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Why you shouldn't jumpstart a modern car

By Wade O'Leary, Advocacy & Community Content Manager

Since the turn of the century, cars have shifted from being primary mechanical to predominantly electrical.

Many people would perceive that in terms of the rise of electrical and plug-in hybrid vehicles, but the fact is that every car built in the past two decades has been part of the transformation.

The change has been enormously positive – pardon the pun – for drivers, who enjoy the convenience, safety and efficiency gains derived from advances such as stop-start technology, traction control and sat-nav.

But the greater complexity manifests negatively when a car breaks down.

The trusty jumper leads that a handy motorist would once have kept in the boot in case of a flat battery have become taboo for modern cars, and NRMA technical trainer Darrin Tucker says that’s with good reason.

“The big change from then to now is computers and it’s not as if a modern car has just one – a Mercedes-Benz S-Class from a few years ago has 64 ECUs (Electronic Control Units),” he said.

“Jaguar warned us that you could damage the whole wiring harness if you try to jump-start one of their cars, and quite frankly that warning is probably applicable to most vehicles built within the past five years.

“A five-year-old Audi was recently taken to an NRMA Approved Repairer with an electrical system so badly damaged by an attempted jumpstart that, in the end, it was actually cheaper to write the car off rather than repairing the damage.”

But what has driven this destructive change?

The simplest answer is … computers.

Digital revolution

The ECUs that control the systems maximising fuel efficiency, traction and safety operate via binary code that relies on a stable voltage from the battery.

Zapping them with a random blast from jumper leads is like shocking a human heart: an interruption of the correct levels and rhythms can be fatal.

An NRMA technician can revive a flat battery in a modern car by supplying power to the battery but only by following the official jumpstart procedure.

Some patrol vehicles are equipped with Anderson plugs and surge protectors which serve to regulate the flow of energy - technology well beyond the grasp of the regular motorist and their vehicle.

The proliferation of complex accessories further complicates matters.

Even when a patrolman is following the correct procedure, leaving the keys in the ignition of the Member’s car can result in a damaged transponder chip in the key and potentially cost hundreds of dollars.

Another factor is the fundamental change to the battery’s function within the car’s ecosystem.

“Once upon a time, it would just power the starter motor and you were on your way,” Darrin says.

“Now it maintains all functions at all times – it serves to stabilise and subsidise power supply throughout the vehicle.

“For example, more cars are using electrical power steering rather than a hydraulic system and simply turning the wheel can use up to 60 amps.

“An alternator can’t reliably deliver that amount of current, therefore the battery needs to supply the power to the electric motor.

“A very wise man once told me: everything starts with the battery ... well, now the battery is the most important component in the vehicle.”

Injured electronics

Another phenomenon NRMA Members probably aren’t aware of is ‘injured’ electronics.

That’s when a vehicle continues to function after a conventional jumpstart but then slowly starts to lose its electrical functionality over the following months.

Predictive functions like auto window-winding will stop first before the rot spreads domino-like through the rest of the vehicle’s ECUs.

“Well-meaning car owners can do massive amounts of damage very easily and not realise it until it’s too late,” Darrin says.

“Your mate can’t jumpstart the car anymore – the NRMA has to do it for you.”

Luxury vehicles

Luxury or performance vehicles are often cited as being most likely to fall foul of technological advances but there’s a dynamic in play that effectively sees them being punished for innovating.

Premium marques take pride in pioneering new technologies that deliver new levels of comfort and safety, but this can make them more vulnerable to owner-induced malfunctions as there will be new procedures to follow that most aren't aware of.

“Luxury and performance-focused manufacturers are the most innovative – they tend to pioneer new technologies that other carmakers will introduce into their model lines a few years later,” Darrin says.

“All manufacturers go to companies like Bosch and say ‘I’ll have this system here’ but some manufacturers assist with development of these components.

"Other manufacturers will implement these systems later, which allows time for new processes and information to filter down through the trade and to customers.”

It should be said that none of this is an argument against technological advancement.

Cars are safer and more reliable than ever before, backed by unprecedented levels of pre-costed and scheduled maintenance.

The reality is that, if you truly love cars and can afford to have the best, you may as well invest in the top-line model of your choice and enjoy the benefits they offer.

But be sure to keep the owners’ manual in the car with you at all times and follow maintenance schedules to the letter.

And whatever you do: if you get a flat battery, leave the jumper leads in the boot and call the NRMA.

Your questions answered

We collated a representative group of questions and assertions from NRMA's Facebook community for NRMA technical trainer Darrin Tucker to address - and here are his replies:

Anti zap jumper leads are common place now, or a "booster box"! ... You'd have to go out of your way to buy a set of jumper leads WITHOUT surge protection now. Most definitely not 'technology well beyond the grasp of the regular motorist and their vehicle.' ... Plus they'll all usually have labels telling you the correct order & location for the leads (it's not battery to battery for all 4 connections...). 

Darrin: The problem with some booster batteries and surge protection is that the surge protector protects the booster box, not the vehicle’s ECUs. Only some boosters will protect the vehicle, it depends where the booster is in the circuit. The correct sequence should be: 1) booster battery 2) switch 3) surge protector, so that the surge protector is on the vehicle side of the booster pack switch. 

A battery does not always have constant voltage , sometimes they can over charge. Using the logic of this article means the when your battery fails you need to a new car as it will damage wiring and computers. Also it would mean no modern vehicle could ever be fitted with 2 batteries.

Darrin: A battery doesn’t charge, a charging system does. A battery supplies a stable voltage from the charging system. It’s a complex debate as there are so many different systems but a modern vehicle can be fitted with two batteries, but only if done so correctly.

Every car has circuit breakers/fuses and voltage regulators before any major electrical component.. So if there was a spike these would be the things to blow first.. Once blown any electrical current can't make it to the component.. Only once the circuit is restored will u have power.

Darrin: Circuit breakers and fuses protect against excessive current flow, not voltage spikes. 

Follow the manufacturers instructions on jump starting the car. 

Darrin: If you follow the manufacturer’s instructions, ensure you have the correct test equipment and tools to jumpstart the vehicle safely.

How is an Anderson plug going to assist in spike prevention? And it's not anything special anyway as most people towing a caravan or camper have one. There is no "random blast" from jumper leads. The voltage is never more than about 14V from the working vehicle. Special plugs (Anderson) are meaningless.

Darrin: An Anderson plug will assist in spike prevention in concert with a surge protector. There is a ‘random blast’ if you disconnect while the car is running. Voltage can be significantly higher than 14V.

I've Been an Auto Electrician for 15 years and install ECUs and computers into cars and never have I hurt anything from a jumpstart if the procedure is followed correctly!! Many a time you will find its dodgy leads bought fromServos that don't allow enough current to travel through them due to bad wire or terminations. These modules are generally designed to run from 9v to 15v this very reason.

Darrin: You’re absolutely right. Unfortunately, most of our Members aren’t auto electricians!

It is neither difficult nor unreasonable for manufacturers to fit suppressing devices that totally eliminate the problems associated with jump starting. A couple of capacitors, maybe a few diodes and the correct choice of computer chips is all that is required.

Darrin: You’re correct – you’ve basically described a surge protector.

OK so if I need a jump start will there be anticipated damage if the positives are linked and the black (negative) lead is attached to a part of the motor/chassis? Is this now a no no? 

Darrin: You are describing correct procedure. The negative lead needs to go onto a good known earth, for example unpainted metal.

Does the NRMA have any advice on using jumper leads with surge/spike suppression? I was thinking of buying a set.

Darrin: You should first know how to test the battery and have the correct tools and test equipment to do the job. If you buy a booster pack, ensure the switch and surge protector are in the correct order.

Does this apply to batter chargers as well? And does it affect pre-2010 models?

Darrin: It does affect battery chargers – they have evolved a lot in the past decade. You need to purchase a modern battery charger to charge modern batteries. Our warning extends back to at least the year 2000 and possibly earlier. It’s not only the car but also the battery – you will likely have a modern battery in an older car, where these warnings could still be applicable.

Have you had a bad experience trying to jumpstart a car? Do you believe it's worth modern vehicles being so complex? Let us know your thoughts below.

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