How to jump start a car safely

A flat battery is one of the most common reasons a car refuses to start, and a jump start will often get you moving again in a few minutes. Done in the wrong order, though, jump starting can damage sensitive electronics or, in the worst case, cause a battery to vent gas near a spark. The method below is the safe, widely recommended sequence. Take it slowly and it is straightforward.

Before you begin, make sure the flat battery really is the problem. If the car is completely dead with no lights and no click, a flat battery is likely. If you are not sure what is happening, our guide on why your car will not start can help you narrow it down.

What you need and a few safety checks

You need a set of jump leads in good condition or a charged jump pack, and if you are using leads a second running car with a battery of the same voltage. Nearly all cars are 12 volts, but do not attempt this if either battery looks damaged, cracked, leaking or frozen. Remove any loose scarves or ties, take off metal jewellery, and if you have safety glasses to hand, wear them.

Position the working car close enough that the leads reach comfortably, but make sure the two cars are not touching. Put both in park or neutral, apply the handbrakes, switch off the ignition on both, and turn off lights, heaters and other electrics. The Highway Code covers the basics of looking after a vehicle safely in its vehicle maintenance and safety annex.

Where the battery actually is

Do not assume it is under the bonnet. Plenty of cars carry the battery in the boot, under the boot floor, or under a seat, and on those models the manufacturer usually provides a remote jump point in the engine bay instead: a red positive post under a plastic cover, and a marked earth stud. Use those points if your car has them. If you cannot find either, spend two minutes with the handbook before you start clipping leads onto things.

Jump leads or a jump pack?

Both do the same job. The difference is what you need to have with you and how much protection you get if something goes wrong.

If you buy leads, buy heavy ones. Thin budget leads with small clamps are the single most common reason a jump start fails on a diesel or a larger engine: they simply cannot carry enough current, and they get hot trying. If you buy a pack, put a reminder in your phone to top it up a few times a year, and keep it in the car rather than the garage.

No leads, no jump pack, and nobody to help

This is the situation most guides skip, and it is the one people actually find themselves in at seven on a Monday morning. Here is what is left, roughly in the order worth trying.

Ask for a jump pack rather than a jump. Enough people carry one now that it is worth asking, and it saves a stranger having to park nose to nose with you and hand over their electrical system. If someone offers leads from their own car, look at the leads before you accept. Thin ones with small clamps on a cold diesel will get warm and achieve nothing, and you will both have wasted twenty minutes.

Push starting works, but only on a manual, and only on a weak battery. With the ignition on and second gear selected, hold the clutch down, get the car rolling at a brisk walking pace with someone pushing, then lift the clutch sharply. It works because the wheels turn the engine instead of the starter motor doing it. That means it rescues a battery with something left in it, since the ignition and fuel systems still need power, and it does nothing for a battery flat enough to leave the dashboard dark. It will not work on an automatic, as explained below, and on many newer cars an electric parking brake or electric power steering makes it a bad idea anyway.

Call recovery, but do the sums before you agree to a roadside battery. If you have breakdown cover, use it, and ask the patrol to test the charging system rather than just getting you going. If you do not, a one-off callout is not cheap, and a battery fitted at the roadside is normally dearer than the same battery fitted at a garage, where a standard one runs about £70 to £130 fitted. If the car is somewhere safe and you can leave it until the morning, you usually have the cheaper option.

Do not sit there cranking. Ten seconds at a time with a rest in between is the limit. Holding the key over drains what little is left and overheats the starter motor, which turns a battery problem into a starter motor replacement.

The correct connection order

This is the part that matters most. Connect the four clamps in this exact order.

1. Red lead to the flat battery positive. Clip one end of the red, positive lead to the positive terminal of the flat battery. The positive terminal is usually marked with a plus symbol and often has a red cover.

2. Red lead to the good battery positive. Clip the other end of the red lead to the positive terminal of the working car battery.

3. Black lead to the good battery negative. Clip one end of the black, negative lead to the negative terminal of the working car battery, marked with a minus symbol.

4. Black lead to an earth point on the flat car. Clip the final black clamp to a solid piece of bare, unpainted metal on the engine block or chassis of the car with the flat battery. Do not attach it to the flat battery negative terminal. A bolt or bracket on the engine, well away from the battery, fuel lines and any moving parts, is ideal.

That last step catches a lot of people out, so it is worth understanding why. A flat battery can give off a small amount of flammable gas. Making the final connection away from the battery means that if there is a tiny spark as the clamp touches down, it is nowhere near that gas. It is a simple habit that keeps the whole job safer.

Cars that need a different approach

The sequence above covers the great majority of petrol and diesel cars. A few situations need more care.

Do not use a hybrid or electric car as the donor

The RAC is explicit about this: avoid using a hybrid or electric car to jump another vehicle, because it can cause damage. Their small 12 volt systems are not built to push that much current into someone else's battery. A hybrid or electric car that will not wake up can itself be jump started on its own 12 volt battery, which is a separate thing from the high voltage traction pack. Never touch the orange high voltage cabling or the traction battery.

Stop-start cars

Cars with engine stop-start use an AGM or EFB battery, built for the constant restarting. They jump start normally, but when the time comes to replace one it has to be the same type, and on many models the new battery has to be registered to the car electronically so the charging system treats it correctly. That is a garage job, and fitting an ordinary battery instead is a false economy that shortens its life.

Never bump start an automatic

Push or bump starting works only on a manual, and even then it is a last resort. On an automatic it will not work and can damage the transmission. If you drive an automatic, it is leads, a pack, or a call for help.

Starting the engines

With all four clamps on and secure, start the engine of the working car and leave it running for a few minutes. This lets it feed some charge across to the flat battery. After that, try to start the car with the flat battery. If it fires up, leave both engines running together for around five minutes before you touch the leads.

Crank in short bursts of a few seconds with a pause between them, rather than holding the key over. Long continuous cranking overheats the starter motor and pulls the donor battery down as well.

If it still will not start

What you hear tells you a lot about what is wrong.

• Rapid clicking, then nothing. Usually still not enough current. Check the clamps are biting clean metal, wiggle them onto the terminals properly, and let the donor run a few minutes longer before trying again.
• Slow, laboured cranking. The battery is taking some charge but the engine will not catch, which points at a fuel or ignition problem rather than the battery.
• Complete silence with the dashboard lit. Often a starter motor or ignition switch fault, not the battery at all.
• It starts, then dies as soon as the leads come off. That is the classic sign of a charging fault. See our guide to what an alternator replacement costs.

If two or three careful attempts get you nowhere, stop. Repeated cranking flattens the donor car too, and at that point a breakdown service or a garage will get you moving faster than another go with the leads. A diagnostic check will find a starting fault that is not the battery.

Disconnecting in reverse

Once the previously flat car is running, remove the leads in the reverse of the order you fitted them, and take care that the clamps do not touch each other or any metal while the other end is still attached. Remove the black clamp from the earth point on the previously flat car first, then the black clamp from the working car battery, then the red clamp from the working car, and finally the red clamp from the car you have just started.

Run the engine afterwards

A jump start only gives the battery enough to fire the engine. It does not recharge it. To put charge back you need to drive the car for a decent run, around half an hour on faster roads is a good guide, so the alternator can top the battery back up. Avoid stopping and restarting straight away, and try not to switch off before that charging run is done or you may find the car flat again.

Why it always happens on the coldest morning

It is not bad luck and it is not your imagination. Cold does two things to a car at once, and they work against each other. The chemical reaction inside a lead-acid battery slows down as the temperature drops, so it has less to give, and at the same time the engine oil is thicker and the engine is physically harder to turn, so the starter motor asks for more. A battery that was quietly marginal in October gets found out on the first frost.

The scale of it shows up in breakdown data. The RAC reported that its patrols dealt with 3,600 battery-related breakdowns on Monday 7 January 2019, which was 31% of all its call-outs that day. Nearly a third of a national breakdown service's work on one winter morning, from a single part.

Two things follow from that. The first is that a jump start in January is far more likely to be a symptom than an accident, so treat it as information rather than an inconvenience. The second is that the fix is seasonal: a battery test in autumn is worth ten in February, which is why it sits near the top of our winter car checks. Short journeys make it worse still, because a fifteen minute commute never puts back what starting the car took out.

When a jump means the battery needs replacing

An occasional flat battery can be down to a one off, such as leaving the lights on overnight or a very short journey pattern. But if your car needs jump starting again and again, or struggles to start every cold morning, the battery is likely near the end of its life or something is draining it. Most car batteries last roughly five to seven years, so age is often the answer.

It is worth learning the car battery warning signs so you can plan a replacement rather than get caught out. Repeated jump starts are hard on the car and inconvenient, and a battery that keeps dying should be tested by a garage. They can check the battery, the alternator that charges it, and whether anything is drawing power when the car is off, then fit a new battery if that is what is needed.

The three things a garage will check

A proper battery diagnosis is not just a voltage reading. There are three separate faults that all look the same from the driver's seat, and knowing which one you have decides what you pay for.

1. The battery itself. A load test shows whether it can still deliver cranking current, not just whether it reads twelve and a bit volts sitting still. A tired battery often passes a voltage check and fails a load test.
2. The alternator. If it is not putting charge back in, a brand new battery will go flat too. This is why replacing the battery without testing the charging system sometimes fixes nothing.
3. A parasitic drain. Something staying awake after you lock the car, often an aftermarket accessory, a dashcam wired to a permanent live, or a faulty module. It is found by measuring the current draw with everything off, and it is the reason a car goes flat over a few days of standing.

Cold weather makes all three worse, because a battery has less to give and the engine is harder to turn. If yours is already marginal in October it will let you down in January, which is why a battery check belongs on your winter car checks before the first frost rather than after it.

What it costs to put right

Once you know which of the three faults you have, the money is fairly predictable. These are UK fitted prices, and they are the ranges we use across our cost guides.

The ranges are wide because the car matters more than the part. A dedicated guide covers each one: what a new car battery costs, what an alternator replacement costs and what a starter motor costs. The one thing worth insisting on is a test before a part is ordered, because these three faults are close to indistinguishable from the driver's seat and two of them are cheaper than the one most people assume.

Does the MOT check any of this?

No, and it catches people out every winter. Nothing in the MOT tests whether your car starts. The section of the DVSA inspection manual covering lamps, reflectors and electrical equipment does look at the battery, where being insecure in its carrier or leaking is a recorded defect, and at visible wiring. It does not test the battery's condition, it does not test the charging system, and there is no check of the starter motor at all.

So a fresh pass certificate tells you nothing about whether the car will start next Tuesday. Starting reliability belongs to servicing rather than testing, which is one of the clearer differences between the two and is set out in our comparison of an MOT against a service. Our guide to what is checked in an MOT test covers the full scope, and you can check when your MOT is due in seconds.

The bottom line

Red to the flat positive, red to the good positive, black to the good negative, and the last black clamp to bare metal on the flat car. Start the donor, then the flat car, disconnect in reverse, and drive for half an hour. That gets you moving today.

What it does not do is fix anything. A car that needed jumping once after a light was left on is fine. A car that needs jumping twice has a fault, it is one of three, and guessing costs more than testing. If yours has been on the leads more than once, compare local garages and book a battery and charging test. It is a short job, and no payment is taken on this site.