The last clamp never goes on the battery, and on a modern car the reason has changed
Jump leads are the last piece of car equipment almost everybody still uses from memory.
Nobody opens a handbook with a flat battery and someone waiting behind them. The sequence most drivers carry in their heads was learned on a car with a lead-acid battery bolted into the engine bay, two terminals in plain sight and no electronics that cared what you did to them. That car went out of production two decades ago. On most things built since roughly 2010 the battery has moved to the boot, under a seat or under the floor, and what you find under the bonnet instead is a positive post under a red plastic cap and an earth stud bolted to the body, both put there by the manufacturer for exactly one purpose: so that you never touch the battery at all. The reason behind the old rule has quietly gained a second half, too. Keeping the final connection away from the battery has always been about hydrogen, and it still is. What is new is that on a modern car a small sensor sits clamped around the negative battery cable, counting every amp going in and out, and a jump lead hung on the negative post feeds current straight past it. The engine starts either way. What the car believes afterwards is not the same.
Which end goes where, and which one goes last
There are four connections and their order is the whole safety procedure. Red goes first to the positive terminal or jump post of the donor car, then to the positive post of the flat one. Black goes to the donor's negative terminal or its earth point, and only then, last of all, to a piece of bare unpainted metal on the flat car, well away from its battery. That fourth clamp is the one that completes the circuit, which means it is the one that sparks, and the point of putting it on the far side of the engine bay is that the spark happens somewhere other than directly above a battery that is quietly giving off hydrogen. Park the cars close but never touching, since bodywork contact makes an earth path nobody planned. Start the donor, leave it running for two or three minutes so the flat battery takes a surface charge, then crank the dead car in bursts of no more than ten to fifteen seconds with half a minute between attempts, because a starter motor held on load overheats fast. Everything comes off in exact reverse order, and the loose clamps are kept apart while you do it.
The sensor on the negative cable is why the rule got stricter
Look at the negative battery cable on almost any BMW, Volkswagen, Mercedes or Volvo of the past fifteen years and you will find a plastic module moulded around it, roughly the size of a thumb. That is the battery monitoring sensor, and it measures current, voltage and temperature so the car can keep a running estimate of state of charge. Almost everything sensible your car does with electricity depends on that estimate: how hard the alternator works, whether stop-start is offered at the next junction, whether the seat heaters get switched off to protect the start after a short trip, when it warns you. The sensor only sees what passes through the negative cable, so current arriving at the terminal post above it is invisible. Hook up there and you have poured energy into a battery the car does not know has been charged, which is why manufacturers do not merely suggest the designated points but specify them, and why an owner who jump starts at the terminals often finds stop-start refusing to work for days afterwards for no obvious reason.
The leads in your boot may be too thin to turn a diesel over
Jump leads are sold on the length of the cable and the size of the clamps, and the number that actually matters is printed in small type: the conductor cross-section in square millimetres. A petrol engine up to around 2.5 litres wants at least 16 mm². Anything larger, and every diesel without exception, needs 25 mm² or more, because a cold diesel with high compression asks its starter for a great deal more current than a petrol engine ever does. Undersized copper does not fail politely. It drops so much voltage along its own length that the engine barely turns, the cable gets hot in your hands, and in the worst case the insulation softens while you are leaning over it. Aluminium-cored leads, which are cheap and light, need a bigger section again to carry the same current. Longer leads need more copper than short ones. If you cannot find a cross-section printed anywhere on the packaging, that is the answer about the cable.
An electric car will accept a jump start and must never give one
Electric cars have a conventional 12-volt battery, and it strands them just as readily as it strands anything else. It runs the locks, the screens and the control modules, and crucially it provides the power to close the high-voltage contactors, so a flat 12-volt battery means the traction pack sitting fully charged behind you cannot be reached. Jump starting that 12-volt system is normal and is described in the handbook, usually with a jump post in the fuse box under the bonnet, and the orange cables are never part of the procedure. The direction that is forbidden is the other one. An EV or a hybrid should not be used as the donor car: its 12-volt side is fed by a DC to DC converter sized for the car's own modest loads, not for the several hundred amps a stranger's diesel starter is about to ask for, and Tesla among others states plainly in its manual that the vehicle must not be used to jump start another car. The repair is a converter, not a battery, and it is not a warranty matter.
A running engine is not a recovered battery
The most common mistake happens after the engine fires. An alternator is a voltage-regulated device, not a battery charger, and a deeply discharged battery accepts charge slowly, so the gap between flat and full is measured in hours of charging rather than minutes of driving. On a car with the sensor described above it is worse, because intelligent charging deliberately lets the voltage fall back to save fuel whenever it thinks the battery is healthy. The half-hour drive that everybody prescribes returns a fraction of what is missing, the car goes cold overnight, and the same driver is standing there again the next morning with the same leads. Put the battery on a proper mains charger the same evening. Then ask the more useful question, which is why it went flat at all: if nothing was left switched on, you are looking at either a battery at the end of its life or a drain that never sleeps, and the short-trip and cold-weather pattern behind most European breakdowns covers most of the rest. If the answer turns out to be a new battery, that opens a separate job, because on these cars the replacement has to be registered to the charging system or the car keeps treating it like the worn-out one.
What to actually do
The useful half of this is the part you do on a dry afternoon, not the part you do in the rain at seven in the morning.
Open the bonnet now and find the two points before you need them: the positive post under the red cap and the marked earth stud. If your battery is under the bonnet in plain sight, find the earth point anyway, because that is still where the black clamp goes.
Match the leads to the engine, not to the price. At least 16 mm² for a petrol engine up to about 2.5 litres, 25 mm² or more for anything bigger and for every diesel.
Make the earth connection on the flat car last, on bare metal, away from the battery. Reverse the order exactly when you take the leads off.
Let the donor run two or three minutes before you crank, then crank in bursts of ten to fifteen seconds with thirty seconds between them.
Keep the cars apart, keep the loose clamps apart, and switch off the lights, blower and heated screens on both cars first.
Never jump a battery that is cracked, leaking, visibly swollen or frozen solid after a hard night. A frozen battery can burst rather than start.
Do not attempt any of this on a motorway hard shoulder. Everybody goes behind the barrier and the breakdown service comes to you, which is the same rule that now governs warning triangles across Europe.
Put the battery on a mains charger the same evening. The drive home is not the charge, it is only the trip home.
Treat a second flat morning as a diagnosis, not a repeat performance. A battery that goes flat twice with nothing left switched on is telling you something specific.
Frequently asked questions
Should the donor car's engine be running or switched off?
Connect everything with both cars switched off, then start the donor and let it run. The old warning about damaging the donor's alternator dates from cars with far cruder charging systems, and the modern risk runs the other way: with the donor stopped you are asking one flat battery and one healthy battery to turn a cold engine between them, which drags both down and pushes very high current through the leads. A few minutes of the donor idling before you crank does most of the work, because the flat battery takes some charge and then supplies part of the cranking current itself instead of the cables carrying all of it.
My car has stop-start. Does that change anything?
The jump start itself is the same procedure, and the designated points matter even more, because a stop-start car is exactly the kind that runs a battery monitoring sensor and manages charging around it. Two things follow. Stop-start will often stay unavailable for a day or more after a deep discharge, and that is the car protecting itself rather than a fault. And when the battery is eventually replaced, it takes an AGM or EFB type rather than an ordinary one, and the new unit has to be coded to the car or the charging strategy keeps aiming at a battery that is no longer there.
Is a lithium jump pack as good as leads and a second car?
For most people it is better, because it removes the second car and the second driver from the problem entirely, and the good ones have reverse polarity protection that forgives the mistake this whole article is about. The connection points are identical, earth clamp last and on bare metal. The catch is storage. A pack self-discharges quietly in a boot, hates the heat of a summer parcel shelf and delivers noticeably less when it is cold, which is when you want it, so it needs topping up two or three times a year and it is worth checking that the peak current rating actually suits your engine rather than the one in the advertisement.
Can I still bump start a modern car by pushing it?
Rarely, and never on an automatic or an electric car, where there is no mechanical path from the wheels to a starting engine at all. On a manual the gearbox can still turn the engine over, but a truly flat battery leaves the fuel pump, the injection and the engine management without the voltage they need to do anything useful, so the car turns and does not fire. There is also the modern penalty: dumping unburnt fuel into a hot catalytic converter is an expensive way to save a phone call. If there is enough charge left for the dashboard to light up properly, leads or a pack will work anyway, and if there is not, pushing will not help.
It started fine, then it was dead again the next morning. What now?
That is the single most useful symptom in the whole subject, because it separates the two causes. If the car ran normally all day and died overnight, either the battery no longer holds charge, which is ordinary at five or six years old and common much earlier on short journeys, or something is drawing current while the car sleeps: an aftermarket dashcam wired to a permanent live, a tracker, a boot light, a module that never goes to sleep properly. A garage can measure that drain in minutes with a clamp meter. What it is almost never is the alternator, because a failing alternator makes the car die while you are driving it, with the battery warning light on, rather than politely overnight in your own driveway.
