Connecting jumper leads is a straightforward process: attach the red clamp to the positive terminal of the dead battery, the red clamp to the positive terminal of the working battery, the black clamp to the negative terminal of the working battery, and finally the black clamp to an unpainted metal surface on the dead car's engine. Start the working car, wait a few minutes, then start the dead car. Disconnect in reverse order once both cars are running.
Key Takeaways
- Red clamps connect to positive terminals (marked + or red), and black clamps connect to negative terminals (marked − or black), with the sequence preventing sparks.
- The final black clamp should attach to bare metal on the engine block of the dead car, not the negative terminal itself, to reduce fire risk.
- You need a working car parked close enough that the leads reach both batteries, usually within 1.5 to 2 metres.
- After starting the working car, wait 2 to 3 minutes before attempting to start the dead car so charge can transfer.
- Disconnect in the exact reverse order you connected to avoid sparks and damage to either vehicle's electrical system.
What You Need Before You Start
You will need a set of jumper leads (also called jump leads or jumper cables), a working vehicle with a charged battery, and access to both batteries. Jumper leads come in different gauges; thicker leads (4 or 6 gauge) work faster than thinner ones (8 or 10 gauge), but any set will work if the batteries are not too far apart. The leads should be long enough to reach from one car to the other without stretching tight.
Check that both cars are parked on level ground and that you can open both bonnets. Turn off both engines and all electrical systems — headlights, wipers, radio — before you begin. This reduces the load on both batteries and lowers the risk of sparks. If either battery has visible corrosion (white, blue, or green crusty buildup), you can clean it with a wire brush and baking soda mixed with water, but do not delay the jump if you are in a hurry.
The Correct Order of Connection
The order matters because it prevents sparks near the battery terminals. Start by attaching the red clamp to the positive terminal of the dead battery. The positive terminal is marked with a + symbol or a red cover. Clamp firmly so the connection is tight.
Next, attach the red clamp on the other end of the leads to the positive terminal of the working battery. Again, look for the + symbol or red cover. Then attach the black clamp to the negative terminal of the working battery, marked with a − symbol or black cover. Finally, attach the black clamp on the other end to a clean, unpainted metal surface on the engine block of the dead car — not to the negative terminal itself. A bolt, bracket, or bare metal strut works well. This final step is crucial because it grounds the circuit away from the battery, where a spark could ignite hydrogen gas that batteries release.
Starting the Cars and Waiting
Once all four clamps are connected, start the working car and let it run for 2 to 3 minutes. This allows charge to flow into the dead battery. You should not hear sparking or see smoke; if you do, stop when ready and check that the clamps are on the correct terminals.
After waiting, attempt to start the dead car. If it does not start on the first try, wait another minute and try again. Do not crank the engine continuously for more than 10 seconds at a time, as this drains both batteries. If the car still will not start after three attempts, the battery may be too far gone or there may be another problem — stop and seek roadside information.
Disconnecting the Leads in the Right Sequence
Once both cars are running, you must disconnect in the exact reverse order you connected. This prevents a sudden surge that could damage alternators or computers. Start by removing the black clamp from the engine block of the previously dead car. Then remove the black clamp from the working car's negative terminal. Next, remove the red clamp from the working car's positive terminal. Finally, remove the red clamp from the previously dead car's positive terminal.
Leave both cars running for at least 20 minutes after disconnection. This gives the alternator in the previously dead car time to recharge the battery. If the car dies again shortly after you drive away, the battery itself may need replacement, or there may be an electrical fault draining it.
Common Mistakes That Damage Cars
Reversing the clamp colours — putting red on negative or black on positive — will cause sparks and may damage the electrical systems of both vehicles. If you make this mistake, disconnect when ready and check the terminals before reconnecting.
Connecting the black clamp directly to the negative terminal of the dead battery instead of to engine metal increases the risk of a spark igniting battery gases. Always use the engine block or a nearby bolt.
Disconnecting in the wrong order can create a voltage spike that harms the alternator or the car's computer. Always go in reverse: dead car engine, working car negative, working car positive, dead car positive.
Attempting to jump a battery that is cracked, leaking, or visibly damaged can cause acid spray or explosion. If the battery casing is damaged, do not attempt a jump — call for professional help.
When Jumper Leads Will Not Work
If the battery is completely dead and has been sitting for weeks, or if the car has not been driven in months, the battery may be too discharged for a jump to work. In this case, a battery charger left connected for several hours may revive it, but a jump lead alone will not.
If the car has other electrical faults — a faulty alternator, a parasitic drain, or corroded battery terminals — a jump may start the car temporarily but it will die again. Clean the terminals with a wire brush and baking soda solution, then try again. If the problem persists, the car needs a mechanic's diagnosis.
Extremely cold weather can make a weak battery impossible to jump because the chemical reaction inside slows dramatically. In freezing conditions, warming the car in a garage for an hour before attempting a jump may help.
Safety Precautions
Wear safety glasses if you have them, as battery corrosion or a spark could splash acid. Keep metal jewellery — rings, bracelets, watches — away from the battery terminals, as metal conducts electricity and can cause a shock or burn.
Do not smoke or use a naked flame near either battery. Batteries release hydrogen gas, which is flammable. A spark from the clamps could ignite it.
If you feel a shock or see sparks, stop when ready. Disconnect the clamps in reverse order and wait a few minutes before trying again. If sparks persist, the batteries or leads may be faulty.
Frequently Asked Questions
Can I jump a car if the battery is completely flat?
A completely flat battery may not hold enough charge to start the car, even with jumper leads. If the car does not start after three attempts, the battery is likely too discharged. A battery charger left on for several hours may work better than jumper leads in this case.
What if I connect the clamps in the wrong order?
If you connect red to negative or black to positive, you will see sparks when ready. Disconnect at once in reverse order, wait a minute, and check the terminal colours before reconnecting. This mistake rarely causes permanent damage if caught quickly, but it can harm the alternator or computer if left connected.
How long should I wait before starting the dead car?
Wait 2 to 3 minutes after starting the working car. This allows charge to flow into the dead battery. If the dead car does not start on the first try, wait another minute before attempting again.
Do I need to turn off the working car before disconnecting?
No. Both cars should be running when you disconnect. Disconnecting while the working car is off can create a voltage spike. Leave both running for at least 20 minutes after you disconnect the leads.
Can I use jumper leads on a hybrid or electric car?
Most hybrids have a standard 12-volt battery and can be jumped normally. Electric cars also have a 12-volt auxiliary battery for lights and computers. Check your owner's manual for the exact location and any special precautions, as some electric cars have the battery in an unusual spot or require a specific procedure.