What happens when you jump start a car
Jump starting a car means using another vehicle's battery to supply power to your dead or dying battery so your engine will turn over. You connect the two batteries with jumper cables—thick wires with metal clamps on each end—and let the working battery push current into the dead one for a few minutes. Then you start your car. If it works, you drive it for at least 20 minutes so the alternator can recharge the battery while the engine runs.
This is a temporary fix, not a repair. Jump starting gets you moving again, but it does not fix whatever drained your battery in the first place. If your battery dies repeatedly, or if your car won't start even after a jump, you likely have a battery that needs replacement or an electrical problem that needs diagnosis.
Key Takeaways
- You need a second vehicle with a working battery, jumper cables, and about 10 minutes of time in a safe location away from traffic.
- The order of cable connections matters: positive to positive, negative to negative, then negative to a metal part of the dead car's engine—never directly to the negative terminal.
- Start the working car first and let it run for 2 to 3 minutes before trying to start your car, then wait another minute after your car starts before disconnecting.
- After a successful jump start, drive for at least 20 minutes without stopping so your alternator can recharge the battery.
What you need before you start
Gather these items before you begin: jumper cables (usually 8 to 10 feet long), a second vehicle with a working battery, and a safe location away from traffic. Check that both cars are parked close enough that the cables will reach from one engine bay to the other—usually within 3 to 4 feet. Turn off both engines and all electrical systems (lights, radio, climate control) in both vehicles.
Inspect the jumper cables for damage. The clamps should be clean and free of corrosion, and the wires should have no cuts or exposed copper. If the cables are damaged, do not use them—a short circuit can damage both batteries or cause a spark near the battery, which is dangerous because batteries release hydrogen gas.
Open both engine hoods and locate the batteries. Car batteries have two terminals: a positive terminal marked with a plus sign (+) and usually a red cap, and a negative terminal marked with a minus sign (−) and usually a black cap. Identify these on both vehicles before you touch the cables.
The correct order for connecting cables
The order of connections prevents sparks and protects both vehicles' electrical systems. Follow these steps in this exact sequence:
- Clamp the red cable to the positive (+) terminal of the dead battery.
- Clamp the other end of the red cable to the positive (+) terminal of the working battery.
- Clamp the black cable to the negative (−) terminal of the working battery.
- Clamp the other end of the black cable to an unpainted metal surface on the engine block or frame of the dead car—not to the negative terminal itself. Look for a bolt, bracket, or bare metal area away from the battery.
This sequence ensures that if a spark occurs, it happens away from the battery where hydrogen gas collects. Connecting the final black cable to bare metal instead of the negative terminal is the safety step that matters most.
Starting the cars in the right sequence
Start the working car first and let it run for 2 to 3 minutes. This gives its alternator time to build up charge in its own battery before it begins supplying power to your car. You should see the engine settle into a steady idle.
After 2 to 3 minutes, try to start your car. Turn the key or press the start button as you normally would. If the engine turns over but does not start, wait 30 seconds and try again. If it still does not start after three attempts, stop and wait 5 minutes—trying repeatedly can drain both batteries further and overheat the starter motor.
Once your car starts, let both engines run for another minute before you disconnect the cables. This gives your alternator a moment to begin charging your battery and stabilizes the electrical system.
Disconnecting the cables safely
Remove the cables in reverse order of how you connected them. This prevents a spark at the moment of disconnection:
- Remove the black cable from the metal surface on your car's engine.
- Remove the black cable from the negative terminal of the working car.
- Remove the red cable from the working car's positive terminal.
- Remove the red cable from your car's positive terminal.
Coil the cables loosely and store them in the working vehicle. Do not let the metal clamps touch each other while the cables are still connected to either battery—this creates a short circuit.
What to do after your car starts
Drive your car for at least 20 minutes without stopping. Highway driving is better than city driving because it keeps the engine running steadily and gives the alternator consistent work. The alternator is the part of your engine that recharges the battery while the car runs, and it needs time to restore the charge that was lost.
Do not turn off the engine and try to restart it right away. If you stop for gas or food within the first 20 minutes, keep the engine running or accept that you may not be able to restart without another jump. If your battery dies again within a few days, or if it dies repeatedly, the battery itself is likely failing and needs replacement. A battery that holds a charge should not need jump starting more than once every few years.
When jump starting will not work
Jump starting fails when the battery is too damaged to accept a charge, when the cables are not making good contact, or when the problem is not the battery at all. If your engine cranks (turns over) but will not start, the issue is likely fuel or ignition, not the battery. If the engine does not crank at all and the dashboard lights are dim or off, the battery is the problem—but a completely dead battery sometimes cannot accept enough charge to start the engine, even with jumper cables.
If jump starting does not work after two attempts, stop and call a tow truck or roadside service. Continuing to try can damage the starter motor or the alternator in the working vehicle. A professional can test whether the battery is salvageable or needs replacement, and can diagnose other electrical problems.
Frequently Asked Questions
Can I jump start a car if the battery terminals are corroded?
Light corrosion (white or blue-green powder) will not stop a jump start, but heavy corrosion blocks electrical contact. If the terminals are heavily corroded, scrape away the corrosion with a wire brush or the edge of a coin before connecting the cables. If you cannot clean them enough to see bare metal, the clamps will not grip properly and the jump start will fail.
What if the jumper cables are not long enough to reach between the cars?
Park the working car closer, or ask the driver to pull forward or backward a few feet. If the cars still cannot get within 3 to 4 feet of each other, you need longer cables or a different location. Stretching short cables to their limit can cause them to overheat or lose contact during the jump start.
Is it safe to jump start a car in the rain?
Light rain is generally safe because the cables and battery terminals are not submerged. Heavy rain or standing water around the battery is riskier because water conducts electricity. If possible, move the car to a covered area or wait for the rain to lighten. If you must jump start in heavy rain, work quickly and keep the cables away from puddles.
How long does a jump start usually take?
The entire process—connecting cables, running the working car, starting your car, disconnecting cables, and driving away—takes about 10 to 15 minutes. If your car does not start after two attempts, stop and wait or call for help rather than continuing to try.
Can I jump start a car with a different type of battery, like a hybrid or electric car?
Most hybrid cars have a standard 12-volt battery that can jump start another car or be jump started the same way. Electric cars and some newer hybrids have high-voltage systems that are not designed for traditional jump starting. Check your owner's manual or the label under the hood before attempting a jump start with an unusual vehicle type.