How to Jump-Start a Car

A dead battery usually gives one of three answers when you turn the key: a single loud click and then nothing, a rapid clicking sound, or an engine that cranks slowly and weakly. Dim or flickering interior lights at the same time point at the battery too.

If the lights stay bright and the engine cranks at full speed but never fires, the problem is fuel, spark, or the starter, and jumper cables will not help. Before you ask anyone for a jump, walk the car: a courtesy light that stayed on because a door switch is bent, a boot lid that never quite latched, or a charger left plugged into the 12-volt socket overnight will flatten a battery that is otherwise healthy. Those are the cheapest faults to find, and they are the most common reason a car that drove fine yesterday will not turn over today.

Look at the date sticker on the top of the case or one of the ends. Most batteries carry a letter and number pair, and the useful life is usually three to five years. On many brands the letter is the month and the digit is the year, so B7 reads as February 2017; the maker's own key wins if the sticker looks different.

A battery that is five years old and only barely turning over is telling you what it is going to do next, usually in colder weather. While you are there, look for a swollen or cracked case, a low fluid level on batteries with caps, and a crusty white or blue-green powder on the terminals. A case that has lost its flat, square shape has been overcharged or frozen, and either of those has already done damage inside that no charger will undo.

Set a multimeter to DC volts and touch the probes to the battery posts with everything switched off. A healthy, rested battery reads about 12.4 to 12.7 volts. Below 12.0 volts, the battery is discharged enough to struggle, and below about 10.5 volts it is deep-discharged enough that a slow charger, not a jump, is the better first move.

Start the car if you can and read it again: a charging system that is working holds between about 13.9 and 14.4 volts. If the voltage does not rise while the engine runs, the alternator, the belt driving it, or its wiring is the fault, and a jump start will only ever be a temporary favour. Write both numbers down before you visit a parts shop; a resting reading and a running reading tell a technician more in ten seconds than a description of the symptoms.

You need jumper cables rated for your engine, not the thinnest pair in the bin. For a normal petrol car, 4 or 6 gauge cable in a length that reaches both bays is a sensible choice; diesel and larger engines want thicker cable. Cheap cable is a common reason a jump fails: it heats up, drops the voltage the starter is asking for, and leaves you cranking twice as long as you needed to.

Add gloves and eye protection, and a torch if it is dark. If you are using a booster pack, check that it is charged and that its clamps match your car's polarity layout. Lithium packs are far lighter than the old lead-acid trolleys, but each one has its own cut-off behaviour and its own connection order, so read the pack's instructions once in daylight rather than for the first time in a car park at nine at night.

Put the donor car facing the dead car so the cables reach the posts comfortably, with no slack hanging near a fan or a belt. Turn both ignitions off, set the parking brakes, put automatics in Park and manuals in first gear with the clutch in, and switch off everything electrical, including lights, radio, and air conditioning.

Do not let the two vehicles touch, and keep flames, cigarettes, and phones out of the bay. Do the job off the road when you can reach a lay-by, a forecourt, or a driveway: on a hard shoulder the traffic is the bigger danger, and you will be leaning over an open hood with both hands busy. If the car cannot be moved, set a warning triangle well behind it, wear a high-visibility coat, and stand clear of the carriageway while engines are running.

The positive post is marked with a plus sign, a red cover, or red paint, and it is usually the larger of the two. The negative is marked with a minus sign or black. On the dead car, identify a solid, unpainted metal point on the engine block or a mounting bracket well away from the battery, the fuel rail, and anything that moves. That is where the last clamp goes.

If the posts are buried under corrosion, wipe them with a cloth or scrape them with a terminal brush before clamping; a clamp biting on powder makes a joint that looks solid and conducts almost nothing. Many modern cars hide the battery in the boot or under a seat and leave two marked studs under the hood instead. Use those rather than hunting for the battery itself, and check the manual for where the negative point should be, because on some cars it is one specific bracket rather than any bare metal.

Work through the sequence slowly, one clamp at a time, and make sure each one bites onto bare metal rather than on corrosion or paint. First red to the dead battery's positive post. Second red to the donor battery's positive post. Third black to the donor battery's negative post. Fourth black to the ground point you chose on the dead car's engine, not to its negative post. Give each clamp a gentle tug before you move on.

Keep the free end of each cable clear of the car as you work, so a live clamp cannot rest on a wing or drop into a belt. Insulated clamp sets click or light up when the joint is good, which is worth learning before you need it in the dark. Once all four are on, have the donor driver start the engine and hold a steady idle while you get settled to crank your own.

Start the donor engine and let it settle into a steady idle for a couple of minutes. Then try the dead car. If it catches within a few seconds, let both engines run connected for another two or three minutes before you disconnect.

If nothing happens after three or four attempts, stop: the cables, the clamps, or the battery is not doing what you think, and repeated cranking only heats the starter and drains the donor. Check the ground clamp first, since it is the connection that fails most often, then re-seat both positive clamps and try once more. A car that fires instantly on good connections had a flat battery; one that still will not fire after that has a starter, immobiliser, or fuel problem, and jump leads were never going to solve it.

Leave the revived engine idling for a few minutes with the cables off and the headlights and blower switched off, so the alternator is putting everything it has into the battery rather than into accessories. Listen for a change in idle quality and watch the dashboard: a battery light that stays on after starting means the charging system is not doing its job, whatever the battery's age.

If the car has a voltmeter rather than a warning light, the needle should sit just under 14.5 volts at idle and dip only slightly when you switch on the demister. A reading that stays near 12 volts means the engine is running on the surface charge the cables left behind, and it will not restart once that is gone. Close the hood properly before you set off, and check that no cable end or loose clip is hanging near a fan.

A jump start gives enough charge to start the engine, not enough to run it for a week. Take the car on a real drive, ideally thirty minutes on a road where you are not stopping every few hundred metres. Short hops to the shops recover almost nothing, because a stop-start trip spends most of its time at idle, when an alternator is producing a fraction of what it can at 2,000 rpm.

If the car will sit for a while afterwards, park it and leave it off. A battery that will not hold a charge overnight has a fault to investigate, not a drive to take. Treat the thirty minutes as a diagnostic as much as a repair: if the car fires normally the next morning, the battery is still holding current, and if it is flat again you have a parasitic draw or a battery at the end of its life, and one of those two answers decides what you spend money on next.

A resting voltage reading tells you the charge, not the health. A load test, which most parts shops and garages will do free, draws current from the battery and shows whether it holds its voltage under strain. Ask for the printed result rather than a thumbs up: the number that matters is the cold cranking amps it actually delivered against the rating printed on the label.

If the battery tests weak, replace it before the weather turns; if it tests good but keeps going flat, the fault is elsewhere in the car. A battery that has sat discharged for weeks sulphates internally and can still read 12.5 volts while giving away almost none of that current, which is why a healthy-looking voltage and a failed load test are such a common pair on a car that has been standing.

Common causes are a light left on, a door switch that keeps the interior light awake, an ageing battery, corroded or loose terminals, a parasitic draw from an aftermarket radio or charger, a slipping drive belt, or a car that spends weeks between short trips. Work through them in that order before you blame the battery. Fitting a new cell to a car with a draw only buys you a flat battery in a different month.

A car that sits for long periods benefits from a trickle charger on a mains outlet, or from a booster pack kept in the house rather than the boot. If you suspect a draw, a technician can clamp the live feed and read the current with everything switched off and the doors shut; anything well above 50 milliamps after half an hour, once the modules have gone to sleep, is worth chasing. Clean terminals, a tight belt, one long drive a week, and a charger over winter keep most batteries inside their expected three to five years.