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I'm aware of the general risks, i.e. igniting hydrogen gas with a spark, and accidentally hooking up the leads backwards. But I have been reading conflicting information about whether one should even try to jump-start a modern vehicle.

One article I read suggested that the amount of electronics connected to the battery in a modern car could result in damage from any kind of transient during a jump-start, and another article I read on how to jump-start a car had a large warning that read "If either vehicle has an electronic ignition system or is an alternatively fueled vehicle, the use of jumper cables may damage it." From my (admittedly limited) research, it appears that most vehicles made after 1974 have electronic ignition systems.

If I'm asked by someone to jump-start their car, should I decline?

Should I decline to let anybody jump-start my car?

What are the risks?

  • Great question! – Pᴀᴜʟsᴛᴇʀ2 Feb 25 '17 at 22:58
  • I'd say the risks are relatively low, use of a jump pack and car to car jump starts are done all the time. I don't have statistics to back this up but you're probably more likely to be hit by a car than having a battery explode during a jump start. As to damage to the car? You're more likely to have nothing eventful happen, less likely you blow a fuse... even less likely your car explodes? – Ben Feb 26 '17 at 1:52
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Damage can come to the person's car who's providing the boost. I have seen blown alternators and other damage. The only lower risk way is disconnect the supply battery from the car. It will clear electronics settings doing this. These days I suggest getting a Lithium battery booster. I'm in my 50's unfortunately I have to turn people down if they want a boost because of damage.

  • You only risk damaging the alternator if you have the engine of the car doing the boosting running... I have sold people alternators who made that mistake so I know... – Solar Mike Feb 17 at 16:47
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    @SolarMike - Not sure what you're saying there. You should always have your "donor car" running when you try to jump start a dead vehicle. I've never had an issue with it. I suspect your person who you sold an alternator to had put the leads on backwards. – Pᴀᴜʟsᴛᴇʀ2 Feb 17 at 23:11
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    @Paulster2 You should only have the donor engine running to try and put some charge into the flat battery: 5 or 10 mins makes a big difference ... Once you have given an initial charge, then switch off the donor engine (alternators have surge diodes in them that blow if they try to keep the voltage up while the starter is engaged (the surge current can be so high, can also blow the rectifier pack... And it was more than one person... and no they did not get the leads wrong... If you have been lucky so far... The day you are not is going to be very expensive... – Solar Mike Feb 17 at 23:21
  • @Paulster2 read the link to "jump start destroying the electrics" and what 9Realms says on June 30 2015 - how it cost him 600 bucks... I studied the internals of the alternators and regulators (well, first ones we looked at were the dynostarts... all the way through to alternators with internal and external regulators...) so I know how I am prepared to jump start cars, even agricultural tractors sometimes... – Solar Mike Feb 17 at 23:29
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Blown fuses from backwards polarity cable installation is the most cause of vehicle damage at this time.

The risk of explosion is lower than in older battery designs due to less venting of hydrogen gas in current designs. It can still happen and should be guarded against. The most common injury is blindness due to acid burns.

The concern in the question from jump stating is that of high amperage arcing causing power to flow through the ground side getting into the circuits in the many IC circuit controlled devices on modern vehicles. Even in this case the risk is low; partly due to better ground side surge protection in the devices and low incidence of arcing during connection. One can reduce the risk of damage by one of two ways.

1) Remove both cables from the battery before connecting to the alternate power source, this is the guaranteed way to ensure no problems.
2) Use a modern battery charger with a "soft start" feature that slowly applies power when turned on.

That said, we still use jump boxes daily with no problems. Ensuring that the key is off and the cable polarity is correct. We do not use other vehicles as the power source and do use a soft start charger whenever possible.

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    I don't understand what you mean by arcing causing power to flow into circuits. It doesn't make sense. The arc can happen only if there is large voltage difference between empty battery and donor car battery. But it will never cause damage to components because the voltage won't exceed the limits of components. If arcing caused problems, you would have problem when replacing battery also. Because you may have arcing when replacing battery or even when disconnecting and reconnecting a battery. – Evren Yurtesen Feb 27 '17 at 10:50
  • @evren yurtesen. Arcs can induce high voltage in circuits. I have measured voltages as high as 50 volts for short durations during battery connection – Fred Wilson Feb 28 '17 at 3:26
  • I would prefer if you could point a reputable research article explaining this. Because I think it is against laws of physics, electrons won't flow faster in air compared to a metallic connection at same source voltage. Also the enegy will be absorbed by empty battery. How did you measure this? Because it would be difficult without special equipment. I saw arcs when I disconnect/reconnect battery of the car (it's own battery) if what you said was correct, simply connecting battery to car would break electronics. – Evren Yurtesen Feb 28 '17 at 8:30
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    @Paul I had a battery explode as I was connecting the terminals. It was an unforgettable experience that resulted in acid burns and hearing loss. We work on over four thousand vehicles per year at our shop and have proven often that if something could possible happen, it eventually will. Please heed the warnings and take the precautions. – Fred Wilson Mar 19 '17 at 19:40
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    @Paul like Fred, I have seen the effects of a battery exploding as the jump leads were being connected - perhaps you should consider that you and your ebgineers are in a controlled environment... My colleague ended up with battery acid over his face and in his eyes - he was lucky because we carried him to the washroom (running) and washed his face - he was then fine. You should consider a spark caused by the jump lead connection being made and broken as the operator’s hand shakes a bit.... – Solar Mike Feb 18 at 6:34
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There is absolutely no reason for a car to break down because of jump starting if:

1- The cables were connected correctly. Cars use same voltage etc.

2- The battery is not broken (eg. cant accept charge anymore)

1 is obvious. The reason for 2, if battery cant absorb energy, removing jump leads can lead to a voltage spike and this is of course harmful. You would have same effect if you disconnect the battery of a car while engine is running.

So, it is safest to charge the empty battery and then restart the car on its own power. Because you can also see if your battery can hold charge (absorbing power) or not.

Here, you have to understand that the problem is not related to soft start charging, arcs etc. It is not even related to jump starting. Considering if you did everything else right. The only dangerous part left is removing the jump leads. Do a web search for "removing battery when car is running". If your battery is completely broken, you may have the same effect when removing jump leads. Because you are disconnecting the donor battery which may be the only working battery in system.

I found this thread to be interesting also : Jump Start Destroying the Electronics - Myth?

I believe most often the damage is caused by people too tired or dumb enough to connect cables wrong way but not even realizing it even afterwards, then thinking that jump starting broke their car and then telling this story to others. I bet many do not even know that it is important to connect + to + and - to - . It takes only one person to tell that jump starting broke his car and the fear spreads like a disease.

I also think that in the past some people attempted jump starting 12V systems with 24V systems then decided jump starting is dangerous for electronics...

In addition, of course marketing departments are not helping either. They know that people are scared of jump starting cars because of electronics damage, wrong connections etc. So, they promote the fear even more so they can make sales on expensive cable sets. Honestly, probably the cables which can detect incorrectly made connections and refuse to work are probably useful in case you are color blind or in case if the user is expects that the cable will do everything for them automatically.

When all these information is passed from person to person, some of the facts are lost and usually one thing is left : 'jump starting broke the car'. Of course when the factors leading to result are lost, and you do not have understanding of how electric behaves, it is easy to believe in this myth.

Also some fun info, do not use defibryllator to jump start your car : MythBusters - Defibrillator Jumpstart

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I jump at least 3 cars a day from my rav4 and I have been doing so for about 2 years now. I haven’t had a single issue or anything blow on my car and it’s been completely fine with no issue jumping others.

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