Check both batteries for visible damage before you touch the cables
Before you connect jumper cables, look at both the dead battery and the working battery for cracks, leaks, or corrosion. A cracked battery case can leak acid and make the jump unsafe. Corrosion looks like white, blue, or green crusty buildup around the battery terminals — you can still jump the car, but you'll need to clean it off first so the cables make good contact.
If you see fluid leaking from either battery, do not attempt the jump. A leaking battery is damaged and jumping it can cause a fire or explosion. Call a tow truck or roadside service instead. If the battery case is intact but heavily corroded, use a wire brush or old toothbrush to scrub the white or blue buildup off the terminals until you see bare metal. This takes two minutes and makes the jump much more likely to work.
Make sure both cars are turned off and parked safely
Turn off the engine in both vehicles and turn off all electrical systems — headlights, radio, interior lights, everything. This protects the alternators and electrical systems in both cars from voltage spikes when you connect the cables. A running engine creates electrical noise that can damage sensitive components.
Park both cars close enough that the cables will reach from one battery to the other, but not touching. You need at least a foot of space between the vehicles so you can work safely and so the cars don't accidentally touch metal to metal while you're handling the cables. If the cars are on a slope, make sure the parking brakes are set firmly on both.
Key Takeaways
- Inspect both batteries for cracks, leaks, or corrosion before you touch the cables, and do not jump a leaking battery.
- Turn off both engines completely and switch off all lights and electronics in both vehicles before connecting anything.
- Park the cars close enough for the cables to reach but far enough apart that they cannot touch each other.
- Clean heavy corrosion off the battery terminals with a wire brush so the cable clamps make solid contact.
- Wear safety glasses if you have them, because battery acid can splash if a battery is damaged during the jump.
Locate the positive and negative terminals on both batteries
Every car battery has two terminals: the positive terminal is marked with a plus sign (+) or a red plastic cover, and the negative terminal is marked with a minus sign (−) or a black plastic cover. On the dead battery, you need to see both terminals clearly. On the working battery, you also need clear access to both. If either battery has a plastic cover over the terminals, remove it by hand or with a wrench — it's designed to come off.
Write down which terminal is which on each battery before you touch the cables. This sounds basic, but in a stressful moment it's straightforward to mix them up, and connecting the cables backward can damage both cars' electrical systems. If you cannot see the terminals clearly because of corrosion or debris, clean them now rather than trying to work around the problem.
Check that the jumper cables are in good condition
Look at the cables themselves before you use them. The insulation (the rubber coating) should not be cracked, melted, or missing. The metal clamps at each end should be clean and should close firmly when you squeeze them. If the clamps are loose or the insulation is damaged, the cables can overheat or fail to conduct electricity properly.
Test the clamps by squeezing them together — they should snap shut with a click and stay closed until you deliberately open them. If a clamp is bent or won't close properly, do not use those cables. Damaged cables can cause sparks or fail to deliver enough current to start the car. If you're borrowing cables from someone else, ask how old they are and whether they've been stored in extreme heat or cold, both of which degrade the insulation over time.
Confirm the working car's battery is actually charged
Before you connect the cables, start the working car and let it run for a minute to confirm it actually starts and runs. This sounds obvious, but if the "good" battery is also weak or dead, the jump will not work and you'll have wasted time. Listen for the engine to turn over smoothly and run without stalling. If it struggles to start or dies after a few seconds, that battery is not strong enough to jump another car.
Once you've confirmed the working car runs, turn it off again. You'll start it again after the cables are connected, following the specific order of steps that comes next. Turning it off now prevents accidental electrical surges while you're handling the cables.
Wear safety glasses and remove metal jewelry
If you have safety glasses or regular eyeglasses, wear them. A battery can produce sparks or spray acid if something goes wrong, and your eyes are vulnerable. You don't need expensive equipment — regular glasses work fine.
Remove any metal rings, bracelets, or watches before you start. Metal jewelry can accidentally touch both battery terminals at the same time and create a short circuit, which causes sparks and can burn your skin. This is rare, but it takes only a second to remove jewelry and it eliminates the risk entirely.
Know what to do if you see sparks or smell burning
If you see sparks when you connect the cables or smell a burning or chemical smell, stop when ready and disconnect the cables in reverse order: negative clamp off the dead battery first, then negative off the working battery, then positive off the working battery, then positive off the dead battery. Do not panic — sparks are usually just static discharge and are not dangerous, but a strong burning smell means something is wrong.
If you smell burning plastic or a strong chemical smell, the battery may be damaged or the cables may be faulty. In that case, do not attempt the jump again. Call roadside service or a tow truck. If you see sparks but no smell, wait 30 seconds and try connecting again, making sure the clamps are fully closed and making solid contact with the terminals.
Frequently Asked Questions
Can I jump a battery if it's covered in corrosion?
Yes, but you should clean the corrosion off the terminals first. Use a wire brush or old toothbrush to scrub away the white, blue, or green buildup until you see bare metal. This takes a few minutes and ensures the cable clamps make good electrical contact. Without clean contact, the jump may not work.
What if I can't remember which terminal is positive and which is negative?
The positive terminal is always marked with a plus sign (+) or has a red plastic cover. The negative terminal is marked with a minus sign (−) or has a black plastic cover. If the covers are missing, look for these symbols stamped or printed on the battery case itself. When in doubt, take a photo of both batteries before you start so you can reference it.
Do I need to let the working car run before I connect the cables?
No. Start the working car, confirm it runs smoothly for a few seconds, then turn it off again. This verifies the battery is strong enough to help. Then connect the cables while both cars are off, and start the working car again after the cables are fully connected.
Is it dangerous if the two cars touch each other while I'm jumping?
It can be. If metal parts of the two cars touch while the cables are connected, you create an unintended path for electricity that can damage both vehicles' electrical systems. Park them close enough for the cables to reach comfortably, but with at least a foot of space between them.
What should I do if the battery smells like rotten eggs?
That smell is hydrogen sulfide gas, which means the battery is overcharging or damaged internally. Do not jump the car. Call roadside service or a tow truck instead. A battery that smells this way can overheat or explode during a jump.