How to Jump-Start a Car Safely
To jump-start a car safely, connect the red lead to the flat battery's positive terminal, the other red end to the good battery's positive, the black lead to the good battery's negative, and the final black end to an unpainted metal earth point on the flat car, not its battery. Start the working car, then the flat one.
Before you start
First, make sure jump-starting is safe and appropriate. Both cars should be off, in park or neutral with handbrakes on, and not touching each other. Use jump leads that are in good condition and heavy enough for the job.
Do not attempt to jump-start a battery that is visibly damaged, leaking, or frozen solid (which can happen in severe cold), as it can be dangerous. If in doubt, call for assistance rather than risk it.
The correct connection order
Order matters for safety. With both engines off, connect in this sequence.
- Red lead to the positive (+) terminal of the flat battery.
- Other red end to the positive (+) of the good battery.
- Black lead to the negative (-) of the good battery.
- Final black end to a solid, unpainted metal earth point on the flat car (such as a bolt or bracket), away from the battery.
Earthing the last lead away from the flat battery reduces the spark risk near any battery gases.
Starting and disconnecting
With the leads connected, start the working car and let it run for a few minutes to put some charge into the flat battery. Then try to start the flat car. If it starts, leave both running for a few minutes before disconnecting.
Remove the leads in the reverse order of connection, taking care they do not touch each other or any metal while still attached. Once disconnected, keep the revived car running or take it for a decent drive to recharge the battery.
What not to do
A few mistakes can damage the cars or cause injury, so avoid them.
- Never connect the final black lead directly to the flat battery's negative terminal, due to the spark and gas risk.
- Do not let the leads, or the cars, touch each other.
- Do not mix up positive and negative, which can cause serious electrical damage.
- Do not jump-start a frozen, leaking or damaged battery.
After a jump-start
A jump-start gets you going, but the underlying battery is still flat or weak. To recharge it, drive for a good 20 to 30 minutes or run the engine for a while; short trips will not put enough back in.
If the car starts fine next time, the battery may simply have been drained (for example, by lights left on). But if it keeps going flat, jump-starting is only a temporary fix and the real cause needs finding.
When jumping is only a sticking plaster
Repeated flat batteries point to a real problem: an old, failing battery that no longer holds charge, or a charging fault where the alternator is not recharging it. Jumping the car each morning just postpones the inevitable breakdown.
If your battery keeps going flat, have it and the charging system tested rather than relying on jump leads. We offer battery testing and same-day fitting, and can check the alternator too, so you get reliable starting back.
Frequently Asked Questions
What is the correct order to connect jump leads?
Red to the flat battery's positive, the other red to the good battery's positive, black to the good battery's negative, and the final black to an unpainted metal earth point on the flat car, away from its battery. Start the working car, then the flat one, and disconnect in reverse.
Why should the last lead not go on the flat battery?
Connecting the final black lead to a metal earth point away from the flat battery, rather than its negative terminal, keeps any spark away from the gases a flat or charging battery can give off, reducing the small risk of igniting them. It is the safest place for that last connection.
How long should I drive after a jump-start?
Drive for a good 20 to 30 minutes, or run the engine for a while, to put meaningful charge back into the battery. Short trips will not recharge it enough. If the car starts fine afterwards the battery may just have been drained, but if it keeps going flat, have it tested.
When should I not jump-start a car?
Do not jump-start a battery that is visibly damaged, leaking, or frozen solid, as it can be dangerous. Also avoid it if you are unsure about the leads or connections. In those cases, call for breakdown assistance rather than risk injury or damaging the car's electrics.
My car keeps needing a jump-start - why?
Repeated flat batteries usually mean the battery is old and failing, or the alternator is not recharging it. Jump-starting only postpones the problem. Have the battery and charging system tested to find the real cause, rather than relying on jump leads every morning.
Book With Norwich Tyres & Auto Service
Need a hand from a real, independent Norwich garage? Call 07933 900901 or pop into Ber Street, NR1 3ES. Same-day tyre fitting is available on most common sizes, with free parking on site.
Hero image: “NCDOT State Farm Safety Patrol” by NCDOTcommunications (source), licensed under CC BY.