How do you jump start a car?

What’s the best way to start a car with a flat battery? And can an electric car be used to jump start another car? We answer all your questions.

Jump starting a car is as brilliantly simple as it is potentially catastrophic. Exceptionally handy in certain situations, do things incorrectly, and it could end very badly for individuals and vehicles alike.

So how do you jump start a car? And how do you avoid disaster attempting it?

The principle behind using one car with a charged battery to jump start another car with a flat or discharged battery is clever and easy to understand.

A standard vehicle’s 12-volt rechargeable lead-acid battery has a simple job: to provide enough cranking power – rated as cold cranking amps or CCA – to crank or turnover an engine to start it.

The battery does this by powering the starter motor, which turns the engine’s crankshaft by first turning the flywheel.

This process requires a decent amount of power from the battery, hence the need for an alternator.

Connected to both the battery and the engine, when the engine is running and the alternator is working correctly, the alternator not only helps to power a car’s electrical system, it also sends charge back to the battery to ensure there’s enough charge for the battery to once again be able to power the starter motor and start the car.

This is why leaving your lights on or having the radio on with the engine off drains the battery, while doing the same with the engine on means the majority of power is coming from the alternator.

When a battery is low on charge, there may be enough power to run a car’s interior lights, headlights, and even the audio system, but not enough to crank over the engine.

As a battery loses charge, a vehicle’s start-up sound can audibly begin to labour. From taking longer to start the car, to requiring several attempts to crank over, the textbook tell-tale sign of a properly flat battery is the always shoulder-slumping ‘click click’ emanating from the starter motor solenoid located in the engine bay. Or worse still, no click at all.

You may also notice a battery warning light illuminated on the dash. This, however, is more likely related to an alternator issue rather than merely a flat battery, with most owner’s manuals suggesting the warning light indicates a fault or problem with the charging system and to visit to your nearest dealership or vehicle specialist.

The good news is a flat battery can often be jumped into action. Of course, expectations need to be measured, as an altogether ‘dead’ battery may be too far gone to be revived. In this case, a new battery is the best course of action.

So how do you jump start a car?

First things first: safety is imperative.

Be careful, diligent, and take your time. Improper procedure or incorrectly connected jumper leads could cause damage to fuses or a car’s electronics, and more seriously, risks electric shocks and possible fires.

To jump start a car with a discharged or flat battery, all you need is a car with a charged battery, and a set of jumper leads.

Before you attempt to jump start a car, check and double check:

• the specific location of the charged and discharged batteries you intend to connect – they’re generally located in the engine bay, but some vehicles may have them positioned in the boot or even hidden in the cabin #UteLife
• the condition of both batteries, keeping an eye out for any obvious damage, leaking fluid, swelling, or corrosion
• both cars are close enough for the jumper leads to comfortably reach each battery, but definitely not so close they’re in contact at any point
• both cars are in ‘Park’ or neutral for a manual car, handbrakes are on, and both cars are completely off – meaning the ignition switch in both cars is also in ‘OFF’
• both cars’ headlights and audio systems are off, so as to avoid unnecessary drain on batteries

It’s also worth checking to see if the charged battery doing the jumping is the same voltage as the flat battery being jumped.

From here, the process of jump starting a car is simple and safe as long as each step is followed to the letter.

Step One

Attach one of the red clamps at the end of the red positive jumper lead to the positive-marked or ‘+’ terminal of the flat battery.

Step Two

Attach the other red clamp at the other end of the red positive jumper lead to the positive-marked or ‘+’ terminal of the charged battery.

Step Three

Attach one of the black clamps at the end of the black negative jumper lead to the negative-marked or ‘-’ terminal of the charged battery.

Step Four

Attach the other black clamp at the other end of the black negative jumper lead to a safe ground or ‘earth’ on the car with the flat battery. Often a clean and unpainted piece of metal on the engine block or body of the car can be found a good distance away from the battery itself for just this occasion.

Be absolutely sure not to attach the black negative jumper lead to the negative terminal of the flat battery, as this could risk the release and sparking of highly flammable volatile gases such as oxyhydrogen, which could in turn trigger a possible explosion, not to mention potential exposure to sulfuric acid and other harmful materials.

Further, never allow the red positive clamps and black negative clamps to touch, as this too will cause sparking.

Step Five

After ensuring all clamps are correctly connected and all jumper leads are clear of any moving parts in the engine bay, start the car with the charged battery.

Step Six

After a few minutes allowing the car with the charged battery to idle, start the car with the flat battery. If it doesn’t start first time, switch it off, and try again after several more minutes idling the car with the charged battery.

Step Seven

Once the car with the flat battery has been started, leave it to idle with the headlights turned on to help avoid any unwanted voltage spike.

Step Eight

From here, simply disconnect the jumper leads in the reverse order to how you connected them.

This means disconnecting the black clamp at the end of the black negative jumper lead from the safe ground or ‘earth’ on the car that had the flat battery, then disconnecting the other black clamp at the other end of the black negative jumper lead from the negative-marked or ‘-’ terminal of the charged battery.

Next, with the black negative jumper lead disconnected at both ends, disconnect the red clamp at the end of the red positive jumper lead from the positive-marked or ‘+’ terminal of the charged battery, then finally, disconnect the other red clamp at the other end of the red positive jumper lead from the positive-marked or ‘+’ terminal of the car that had the flat battery.

Once you’ve successfully jump started the car, be sure to take it for a decent 15-30-minute drive before turning it off again, to give the alternator a chance to begin charging the previously flat battery.

Alternatively, if you have access to a quality battery charger such as from CTEK or NOCO, putting the car on charge overnight or even for several hours can be enough to refresh and recharge an otherwise undamaged battery.

Sadly, if jump starting hasn’t worked at all, or the battery refuses to maintain or hold charge, as mentioned, the battery will most likely require replacing.

Which jumper cables are the right jumper cables?

More often than not, when you go to start your car, you expect it to start. So being prepared for a flat battery is not always front of mind.

That said, having a good set of jumper leads in your car could prove helpful for not only you, but also others caught out by the flat-battery bandit.

Now, although prices vary considerably, from around $25-$200, jumper leads worth having will have insulation around the clamps, clamps that securely grip and maintain a good clasp on terminals, and be rated for a total amperage load of at least 400-450 amps, with higher ratings – 750-1000 amps – providing a wider range of flexibility for jumping different vehicles.

With safety always paramount, you should only use jumper leads with in-built surge protection or ‘spike guard’ technology, designed to protect people as well as electronic vehicle systems, and some will offer additional features including reverse polarity prevention and possible temperature and overload warnings.

Lengths generally range from 2.5-6.0 metres, and some are specific for 12-volt applications only, while others support both 12- and 24-volt applications.

And be sure to look after them. Avoid moisture and direct sun, and keep them neatly wound in a carry case or securely stored to aid longevity and ease of use.

Will jumping someone else’s car damage my car or my battery?

Provided the correct procedure is followed, being the jumper rather than the jumpee shouldn’t cause any negative effects to your car or your battery, however, electric vehicles may be a different story.

Should you jump start an EV if it has a flat battery?

Despite the number of lithium-ion batteries on board your average electric vehicle, EVs still require a 12-volt battery as you’d find in any internal combustion engine car powered by petrol or diesel, again, to help power vehicle accessories.

Some EVs will take just as kindly to being jump started as an ICE vehicle, however, caution is king and consulting the owner’s manual or checking with the vehicle manufacturer or dealer is often a good first step.

In short, technically, if it’s only the 12-volt battery that’s flat, you most definitely could. If it’s the car’s main batteries, then no.

Jump starting the 12-volt battery in an EV with low or no charge will not recharge its main batteries, and the vehicle will need to be recharged the same way it normally would be from a home outlet, a wall charger, or a public charging point. Some EVs do allow vehicle-to-vehicle charging, however, check all technical specifications and manufacturer recommendations before going down this path.

According to Tesla, “The procedure for jump starting differs depending on whether the low voltage battery is lead-acid or lithium-ion,” so you’d need to check, and Volvo says in relation to its Pure Electric models, “If the 12V battery is discharged for any reason, jump-starting can still be used.”

Similarly, Nissan says, “a gasoline engine vehicle can be used to jump start [the] LEAF’s 12-volt battery.”

Can you jump start a car with an EV?

This really comes down to the specifics of the EV in question, although Tesla says the Model 3 “cannot be used to jump start another vehicle” for example, and that doing so “can result in damage”.

Volvo states that, “The car’s charging points are only intended for jump starting the car itself,” with the Scandinavian safety supremo adding, “The charging points are not intended for jump starting another car. Using the charging points for jump starting another car may cause a fuse to blow, which means the charging points will stop working.”

And Nissan tells LEAF owners that the vehicle “cannot be used as a booster vehicle because it cannot supply enough power to start a gasoline engine”, while also pointing out that one should “not attempt to perform a jump start on the 12-volt battery at the same time that the Li-ion battery is being charged” as this “may damage the vehicle or charging equipment and could cause an injury”.

What if I don’t have another car to jump start mine?

All is not necessarily lost, but it could depend on the car you’re driving…

One option worth considering may be tracking down, borrowing, or already having a jump pack.

A variety of jump packs are available from auto parts retailers, priced from around $100-$1000 and above, with smaller lithium jump starter units priced from around $180 and beyond.

Super powerful and convenient, they allow you to jump start your battery without the need for another car, a set of jumper leads, or a helpful bystander providing assistance.

Additionally, if you drive a manual car, you may be able to ‘kick start’ or ‘bump start’ the vehicle despite having a flat battery.

Effectively, kick starting a car is using the momentum of an already moving car – one slowly rolling down a slight hill or being pushed along by a friend or obliging stranger – to turn the flywheel and the crankshaft to force a car into starting while bypassing the starter motor.

Achieved by simply depressing the clutch pedal, selecting second gear, ensuring the ignition is ‘ON’, and then abruptly ‘popping’ or releasing the clutch once the car has momentum, kick starting can be a handy Get Out of Jail Free card in the right – or wrong – circumstances.

And no, before you ask, an EV should not be kick started. As Nissan explicitly says, “Do not attempt to start the system by pushing the vehicle”. “Attempting to do so may cause traction motor damage.” Not good.

Remember too, as tempting as it may be to think of first, a flat or dead battery may not be the only cause for a car to not start.

Look for any illuminated warning lights on the dash, check under the bonnet for any other obvious issues, and consider other possible reasons why the car may not start such as the alternator, a fuse, an alarm fault, a lack of spark, or even a fuel-related issue. In the end, no matter what you try, there’s always the potential requirement for a tow truck.

Take care, be safe, and ensure you’re fully aware of all steps required before attempting to jump start a vehicle, and indeed if the vehicle can and should be jump started. Good luck.

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