How to jump start a car

There’s nothing worse than getting in your car and finding out the engine just won't turn over. In situations like this, you may need to jump start the vehicle to give it a boost.

If you’ve never needed to know how to jump start a car before then it can feel like an intimidating process.

However, the good news is that by following the steps below, you’ll be equipped with the knowledge to get your motor up and running again.

What are the signs that your car needs a jump start?

You need to jump start a car if the battery voltage has dropped below what is needed to turn the start motor or keep the car running if it is using electronic systems to run the pump(s), injectors and so on. But what can cause this to happen?

The engine doesn’t make any noise, cranks slowly or doesn’t crank at all. This means you could have a weak or dead battery.

You have dim or flickering interior and exterior lights. Your battery may not have enough power to start the engine.

When you turn the key, you hear a clicking sound. The starter motor is trying to engage but may not be receiving enough power from the battery.

Your electrical accessories stop working properly. If the power windows, radio, power seats or other electrical accessories are malfunctioning or operating sluggishly then you may have a weak battery.

You see warning lights on the dashboard. The battery icon or check engine light may flash up on your dash, indicating that there are battery or charging system issues.

Your engine stalls while you’re driving and you can’t restart it. This is a sign that a failing battery could be the cause.

Unusual odours or sounds could come from the engine. If you smell a sulphurous or rotten egg smell, or hear unusual sounds from the engine bay, you could have a battery problem.

It’s cold outside and your car is struggling to start. Batteries are more likely to fail during the colder months, and a weak battery is often behind the issue.

If you leave your car idle for a while, the battery may discharge. You may not use your vehicle often, or it may have been stored for a while. This can cause battery problems.

What can cause car battery issues?

It’s likely you will recognise one of the issues below when you’re thinking about why your battery is having problems – whether you’ve got an old car, you’ve been letting the battery rattle around for a while or you suspect a parasitic drain...

The age of the battery. Most car batteries have a lifespan of three to five years. As they age, their capacity to hold a charge decreases, leading to reduced performance and eventual failure.

Extreme temperatures. Extremely hot or cold weather can have a significant impact on battery performance. Cold weather can reduce a battery's capacity, making it harder to start the engine, while hot weather can lead to the evaporation of battery fluids and internal damage.

Parasitic drain. Some electrical components in a vehicle can continue to draw power from the battery even when the car is turned off. This is known as parasitic drain and can lead to a dead battery if the vehicle is not driven for an extended period.

A faulty charging system. Problems with the alternator, voltage regulator, or other components of the charging system can prevent the battery from charging properly. This can lead to a dead battery, even if the battery itself is in good condition.

Corrosion. If the battery terminals or cables become corroded, this can interfere with the flow of electricity, reducing the battery's ability to charge and discharge effectively.

Loose or damaged cables. These can disrupt the connection between the battery and the electrical system, leading to voltage drops and starting problems.

Taking too many short trips. Frequent short trips without giving the battery enough time to recharge fully can lead to a gradual depletion of the battery's charge, reducing its lifespan.

Overusing electrical accessories. Using electrical accessories like lights, radios, or power outlets when the engine is off can drain the battery. Leaving lights on overnight is a common cause of battery drain.

Excessive vibration. Often caused by rough driving conditions or loose battery hold-downs, excessive vibration can damage the internal components of a battery and lead to premature failure.

Sulphation. Over time, lead-acid batteries can develop sulphate buildup on the plates inside the battery, reducing their ability to hold a charge.

Manufacturing defects. In rare cases, a battery can have manufacturing defects that lead to premature failure.

Before you dive in and dig out the jump leads, you’ll need to reduce your risk of damaging the vehicle – and yourself...

Safety first!

Park both vehicles in a way that their batteries are close enough for the jumper cables to reach but not touching each other.

Turn off both cars' engines and put them in neutral or park. Don’t forget to take the key out of the ignition.

Engage the parking brakes on both vehicles.

Ensure that all lights, radios, and other accessories are turned off to minimize electrical load.

Ensure the battery isn’t damaged or leaking. You will need to seek help from a mechanic if this is happening.

Don’t smoke a cigarette or flip open a lighter while you’re working on the job.

Don’t dangle clothing or metal items, such as jewellery, near the battery while you’re working on it.

Stop using jump leads if they become hot during the process.

Don’t remove the jump leads while either car engine is running as you could end up damaging the vehicle electronics.

Should you jump start a hire car?

It’s all well and good if you are jumping your own car so you can get to work on time or leave a car park, but if you’re encountering this issue with a hire car then you should proceed with caution. Consider the information below before you try to fix the issue yourself.

The rental agreement. Most rental car companies have specific terms and conditions outlined in their rental agreements. Jump-starting a rental car may violate these terms, which could result in penalties or fees. Check over the small print before you get the jump cables out.

Liability. Because you are renting the vehicle, you may be exposed to liability if you accidentally damage the vehicle or cause any electrical issues from jump-starting it. Car hire excess insurance can save you £100s if you have caused damage to a hire car, but you may not be able to claim in a situation like this.

Roadside assistance. Many rental car companies offer roadside assistance as part of their rental packages. If your hire car's battery is dead, it's usually best to contact the rental company's roadside assistance service, which can send a professional to assist you. This service is often included in the rental fee. You may be in a rush to make your flight on time, but you could end up paying out for damages if you can’t wait for this service.

Unknown vehicle history. You may not have information about the maintenance history or condition of the rental car's battery, which can make jump-starting riskier.

Safety concerns. Jump-starting a rental car may involve unfamiliar procedures, and if not done correctly, it can pose safety risks to you and the vehicle.

If you find yourself in a situation where your hire car battery is dead, it's generally advisable to follow these steps:

Contact the car hire company. Call their customer service or roadside assistance hotline. They can provide guidance on how to proceed and may send a professional to assist you.

Follow their instructions. The company will provide instructions on what to do next, which may include waiting for a service technician to arrive or providing further assistance over the phone.

Document the situation. Take pictures of the hire car, its mileage, and any damage or issues to document the situation in case of disputes or claims later.

What you’ll need

If you have decided to jump start your car, here’s what you will need for the job:

Jump cables. Obviously, this is the primary tool for jump-starting a car. They consist of two insulated cables with clamps on each end. These clamps are used to connect the batteries of the two vehicles.

Another functioning vehicle. You'll need access to another vehicle with a charged and functioning battery. This vehicle will provide the power needed to jump-start the dead battery in your car.

Safety gear. While not absolutely necessary, it's a good idea to wear safety glasses and gloves when working with batteries and jumper cables to protect your eyes and hands from sparks and battery acid.

Owner's manual. It's helpful to have the owner's manual for both your vehicle and the donor vehicle. These manuals can provide specific jump-starting instructions and important safety information.

Ensure that the jumper cables you use are in good condition, with no exposed wires or damage to the insulation.

It's also crucial to choose the appropriate gauge of jumper cables for your vehicle. Thicker gauge cables can carry more current and are suitable for larger vehicles with larger batteries.

How to jump start a car with jump leads

Now it’s time to jump your car. Follow the steps below to get the job done:

Open the bonnets.

Open the bonnets of both vehicles.

Identify the batteries.

Locate the batteries in each vehicle. They are typically rectangular or square boxes with two terminals: one positive (+) and one negative (-).

Identify the jumper cables.

Jumper cables come in pairs. One cable is colour coded red, and the other is colour coded black. Red is for positive (+) and black is for negative (-).

Each cable has two clamps (four in total), so you have two red clamps and two black clamps.

Connect the jumper cables.

Follow this sequence carefully:

Step 1. Connect one red (positive) clamp to the positive terminal of the dead battery.

Step 2. Connect the other end of the red (positive) cable to the positive terminal of the working battery.

Step 3. Connect one black (negative) clamp to the negative terminal of the working battery.

Step 4. Attach the other black (negative) clamp to an unpainted, metal surface on the dead car's engine block, away from the battery. This acts as a ground and reduces the risk of sparks near the battery.

Now wait for five minutes.

Start the working vehicle.

Start the engine of the working vehicle and let it run for a few minutes to charge the dead battery.

Attempt to start the dead car.

Try starting the dead car. If it starts, let both vehicles run for a few minutes to allow the dead battery to charge.

Remove the jumper cables.

In the reverse order of connection:

Step 1. Remove the black (negative) clamp from the previously dead car's engine block.

Step 2. Remove the other end of the black (negative) cable from the working car's battery.

Step 3. Remove the red (positive) clamp from the working car's battery.

Step 4. Finally, remove the red (positive) clamp from the previously dead car's battery.

Drive the previously dead car.

Let the previously dead car run for a while to ensure the battery is fully charged. Then, drive it around for at least 15-20 minutes to charge the battery further.

Aftercare

Don’t forget that jump-starting is a temporary solution. Consider getting your car looked at by a professional, and bear in mind that you may need to replace the battery.

If you have jump started a hire car, it’s recommended to tell the car hire company so they can get the battery looked at.

About the author

Dom has loved cars since he was five years old. He has worked as a mechanic for years, relishing the opportunity to be around vehicles and keep them in top shape.

Dom trained as a driving instructor once his daughter got her first car so he could help people to stay safe on the roads.

Now he takes every chance he can to share his knowledge of cars with the general public, focusing on vehicle maintenance and safety.