How to Jump Start a Car

What You Need Before You Start

You need a set of jump leads in decent condition or a portable jump starter pack, and a donor vehicle with a charged battery of the same voltage — for almost all cars on UK roads that means 12 volts. Check the leads before anything else: cracked insulation, corroded clamps or frayed cable are all reasons to stop and call for help instead.

Park the donor car close enough that the leads reach comfortably, but the two cars must not touch. Switch both engines off, put both cars in neutral or park, and apply both handbrakes. Turn off everything electrical in the dead car — lights, heater blower, heated screens, phone chargers — so the incoming charge is not fighting a load.

Take a moment to look at the flat battery itself. This is the inspection step most guides skip and the one that matters most. If the case is cracked, swollen or leaking, or you can smell rotten eggs around it, do not connect anything. A damaged battery is not a jump-start candidate; it is a recovery job.

The Lead Order That Matters

There are four connections and they go in a specific order. Memorise it as red-dead, red-donor, black-donor, black-metal:

• Red lead to the positive (+) terminal of the flat battery.
• Other red end to the positive (+) terminal of the donor battery.
• Black lead to the negative (−) terminal of the donor battery.
• Other black end to a bare metal point on the dead car — an engine bracket or bolt — well away from the battery itself.

Starting the Cars and Removing the Leads

With all four clamps secure and not touching each other, start the donor car and let it run for about two minutes. This puts some charge into the flat battery before you ask it to do the hard work of cranking an engine.

Now try the dead car. If it starts, leave both vehicles connected for another minute or two so the charge stabilises, then remove the leads in the exact reverse order: black from the metal point, black from the donor, red from the donor, red from the flat battery. Keep the clamps apart as you remove them.

If the dead car does not start on the first try, wait a couple of minutes and try once more. Do not crank it endlessly — long cranking overheats the starter motor and drains whatever charge you just put in. Two or three sensible attempts is the limit; after that, the problem is probably not a flat battery, and it is time to call us on 07725 943641.

Once running, do not switch the engine off again. The car needs a proper drive of twenty to thirty minutes to recharge the battery — idling on the driveway barely counts. Then get the battery tested at a garage to confirm it is holding charge.

Mistakes That Cause Real Damage

The expensive mistakes are all avoidable. Reversing the leads — positive to negative — can fry electronics in seconds, so double-check every clamp before starting either engine. Letting a connected clamp touch the other car's bodywork creates a short with the same result.

Jump starting a battery you have already inspected and found damaged is the other big one. A cracked or leaking battery can vent gas, and the small spark that sometimes occurs when the final clamp connects is exactly what you do not want near it. That is why the final black clamp goes to bare metal away from the battery, not to its negative terminal.

Modern cars deserve extra respect. Vehicles with start-stop systems and complex electronics are more sensitive to voltage spikes, and some manufacturers specify their own jump procedure or a dedicated jump point under the bonnet rather than the battery terminals. If your handbook gives specific instructions, follow those instead of this guide.

When a Jump Start Is Not the Answer

Knowing when to stop is part of doing the job safely. Do not attempt a jump if the battery case is cracked, swollen or leaking, if there is a strong rotten-egg smell, or if the terminals are bridged with heavy corrosion. None of those are flat-battery problems; all of them are recovery problems.

Stop too if the car has needed jumping more than once recently. A battery that will not stay charged is a dead battery, not a flat one, and each failed jump just moves the inevitable stranding to a less convenient location.

And if the engine cranks strongly but will not fire, the battery was never the problem — that points to fuel, ignition or immobiliser trouble. No amount of jumping will fix it. In all of these cases, call 07725 943641. We will recover the vehicle to your home or a garage where it can be properly diagnosed, any time of day or night.