If you have run out of battery, you will want to know how to connect jump leads to the car and start it.

We explain the correct way to connect jump leads to a car when its battery is flat, so you can avoid scares and breakdowns.

One of the most common reasons why roadside assistance services are called out for motorists is not knowing what to do if the car’s battery has run flat.

That feeling that your car has died or, at the very least, fallen into a deep state of lethargy could, in some cases, have been avoided by following our tips for keeping the car battery in good condition.

But it can also be solved by replacing the battery or, temporarily until it is swapped for one fully charged, by connecting the two batteries with jump leads or by using a jump starter.

Before attaching the jump leads, make sure the battery is discharged. Use a multimeter or a voltage tester to confirm that the battery has no electrical charge.

If that is not possible, check that the lighting systems and other services such as the multimedia system or the climate control are not working. Even so, be cautious and do not touch either of the terminals of the battery.

Then comes the moment to connect the two cars, one donor and the other receiving electrical power. A transfer that can be done using the famous battery jump leads.

Preliminary instructions

In any case, the process must be carried out in a specific way to avoid major problems both in the vehicle giving the power and in the one receiving it.

Despite what it may seem, the first step is not to clip the terminals of the charged battery at random and then do the same on the car with the flat battery.

Nor is it to connect the cables to the battery of the car that is going to donate energy and then carry out the same action on the receiving car.

How to connect the batteries of two cars

Therefore, the sequence for connecting the jump leads to the car correctly is as follows:

1. Switch off the car that will act as the donor and, in both vehicles, remove the keys from the ignition.
2. Locate the positive terminals, marked with the + symbol, usually covered by a cap or plug always red in colour, and the negative terminals -, which are black, on both batteries.
3. Identify the cables and their corresponding clamps and direct them towards each of the terminals, respecting the type of charge: the positive ones, red, and the negative ones, black, without clamping them yet.
4. Connect the red clamp to the discharged battery.
5. Clamp the positive terminal of the charged battery.
6. Again, on the charged battery, place the black clamp on the negative terminal.
7. Now, only one negative clamp remains to be connected. This should be attached to a metal part such as the car’s chassis, the bolts of the suspension strut tower or any element that conducts electricity, but do not connect the last clamp, the black one, to the negative terminal of the discharged battery.
8. Insert the key into the donor car and start it. The current will then begin to flow. Wait a reasonable time, between 5 and 10 minutes, so that the car with the previously discharged battery has received and stored enough charge.
9. After that time, get into the car receiving the power and try to start it.
10. If the car in trouble has managed to start again, it is time to remove the cables and clamps.

How to disconnect the battery clamps from a car

• Disconnect the negative clamp that was attached to a metal part of the car.
• Do the same with the black clamp that was connected to the donor car.
• Remove the positive clamp from the receiving car.
• Finally, disconnect the red clamp from the car that allowed the vehicle in difficulty to get going again.

What to do when a car with a flat battery is started

Sometimes, through lack of knowledge, due to the situation or because they believe the problem has been solved, some people immediately switch off either of the two cars involved in the case explained above on how to connect jump leads to a car.

This can cause, on the one hand, the car that has given up energy not to have recovered from the donation and, on the other, the one that has received the electricity not to have sufficient charge to carry out again the operation that requires more energy use: starting.

For this reason, once a car has been started with battery jump leads, neither of the cars involved should be switched off.

In fact, the best thing is to drive for a few kilometres to ensure the energy is regenerated in the case of the car that has provided the electricity, and to go to a workshop or a car parts shop to buy and replace the battery of the car that had been left flat.