How to jump-start a car without wrecking either battery

Before you touch a cable, look at the battery. Bulging sides, a split case, weeping acid around the terminals, or a thick slow slosh when you rock the car. Any of those and you stop. A fully discharged lead-acid battery can freeze at around minus ten Celsius, where a healthy charged one shrugs off minus forty, and pushing current into a frozen battery is how people end up with acid in their eyes. Cold morning, flat battery, frost on the inside of the windscreen. That's exactly the combination.

Assuming the battery looks sound, the next question is whether it's even the problem. Turn the key and listen. A fast machine-gun clicking from the engine bay is a low battery: the starter solenoid pulls in, voltage collapses, it drops out, repeat. One loud solid clunk and nothing is usually the starter itself, and no amount of jumping will help. Complete silence with a dead dashboard points at the battery, a loose terminal, or a blown main fusible link.

Wiggle the terminals first

This takes ten seconds and it fixes maybe one no-start in six. Grab each battery post by the clamp and try to rotate it. If it moves, that's your fault right there. Nip it up with a 10mm spanner and try again before you bother anybody.

White or blue-green fluff on the posts is another one. It looks harmless and it's a genuine insulator. A wire brush and a rag will do it. If you've got nothing else, the edge of a coin will scrape enough contact area clear to get you going.

What you should have in the boot

Cheap jump leads are close to useless and they're sold everywhere. The number to look at is the gauge, and lower means thicker. Six gauge is the sensible minimum for a normal petrol car, four gauge is better, and two gauge is what you want if you're jumping a diesel or anything with a large engine. The trick with bad leads is thin conductors wrapped in fat insulation so the cable feels substantial in the shop. Squeeze the cable near the clamp. If it collapses easily under your thumb, there isn't much copper in there.

Check the clamps too. The jaws want proper teeth so they bite through surface corrosion, and the spring should be stiff enough that you struggle slightly to open it. Floppy clamps make a poor connection, poor connections get hot, and hot clamps melt insulation onto a battery you're standing over.

A lithium jump pack is the better answer for most people. No second car, no arguing about who parks where, and crucially no risk to somebody else's electrics. Keep it charged, top it up every few months, and don't leave it in a freezing boot all winter, because lithium cells give poor current when they're cold. Mine lives in the house from November.

The order, and the reason for it

Both cars off. Handbrakes on. Park them close enough that the leads reach with slack, and make sure the two bodies are not touching anywhere.

Then switch off everything in the dead car. Headlights, blower, heated rear screen, radio, the phone charger in the socket. On a cold morning all of those are usually still switched on from when the car died, and they'll eat a big share of the current you're about to supply.

Now the clamps.

1. Red to the dead car's positive post. Positive is the thicker post, usually has a red cover and a moulded plus sign. Check it twice. Reversing polarity is the single most expensive mistake here and it can take out fusible links, alternator diodes and control modules in one go.
2. Red to the donor car's positive post.
3. Black to the donor car's negative post.
4. Black to bare unpainted metal on the dead car's engine block or a dedicated ground stud. Not the negative battery post.

That last one catches people out, and there are two good reasons for it. A charging or discharging lead-acid battery vents hydrogen, and connecting the final clamp always makes a small spark. You want that spark happening half a metre away from the vent, not directly above it. The second reason is modern: many cars have a current sensor clamped onto the negative terminal, and grounding straight to the post bypasses it, which leaves the battery management system with a wrong idea of what just happened.

Plenty of cars now hide the battery in the boot or under a seat and give you a red jump post under the bonnet with a plastic flip-up cover, plus a marked ground point nearby. Use those. They exist precisely for this.

Starting it

Start the donor car and leave it running at idle for three or four minutes. You're not "jumping" the other car so much as putting a little charge back into a flat battery so its own starter has something to work with. A fast idle of around 1,500 rpm is fine if the donor is very small and the dead car is very big. Don't sit there revving it to 3,000. It doesn't help much and it does put the donor's alternator under real strain, because a deeply flat battery on the other end of those cables looks like an enormous load.

Now try the dead car. Crank for ten seconds maximum, then stop and wait thirty seconds. Starters are duty-cycle rated and they get genuinely hot, and a cooked starter turns a twenty-minute job into a garage bill.

Three failed attempts is your limit. If it hasn't gone by then, something else is wrong and grinding away at it won't reveal what.

Coming off, and the bit everyone skips

Remove the clamps in exact reverse order: black off the engine block, black off the donor, red off the donor, red off the previously-dead car. Keep the loose ends apart and don't let a live red clamp touch bodywork. Keeping the donor running while you disconnect is fine and avoids a voltage sag.

Now the part that matters. A jump start does not charge the battery. It gets the engine turning. Alternators are designed to maintain a healthy battery, not to resurrect a flat one, and forcing one to do that job at full output for an hour is a decent way to kill its diodes.

Driving for thirty or forty minutes at motorway speed will put a useful amount back. Idling in a car park for the same period will not, because at idle the alternator's output is low and the car's own systems are already drinking most of it. The right answer, if you have the option, is a proper mains battery charger overnight. Anything with a maintenance or recovery mode will do more good in eight quiet hours than a week of short journeys.

Two cars you treat differently

If the dead car is a hybrid, you jump the small 12V accessory battery exactly as above and you never go near anything with orange cables. But don't use a hybrid as the donor. Its DC-DC converter isn't built to deliver that kind of current, and most manufacturers say so in the handbook.

And if the car has stop-start, it almost certainly has an AGM or EFB battery. When you replace it, match the type. Fitting a standard flooded battery to a stop-start car is a false economy that usually shows up within a year.

One last thing worth knowing. If the engine fires and then dies the instant you pull the leads off, the battery isn't the whole story. That's an alternator that isn't charging, and you've got roughly the range of whatever is left in the battery to get somewhere useful. Pick your destination accordingly.