How to Jump Start a Car Safely, Step by Step
Written by Petro Olekhnovych, mobile mechanic in Aylesham, Kent
A flat battery on a frosty morning is one of our most common winter calls. Done carefully, a jump start gets you moving. Done badly, it can damage electronics or hurt someone.
Before you connect anything
Take two minutes to check these first. They prevent most jump start problems.
- Look at the battery. If the case is cracked, bulging, leaking or smells of rotten eggs, do not jump it. A battery that has frozen solid can also be damaged inside. Call for help instead.
- Read the handbook. Many modern cars have remote jump points under the bonnet because the battery sits in the boot or under a seat. Use those if fitted.
- Check the leads. They should be thick, with clean clamps and no split insulation.
- Match the voltage. Both cars should be 12V. Do not jump a car from a van or truck with a 24V system.
- Switch everything off. Lights, heater, wipers and radio off in both cars, and keys out of the ignition or the cars fully off.
Park the cars close but not touching, both in neutral or park with the handbrakes on.
The jump leads order, step by step
- Clamp the red lead to the positive (+) terminal of the flat battery.
- Clamp the other end of the red lead to the positive (+) terminal of the good battery.
- Clamp the black lead to the negative (-) terminal of the good battery.
- Clamp the other end of the black lead to a clean, unpainted metal point on the engine or body of the flat car, away from the battery and fuel lines. Many cars have a marked earth stud.
- Start the good car and let it run at a gentle idle for three to five minutes.
- Try to start the flat car. Crank for no more than about ten seconds at a time, with a minute or so between tries.
- Once it is running, remove the leads in the exact reverse order: black from the earth point, black from the good battery, red from the good battery, red from the flat battery.
The last connection is made away from the flat battery because a charging battery can give off hydrogen gas. A small spark at the terminal is not something you want near it.
Need a service or brakes doing?
Servicing, pads, discs and fluids done on your driveway across East Kent.
WhatsApp for a free quote Call 07563 350335 Contact form
Labour £30 to £50 per hour depending on the job. Send your reg number and we will reply with a price before any work starts.
Using a jump pack instead
A jump pack is handy in the boot over winter, especially in quiet villages like Adisham or Barham. Connect red to positive, black to earth, then switch the pack on. Keep it charged, as packs lose charge in a cold car. Our winter car checklist covers what else is worth carrying.
If it still will not start
If the engine turns over slowly, leave the leads on a few minutes longer and try again. If you hear a single click, rapid clicking, or nothing at all, the problem may not be the battery. Common culprits are:
- Corroded or loose battery terminals that stop current getting through
- A failing starter motor, which often clicks but does not turn
- A fuel or glow plug issue on a diesel that cranks fine but will not fire
Our guide on whether it is the battery, starter motor or alternator walks through the clues.
After the jump start
A jump gets you running but does not recharge the battery properly. Drive for at least half an hour at steady speeds, ideally on a road like the A2 rather than stop-start village lanes. If the battery warning light comes on while driving, that points to the charging system. See battery warning light on while driving for what that means.
If the car goes flat again within a day or two, something is wrong. Either the battery is worn out, the alternator is not charging, or something is draining it overnight. Our guide to car batteries in cold weather explains why winter finds out weak batteries.
How we can help
If your car keeps going flat or will not start even with leads, we come to your home or workplace across Aylesham, Canterbury, Dover, Deal and nearby villages. We test the battery, check the charging voltage from the alternator, look at terminals and earth straps, and use a code reader to check for stored faults. Labour is £30 to £50 per hour depending on the job, and you get a quote before any work starts. Send your reg and postcode on WhatsApp 07563 350335 or use our contact page.
Need a service or brakes doing?
Message us on WhatsApp with your reg and postcode, or use our contact form, for a price before any work starts.
WhatsApp for a free quote Call 07563 350335 Contact form
Labour £30 to £50 per hour depending on the job. Send your reg number and we will reply with a price before any work starts.
Frequently asked questions
What order do jump leads go on?
Red on the flat battery positive, red on the good battery positive, black on the good battery negative, then black on a bare metal earth point on the flat car. Take them off in the reverse order.
How long should I leave the car running after a jump start?
Drive for at least 30 minutes at steady speeds if you can, rather than idling on the drive. A short trip round the village will not put back what the starter took out.
Can a jump start damage my car?
It can if the leads go on the wrong way round, touch each other, or if you use a booster pack that is not suitable for your car. Follow the handbook and the order above and the risk is low.
Should I jump start a hybrid or electric car?
Check the handbook first. Many hybrids and EVs have a small 12V battery that can be jumped from set points, but some makers say the car must not be used to jump another vehicle.
