Electrical
How to Jump Start a Car Without Damaging It
Jump starting is straightforward and there are two details that matter: the order the clamps go on, and where the last one goes. Get those wrong on a modern car and you can do real damage to electronics that cost far more than the battery.
What this guide covers
Before You Start
- Check the battery. If it is cracked, leaking or visibly swollen, do not jump start it. Have the car recovered.
- Check both cars are the same voltage. Almost all cars are 12 volt, but not every vehicle is.
- Park them close but not touching. The cars must not be in contact with each other.
- Both engines off, both in park or neutral, handbrakes on.
- Switch off electrical loads in both cars: lights, air conditioning, radio.
On a modern car, do not use the jump start to charge the dead battery by running the donor for a long time first. Modern alternators are not designed to charge another car battery through jump leads, and it can put the donor charging system under a load it was never intended for.
The Correct Order
This sequence exists for a reason and each step should be followed as written.
- Red clamp to the positive terminal of the dead battery.
- Other red clamp to the positive terminal of the good battery.
- Black clamp to the negative terminal of the good battery.
- Final black clamp to an unpainted metal point on the dead car engine or chassis, well away from the battery.
- Start the working car and let it run for a couple of minutes.
- Start the dead car. If it does not go within ten seconds of cranking, stop and wait a minute before trying again.
Why the Last Clamp Does Not Go on the Battery
This is the step people skip and it is the one that matters most for safety.
A discharged battery, and one being charged, gives off hydrogen. Connecting the final clamp always produces a small spark as the circuit completes.
Making that spark directly at the battery terminal means making it in the middle of the gas the battery is producing.
Earthing to a metal point on the engine puts the spark somewhere harmless. It costs nothing and it removes the only genuinely dangerous part of the job.
Disconnecting
Reverse order, exactly.
- Black clamp from the earth point on the previously dead car.
- Black clamp from the good battery negative.
- Red clamp from the good battery positive.
- Red clamp from the previously dead battery positive.
Throughout, do not let a connected clamp touch bodywork or the other clamp. While the leads are live, a clamp touching metal is a direct short.
Modern Car Cautions
- Some cars have a designated jump point under the bonnet rather than an accessible battery, particularly where the battery is in the boot. Use it.
- Do not disconnect a battery while the engine is running to test the alternator. That was a valid test decades ago and it can destroy modern electronics.
- A jump pack is safer than another car on modern vehicles, and worth keeping in the boot in this climate.
- Expect warnings afterwards. Low voltage makes many modules complain, and most clear once the car has run.
When Not to Jump Start
Some situations call for recovery rather than leads.
- A battery that is cracked, leaking or swollen.
- A smell of rotten eggs from the battery, which is a battery gassing badly.
- Any sign of fuel leakage anywhere near the engine bay.
- A car that turns over normally already, because then the battery is not the problem.
That last one is worth repeating. If the engine cranks at normal speed and simply will not fire, jump starting achieves nothing. See what the starting noise tells you.
After It Starts
Getting the car running is not the end of it, because a flat battery is usually a symptom.
- Drive for at least twenty minutes, ideally not in stop-start traffic, to put some charge back.
- Do not switch it off somewhere you cannot get help until you know why it went flat.
- Have the battery and charging system tested. A battery that went flat once usually does it again.
- Have parasitic draw checked if the car had been standing only a short time.
Exotic Auto Services tests battery, charging and draw together, because replacing a battery that was only a symptom is a repair that comes back. See car battery replacement.
Frequently Asked Questions
What is the correct order to connect jump leads?
Red to the dead battery positive, red to the good battery positive, black to the good battery negative, and the final black clamp to an unpainted metal point on the dead car engine or chassis, away from the battery. Disconnect in exactly the reverse order.
Why does the last jump lead go to the engine and not the battery?
Because a discharged battery gives off hydrogen, and completing the circuit always makes a small spark. Connecting the last clamp at the battery terminal puts that spark in the gas the battery is producing. Earthing to a metal point on the engine puts it somewhere harmless.
Can jump starting damage a modern car?
It can if done wrongly. Reversed polarity, clamps touching, or disconnecting a battery while the engine runs can all damage electronics that cost far more than the battery. Done in the correct order with the final clamp earthed properly, it is safe, and a jump pack is safer still than another vehicle.
Should I disconnect the battery to check if the alternator is working?
No. That test was valid on much older cars and on a modern one it can destroy control units, because removing the battery removes the buffer that smooths the charging system output. Charging is measured properly with a meter or diagnostic equipment while everything stays connected.
My car started with a jump. Do I still need it checked?
Yes, because a flat battery is usually a symptom. The common causes are a battery at the end of its life, an alternator not charging properly, or a parasitic draw keeping something awake after the car is locked. All three are measurable, and the car will strand you again if the underlying one is not found.
