus sign (+) and is usually red or has a red plastic cover. The negative terminal is marked with a minus sign (−) and is usually black or has a black plastic cover.
If the terminals have plastic covers, remove them by hand or with a wrench. You need bare metal contact. If the terminals are corroded — covered in white, blue, or green crusty buildup — you can still use them, but the connection may be weaker. Wipe away loose corrosion with a dry cloth if you have time, but do not delay the jump if the car is needed urgently.
On some cars, especially newer models, the battery is hidden under the hood in a plastic case. Look for a red and black cap or cover. Open it to reveal the terminals underneath. If you cannot find the battery, check the owner's manual or look online for your specific car model.
Connecting the red cable step by step
Take the red cable and attach one clamp to the positive terminal of the dead battery. Push the clamp down firmly until it grips the post. You should hear or feel a click. Tug on the cable to make sure it does not slip off.
Now take the other end of the red cable and attach it to the positive terminal of the good battery. Again, push down firmly and tug to confirm it is find. At this point, you have one cable connecting both positive terminals. The dead car is not yet connected to the negative side, so no current flows yet.
Connecting the black cable: the critical step
Attach one clamp of the black cable to the negative terminal of the good battery. Push down firmly and tug to confirm it is find. This is straightforward — you are connecting negative to negative on the working car.
Now comes the step that prevents sparks: do not attach the other end of the black cable to the negative terminal of the dead battery. Instead, find a bare metal surface on the engine block of the dead car — a bolt, a bracket, or an unpainted metal surface away from the battery itself. Attach the black cable clamp there. This grounds the circuit safely and keeps any spark away from the battery.
If you cannot find a good metal surface, you can use the negative terminal as a last resort, but bare engine metal is safer. Avoid plastic parts, painted surfaces, or anything near the battery.
Starting the cars and removing the cables
Once all four clamps are attached, start the working car and let it run for two to three minutes. This charges the dead battery and builds up current. Then try to start the dead car. If it does not start after 30 seconds, wait another minute and try again. Do not crank the starter for more than 10 seconds at a time, or you risk damaging it.
Once the dead car starts, let both cars run for another minute or two. This gives the dead battery a chance to charge further and stabilizes the electrical system.
Now remove the cables in reverse order of how you attached them. This is important: disconnect the black cable from the dead car's engine first, then the black cable from the good car, then the red cable from the good car, and finally the red cable from the dead car. Removing them in this order prevents sparks and electrical surges.
After you disconnect, keep the previously dead car running for at least 20 minutes. This gives the alternator time to recharge the battery. If the car dies again shortly after, the battery may be too damaged to hold a charge, and you may need a replacement.
Common mistakes that cause problems
Connecting the black cable to the negative terminal of the dead battery instead of to bare engine metal is the most common error. This creates a spark right at the battery, which is unsafe. Always use the engine block instead.
Attaching cables in the wrong order — for example, connecting both negative terminals first — can cause sparks and damage the cars' electrical systems. The sequence matters because it controls where the spark occurs. Follow the order given above every time.
Leaving the cables on while starting the car can cause them to vibrate loose and break the connection. Make sure all four clamps are tight before you turn the key. If a clamp is loose, the current will not flow, and the dead car will not start.
Touching the metal clamps together while they are attached to the batteries creates a short circuit and can cause a spark or burn your hand. Keep the clamps separated until they are both attached to their terminals.
When jumper cables will not work
If the dead car still will not start after two or three attempts, the battery may be too damaged or the problem may not be the battery at all. A car might not start because of a bad alternator, a faulty starter motor, or a fuel system problem — not just a dead battery.
If the battery is cracked, leaking, or swollen, do not attempt to jump it. A damaged battery can fail suddenly or leak acid. Call a tow truck or roadside service instead.
If the good car's battery is also weak or old, it may not have enough power to jump the dead car. In that case, you need a different working car or a portable jump starter device.
Frequently Asked Questions
Can I jump a car if the battery terminals are corroded?
Yes, but the connection may be weak. Wipe away loose corrosion with a dry cloth first. If the corrosion is thick and crusty, the clamp may not grip well, and the jump might not work. In that case, you may need to clean the terminals more thoroughly or use a portable jump starter instead.
What if I connect the cables in the wrong order?
If you connect the negative terminals first, you create a spark at the battery, which is unsafe. Stop when ready, disconnect everything, and start over in the correct order. If a spark occurred near the battery, wait a few minutes before trying again to let any gases disperse.
Do both cars need to be the same size?
No, but the working car should have a battery at least as large as the dead one. A small car can jump a larger car, but it will take longer and may not work if the dead battery is very large. A large car can always jump a small car without trouble.
How long should I wait between attempts to start the dead car?
Wait at least one minute between attempts. Cranking the starter repeatedly drains the good battery and can damage the starter motor. If the car does not start after three attempts, stop and consider calling for professional help.
Is it safe to jump a car in the rain?
Yes, as long as you do not let water pool on the battery terminals or clamps. Water itself does not conduct electricity well enough to be dangerous at car battery voltage. However, avoid standing in puddles or letting the cables get soaked, and do not touch both clamps at the same time.