How to Jump Start a Car Safely in the Right Order

You turn the key on a cold morning and get a slow, sad click, or nothing but a dashboard that flickers and dies. A flat battery is the single most common reason for a breakdown call out, and in most cases you can fix it yourself in ten minutes with a set of leads and a friendly neighbour. Knowing how to jump start a car safely means doing it in the right order, because the wrong order is how people get sparks, blown fuses and, now and then, a damaged control unit.

I have jumped more cars than I can count, from old diesel vans to modern stop start hatchbacks. This is the method I use, plus what to do afterwards so you are not stranded again the next morning.

What You Need Before You Start

Gather everything before you open either bonnet. Fumbling with leads in the dark while two engines are running is where mistakes happen.

• Jump leads: at least 3 metres long, with a cable thickness of 16mm² or more for a family car, and heavier still for a large diesel. Thin bargain leads get hot and may not carry enough current.
• A donor car with a battery of the same voltage, which for almost all cars is 12 volts.
• Or a portable jump pack, which avoids needing a second car at all.
• Gloves and a torch, plus eye protection if you have it.
• Your owner's manual, which may show specific jump points under the bonnet.

Check the dead battery before connecting anything. If the case is cracked, bulging or leaking, or you can smell rotten eggs, do not jump it. Call for help instead.

How to Jump Start a Car Safely, Step by Step

Park the donor car close enough for the leads to reach, without the two cars touching. Turn off both engines, lights, radios and heaters. Put both cars in park or neutral with the handbrakes on.

1. Connect one red clamp to the positive terminal of the flat battery, marked + and often under a red cover.
2. Connect the other red clamp to the positive terminal of the donor battery.
3. Connect one black clamp to the negative terminal of the donor battery, marked minus.
4. Connect the last black clamp to a bare, unpainted metal point on the engine block or chassis of the dead car, well away from the battery. Many cars have a marked earth point for this.
5. Start the donor car and let it idle for three to five minutes.
6. Try to start the dead car. If it does not catch, wait another few minutes and try again. Do not crank for more than about ten seconds at a time.
7. Once it starts, remove the leads in exactly the reverse order: black from the earth point, black from the donor, red from the donor, red from the formerly flat car.

Keep the clamps apart from each other at every stage. If two clamps touch while connected to a battery, you will get a shower of sparks.

Why the Last Clamp Goes on Bare Metal

That fourth step is the one people skip, connecting black to black because it seems logical. The reason not to is simple. A battery being charged can give off small amounts of hydrogen gas, and the final connection is the one most likely to spark. Making that connection on metal away from the battery keeps any spark away from the gas.

On many modern cars with the battery in the boot or under a seat, there are dedicated jump terminals under the bonnet instead. Use those. The manual will show you where they are.

Using a Portable Jump Pack

A lithium jump pack costs roughly 50 to 120 pounds and is the most useful item I keep in the boot. You do not need another car, and you avoid any risk of electrical surges between two vehicles.

The order is similar. Make sure the pack is off, clamp red to the positive terminal and black to an earth point, switch the pack on and start the car. Then turn the pack off and remove black first. Keep the pack charged, as most lose some charge over three to six months, so put a reminder in your phone to top it up.

The Advice That Leaves People Stranded Again

Ask around and you will often hear that once the car starts, a twenty minute drive will recharge the battery. That is optimistic. A badly flattened battery can take hours of driving to recover, and short trips with heaters, lights and wipers running may put back very little. Many people jump the car, drive to work, and find it dead again at five o'clock.

Plan on at least 30 to 60 minutes of steady driving, ideally on faster roads, or better still, put the battery on a proper mains charger overnight. Then have it tested. Most garages and parts shops will do a free battery test in a few minutes. A healthy resting battery reads around 12.6 volts. If yours sits below about 12.2 volts after a full charge, or it has needed a jump twice in a month, it is time for a new one. Battery health is one of the items I check alongside the oil in my guide to checking and topping up engine oil.

Cars That Need Extra Care

A few types of car need a different approach.

• Stop start cars use AGM or EFB batteries. They can be jumped normally, but if they go flat often, the replacement must be the same type, or the system will misbehave.
• Hybrids and electric cars still have a 12 volt battery for the electronics. Many can receive a jump through dedicated terminals, but most manufacturers advise against using them as donors.
• Older cars with a positive earth, mostly classics from the 1960s and before, need the polarity reversed. Check before connecting anything.
• Large diesels draw more current on startup and need heavier leads or a bigger pack.

Common Reasons a Battery Goes Flat

A one off flat battery is usually caused by something simple: an interior light left on, a door not fully closed, or a car that has been sitting unused for a few weeks. Cold weather makes it worse, since a battery can lose a large share of its starting power on a freezing morning. If it keeps happening, the cause may be a failing alternator, a parasitic drain from an accessory such as a dashcam wired directly to the battery, or simply an ageing battery. Most last around four to six years. It is worth a check before any long drive, which is why it sits on my list of car checks before a long road trip.

Frequently Asked Questions

Can jump starting damage my car?

Done in the right order with decent leads, the risk is very low. Problems mostly come from reversed polarity, clamps touching, or connecting to a damaged battery. Follow the sequence and keep the clamps apart.

Can I jump start a car in the rain?

Yes, a 12 volt system is safe to handle in the rain. Keep the clamps and battery top as dry as you can and take extra care to avoid slipping.

How long should I leave the donor car running?

Three to five minutes before the first attempt is usually enough. For a deeply flat battery, give it up to ten minutes with the donor engine idling.

What to Remember Next Time

Red to dead, red to donor, black to donor, black to bare metal on the dead car. Start the donor, wait a few minutes, then start yours. Remove everything in reverse, drive for at least half an hour and get the battery tested that week. A set of good leads or a charged jump pack in the boot turns a ruined morning into a ten minute job.