Jump-starting with leads, step by step and safely

Jump-starting a car with leads is a classic, but the devil is in the order. Connecting the clamps in the wrong sequence or reversed risks sparking and even damage to both cars’ electronics. Here’s how to do it safely, step by step, and what to avoid at all costs.

When jump-starting even makes sense

Before you reach for the leads at all, it’s worth establishing whether the problem really lies in the battery. A jump start only helps if the battery is flat. If the cause is something else, the leads will do nothing and you’ll needlessly waste time and tire the other car.

• The starter cranks slowly and sluggishly or just “clicks” — a classic sign of a weak battery, the leads will help here
• The car is completely “dead”, no lights or any sounds — usually a deeply discharged or disconnected battery
• The starter cranks normally and energetically but the engine won’t fire — the problem is elsewhere, not in the battery
• You hear a single click but nothing cranks — a possible starter fault, not the battery itself

If after a jump start the car goes flat again and again, the start alone won’t solve the problem. You then need to establish whether the culprit is a worn battery or an alternator that isn’t recharging it. Those are two different causes and two different repairs.

Watch out with hybrids and electric cars

With ever more hybrids and electric cars on the road, one important rule is worth knowing. These cars have two systems: a high-voltage one for propulsion and an ordinary 12 V one for the electronics. It’s the latter that goes flat, and it’s the one you jump-start — never touch the orange, high-voltage cables.

Importantly, hybrids and EVs can only act as a power donor to a limited extent or not at all, because their small 12 V battery isn’t designed for it. You can, however, safely jump-start a flat hybrid from an ordinary combustion car using the starting points indicated in the manual. If in doubt with an unusual car, it’s better to use a professional than to risk costly electronics.

Before you connect the leads

• Position the cars close together but so the bodies don’t touch
• Apply the handbrakes and set the gearboxes to neutral or park
• Switch off the ignition and all electrical loads in both cars
• Check the leads and clamps are in good condition, with no chafed insulation
• Make sure both batteries have the same nominal voltage (12 V)

Good jump leads are the basis of success. Too thin, cheap leads have a high resistance and don’t carry the current needed to turn the starter, especially in a larger engine. It’s worth keeping solid leads of an adequate cross-section in the boot, because on a frosty morning they decide whether the car will start at all.

The connection order

1. Red clamp to the positive (+) of the flat battery
2. The other end of the red lead to the positive (+) of the good battery
3. Black clamp to the negative (–) of the good battery
4. The other end of the black to a ground point on the flat car (bare, unpainted metal, away from the battery), not to its negative
5. Start the working car and leave it running for a moment to raise the voltage
6. After a few minutes try to start the flat car, not cranking the starter for more than a few seconds at a time

If the flat battery is very weak, let the good car run for a few minutes before trying to start — during that time it will charge the dead battery a little and improve the chances of firing up. Don’t crank the starter continuously for more than a few seconds, as this risks overheating it.

Why the order matters so much

Many drivers treat the connection order as a formality, yet it’s exactly what protects against two real dangers: a spark at the battery and a voltage surge in the electronics. It’s worth understanding where they come from, because then the procedure stops being a memorised rule and becomes obvious.

A flat battery, especially an old and heavily used one, can give off small amounts of hydrogen — an extremely flammable gas that collects just above the battery. If the last clamp were attached directly to its negative, the spark created at that moment could ignite that gas and, in an extreme case, rupture the battery case and spray acid. That’s why the last, black clamp is attached to a body ground point away from the battery — any spark then occurs in a safe place, not directly over the battery.

The second danger is the electronics. Modern cars are full of sensitive control units that cope badly with surges and voltage spikes. Reversing the terminals, that is plus to minus, can in a fraction of a second damage the alternator, the fuses and even the computers of both cars. That’s why the red lead is always plus and the black always minus and ground — a mistake here can be costly.

Disconnect in reverse order

Once started, disconnect the leads in exactly the reverse order to connecting them, taking care that loose clamps don’t touch each other or the bodywork. Don’t switch the started car off straight away — let the alternator begin recharging the battery and drive a longer route right away.

Driving alone after a jump start rarely fully charges a deeply discharged battery. While driving, the alternator only maintains and partly recharges the battery, rather than charging it from zero to full. After an emergency start it’s best to connect the battery to a charger at home for a few hours, especially if the discharge was deep or it happens again. Otherwise the next morning, after a night with the clock and alarm running, the car may fail to start again.

Attach the last black clamp to a ground point, not to the flat battery’s negative — a simple rule that guards against a spark at the battery.

The most common jump-starting mistakes

Even knowing the order, it’s easy to make a mistake that at best leaves the car unstarted and at worst damages the electronics. Here are the ones that happen most often.

• Touching the red and black clamps together with the leads connected — risks a short circuit and a spark
• Attaching the last black clamp straight to the flat battery’s negative instead of a ground point
• Too thin, cheap leads that don’t carry the current needed for the start
• Trying to start with no moment of initial charging when the battery is very weak
• Jump-starting from a car with a noticeably smaller battery than the flat car has
• Cranking the starter continuously for more than a few seconds, which risks overheating it

It’s also worth keeping common sense and basic safety in mind. Don’t try to jump-start a battery that is visibly damaged, swollen, leaking or frozen — this risks it rupturing and spraying caustic acid. When working with the battery, avoid open flames and don’t lean over it with your face at the moment of starting. In such a case the battery is fit only for replacement, not for reviving with leads.

An alternative without a second car

No second car? A portable jump starter, which starts the car on its own, is increasingly popular. And if the battery keeps going flat, it’s probably already worn.

A portable jump starter is worth keeping in the boot especially in winter, when batteries fail most often. It’s small, fits in the glovebox and lets you start the car on your own, without asking anyone for help or positioning a second car alongside. Just remember to charge it every so often and store it warm, because a flat or frozen jump starter is of little use on a frosty morning.

Frequently asked questions about jump-starting

Can I jump-start from another, more powerful car’s battery?

Yes, provided both have a 12 V system, which is the case for the vast majority of passenger cars. What matters is that the donor car doesn’t have a noticeably smaller battery than the flat car, because a weaker battery may not cope with starting a larger engine. Do not, however, connect a 12 V system to a 24 V one, found in trucks and some vans — this risks serious, costly damage to a passenger car’s electronics.

The car started, but won’t go again after switching off. Why?

That’s a typical sign of a worn battery that no longer holds a charge and quickly goes flat. A jump start only briefly masks the problem, because the battery doesn’t regain capacity. If the situation repeats, the battery most likely needs replacing, not another charge. It’s also worth checking whether the alternator is charging at all, and whether anything is draining the battery when parked.

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

Driving alone rarely charges the battery fully. Several dozen minutes of a longer route only partly recharges it. With a deep discharge, the surest solution is connecting the battery to a charger at home for a few hours, which restores its full capacity without loading the alternator.

Can jump-starting damage my car?

Done correctly, it’s completely safe. The risk only arises with mistakes: reversing the terminals, shorting the clamps or a spark right at the battery. That’s why the correct order and attaching the last clamp to a ground point, not the flat battery’s negative, matter so much. If you have an unusual car, a hybrid or simply don’t feel confident, it’s safer to call for professional help than to risk costly electronics.

The battery froze in winter. Can I jump-start it?

No. A frozen battery, recognisable by a bulging case, is damaged, and trying to start it risks rupture and an acid spray. Such a battery is fit for replacement. Freezing is, moreover, a sign the battery was already heavily discharged, because a fully charged electrolyte freezes only at very low temperatures.

If you’d rather not fiddle with leads, we’ll come, start the car and, if needed, replace the battery on the spot.