How to jump start a car safely, step by step

A jump start uses a working battery to supply enough current for the starter motor, then lets the alternator recharge the flat battery once the engine runs. The risks come from sparks, incorrect polarity, and poor earthing, so the setup and cable order matter more than speed. 

What you need

Jumper leads in good condition, with intact insulation and solid clamps. 

A second vehicle with a healthy 12 volt battery, or a suitable booster pack. 

Access to the battery terminals or the manufacturer jump points, which can be under bonnet covers or in a remote location. 

Do not jump start if any of these apply

Battery case is cracked, swollen, leaking, or visibly damaged. 

Battery is frozen, or you suspect it is frozen. 

Jump leads are damaged, frayed, or get hot while connected. 

Safety setup

1. Park both cars close enough for the leads to reach, with the vehicles not touching. 
2. Gearbox in Park for an automatic, or Neutral for a manual, then apply the parking brake on both cars. 
3. Switch off both ignitions before any clamp goes near a terminal. 
4. Switch off nonessential electrics on the flat car, including lights, heated screens, audio, and climate controls. 
5. Remove rings, watches, and jewellery that could bridge terminals or bodywork. 
6. Keep flames, smoking, and sparks away from the battery area. Batteries vent flammable gases. 

Identify the correct terminals and earth point

Positive terminal is marked with a plus sign and often has a red cover. Negative terminal is marked with a minus sign. 

On many modern cars, the safest negative connection is not the negative battery post on the flat car. Use a clean, unpainted metal earthing point on the engine block or chassis, away from the battery. Some manufacturers explicitly specify body earth rather than the battery negative. 

Cable connection order

Connect the leads in this exact order, with both engines off.

Step 1: Red clamp to flat battery positive

Attach one red clamp to the positive terminal on the flat battery. Make the clamp bite metal, not plastic. 

Step 2: Red clamp to donor battery positive

Attach the other red clamp to the positive terminal on the donor battery. 

Step 3: Black clamp to donor battery negative

Attach one black clamp to the negative terminal on the donor battery. 

Step 4: Black clamp to a metal earth point on the flat car

Attach the remaining black clamp to a solid, clean, unpainted metal point on the flat car, such as a thick bracket or engine lifting point. Keep it away from the battery and away from moving parts like belts and fans. This reduces spark risk near the battery and gives a strong return path. 

Quick safety check before starting anything:

Clamps are secure.

Leads are routed clear of belts, fans, and pulleys.

Clamps cannot touch each other or any unintended metal. 

Starting sequence

Step 5: Start the donor car

Start the donor car and let it idle for a few minutes. Keep revs gentle. The goal is stable charging current, not high alternator output spikes. 

Step 6: Start the flat car

Try to start the flat car. Crank in short attempts, then pause. If it does not start, wait a couple of minutes and try again. Repeated long cranking heats the starter and cables and does not charge the battery faster. 

If the flat car still does not start after a few attempts

Stop and reassess. Check clamp contact, check you used a proper earth point, and check for corrosion at the battery terminals. If there is still no start, the fault can sit with the battery, starter motor, alternator, or wiring, so roadside assistance or a workshop test is the safer next move. 

Disconnect order

Once the flat car is running, let both engines run for a short period, then remove the leads in reverse order. Keep clamps apart throughout removal.

1. Remove the black clamp from the earth point on the previously flat car. 
2. Remove the black clamp from the donor battery negative. 
3. Remove the red clamp from the donor battery positive. 
4. Remove the red clamp from the previously flat battery positive. 

After the jump start, stop it happening again on the next corner

Keep the revived car running and recharge properly

Drive for around 20 to 30 minutes so the alternator can restore charge. Short idles often fail to put meaningful charge back into a depleted battery. 

Treat a repeat jump start as a warning sign

If the car needs another jump start soon after, the battery can be near end of life, or the alternator system may not be charging correctly. A battery and charging test gives a clear answer. 

Special cases that catch people out

Cars with remote batteries or jump points

Some vehicles place the battery in the boot or under a seat, with designated jump terminals under the bonnet. Use the manufacturer jump points if present, and follow the handbook procedure for that model. 

Hybrids and vehicles with complex electrical systems

Many hybrids still have a 12 volt system that can be supported via jump leads, yet the correct ground point and connection sequence can be model specific. Follow the model instructions and avoid clamping to the 12 volt battery negative if the manufacturer specifies body ground.