Jump starting a car: how to do it right

In what order do I connect jump leads?

Both engines off, and the two cars must not touch. Then red to the positive terminal of the flat battery and to the positive terminal of the donor battery, then black to the negative terminal of the donor battery and finally to a ground point on the stranded car's engine block. Only then start the donor car.

1. Park both cars, ignition off, handbrake on, and make sure the bodies do not touch
2. Red clamp to the positive terminal of the flat battery
3. Other end of the red cable to the positive terminal of the donor battery
4. Black clamp to the negative terminal of the donor battery
5. Other end of the black cable to a ground point on the stranded car's engine block, not to its battery
6. Start the donor car, then the stranded car, at most three attempts per the ADAC

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How do I disconnect the cables after the engine starts?

Exactly the reverse of connecting, and with electrical loads switched on. The ADAC recommends switching on the blower or the lights in the stranded car before disconnecting, so voltage spikes are absorbed. Then release the black clamp on the stranded car first, then on the donor car, and finally the red cable on both.

1. Switch on a strong load in the stranded car, for example the blower or the dipped beam
2. Release the black clamp from the ground point of the stranded car
3. Release the black clamp from the negative terminal of the donor battery
4. Release the red clamp on the donor car
5. Release the red clamp on the stranded car and make sure the clamps touch nothing

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Which jump leads do I need?

Ones that meet DIN 72553 or ISO 6722, with fully insulated clamps and a sufficient cross-section. The ADAC names at least 25 square millimetres, because diesels and larger engines draw far more current when cranking than small petrol engines. Thin bargain cables get hot and deliver too little starting current.

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Why does the black clamp not go on the flat battery's negative terminal?

Because the final contact creates a spark, and a deeply discharged or defective battery can vent explosive gas. That is why the ADAC explicitly says not to connect the black cable to the stranded car's own battery. On start-stop cars there is a second reason: the battery sensor on the negative terminal can be damaged.

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The car still will not start: what now?

After three failed attempts, stop. That is the ADAC rule, and it protects the starter from overheating. The fault is then usually not the battery: starter motor, fuel supply, immobiliser or rodent damage are all candidates. Are the clamps clean and on bare metal? If so, only roadside assistance or a garage will help.

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How long do I have to drive after a jump start?

The engine must not be switched off again straight away. Per the ADAC both cars keep running for a few minutes before the cables come off, and after that the alternator needs a longer drive to recharge the battery. Short trips with many loads switched on will flatten it again.

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Can an electric car give a jump start?

Receiving one yes, giving one usually not. Per the ADAC many hybrids and electric cars must not serve as the donor vehicle, because their electrical system is not meant to deliver those currents. The other way round, almost every electric car has its own 12 volt battery that can go flat and is jump started normally.

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