How to Jump Start a Car Safely (Step-by-Step UK Guide)

By Salek Tyre & Mechanic · Ashford, TW15

A flat battery is one of the most common reasons a car won't start and in many cases, jump leads and a second vehicle are all you need to get going again. Done correctly, jump starting is safe and straightforward. Done in the wrong order, it can damage expensive electronics or, in rare cases, cause a spark near the battery. Here's the correct procedure.

Before you start: safety checks

Before connecting any jump leads, check the following:

• Inspect the flat battery look for cracks, bulging, leaking fluid, or a strong smell of sulphur (rotten eggs). If any of these are present, do not attempt a jump start. The battery needs to be replaced professionally.
• Check both vehicles are compatible both cars should have the same system voltage (virtually all modern cars use 12V). Do not use a 24V truck or van to jump start a standard 12V car.
• Check your car's handbook some modern vehicles (particularly hybrids, plug-in hybrids, and certain European makes) have specific jump start procedures or must not be jump started at all. When in doubt, check first.
• Position the vehicles park the working car close enough for the jump leads to reach both batteries, but ensure the cars do not touch each other.
• Turn off both vehicles both engines should be off before connecting the leads.

What you need

• A set of jump leads (also called booster cables) thicker cables transfer more current and are more reliable. Cheap thin leads can overheat on larger engines.
• A second vehicle with a healthy, charged battery of the same voltage (12V)

Alternatively, a jump start pack (a portable battery booster) means you don't need a second vehicle. These are worth keeping in the boot, especially in winter.

The correct connection order

The order matters. Connecting leads in the wrong sequence can cause sparks, damage electronics, or in rare circumstances ignite hydrogen gas from the battery. Follow these steps exactly:

1. Red clamp → dead battery positive (+)
   Connect the red (positive) jump lead to the positive terminal of the flat battery. The positive terminal is usually marked with a + symbol and often has a red cover.
2. Red clamp → good battery positive (+)
   Connect the other end of the red lead to the positive terminal of the working battery.
3. Black clamp → good battery negative (−)
   Connect the black (negative) lead to the negative terminal of the working battery.
4. Black clamp → earth point on the dead car
   Connect the final black clamp to an unpainted metal surface on the dead car's engine a bolt, bracket, or metal strut away from the battery, fuel lines, and moving parts. Do not connect it to the dead battery's negative terminal.

Starting the cars

5. Start the working car and let it run for 2–3 minutes. This allows some charge to transfer to the flat battery.
6. Try to start the dead car. If it doesn't start first time, wait another minute and try again. If it still doesn't start after several attempts, the battery may be too far discharged or have an underlying fault.
7. Once the previously dead car is running, let both engines run for a further 2–3 minutes before disconnecting the leads.

Disconnecting the leads reverse order

Remove jump leads in the exact reverse of the connection order:

1. Black clamp from the earth point on the previously dead car
2. Black clamp from the negative terminal of the good battery
3. Red clamp from the positive terminal of the good battery
4. Red clamp from the positive terminal of the previously dead car

Do not let the clamps touch each other or any metal surface while the other end is still connected.

What to do after a successful jump start

The car will run off the alternator once started, but the battery is still depleted. To recharge it:

• Drive for at least 20–30 minutes at a mix of road speeds not just around town at low speed. Motorway or A-road driving recharges the battery faster than urban driving.
• Avoid turning the engine off and on again in the short term (e.g. popping into a shop briefly) until the battery has had time to recharge.

If the car keeps going flat

A one-off flat battery after leaving lights on or not driving for weeks is normal. But if the battery goes flat repeatedly, the cause is usually one of:

• Battery at end of life most car batteries last 3–5 years. An old battery loses its ability to hold a charge.
• Parasitic drain something in the car is drawing power when it's switched off (a faulty module, alarm, or aftermarket accessory).
• Alternator fault if the alternator isn't charging the battery properly, the car will drain it while driving.

A battery and charging system test can identify which of these is the cause. See our battery guide: Car battery warning signs how to tell when it needs replacing →

If a replacement is needed: Battery replacement cost in Ashford TW15 →

When not to jump start call for help instead

• The battery is cracked, bulging, or leaking
• There is a strong smell of sulphur near the battery
• Your vehicle is a hybrid or plug-in hybrid (different procedure check the handbook)
• The handbook for either vehicle states jump starting is not permitted
• You are on a motorway or fast road call your breakdown provider rather than attempting a roadside jump start in traffic