Jump starting modern vehicles

Submitted: Friday, Oct 24, 2003 at 12:13
ThreadID: 8051 Views:20077 Replies:9 FollowUps:3
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Why is it that using jumper-leads on modern vehicles (with computers) causes damage?

Do jumper leads with fuses or current protectors (or whatever is in the box) limit or prevent damage?

We've got a flat battery in our Patrol and not being a member of NRMA, am thinking of buying new jumper leads or a battery charger, instead of paying joining fees (ballpark - $150).
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Reply By: hoyks - Friday, Oct 24, 2003 at 12:40

Friday, Oct 24, 2003 at 12:40
When you get sparking there is the risk of a power surge that can damage the computer.
Turn the ignition off on the vehicle being started, before connecting the leads, this should protect the computer from a surge.
If you are careful and don't stuff around with the conecting the clamps, when putting them on and taking them off, you shouldn't have any problems. Sparks bad, minimise sparks.
AnswerID: 34964

Reply By: Tim - Friday, Oct 24, 2003 at 13:52

Friday, Oct 24, 2003 at 13:52
Another one I have heard is to turn your parking lights on when connecting the jumperleads to your battery as placing a resistive load on the surge will minimise the spike and therefore not damage any sensitive DC electronics in the onboard computer.
And re NRMA membership, I was near a town about 60km SW of Gunnehdah in NSW,stuck, on a property. After 3 hours of trying to free the vehicle (it was not broken down, it was bogged) we rang the NRMA and he was quite happy to come and help me get out even though he had to drive out from Gunnehdah late on a Saturday night. Well worth the annual m'ship fees.
AnswerID: 34974

Reply By: chrisfrd - Friday, Oct 24, 2003 at 15:39

Friday, Oct 24, 2003 at 15:39
It's not just the sparks.... Voltage irregularities are created between the systems and micro-sparks (ones you can't see) cause voltage spikes up to 80-100 Volts DC with AC harmonics upwards of 15,000V at 400-600MHz. This is only transitory, but it can wipe-out computers in the ignition system, car audio, GPS, UHF radios and such.

The best plan of attack it to go and buy a few Metal Oxide Varistors from Jaycar Electronics, solder some leads up and heatshrink them, then connect the leads accross the battery terminals. This will clamp the AC harmonics completely(creates a dead-short for the moment that the MOV shunts) and will stop the DC surge from getting over about 20V, which is more than ample.

I've got two soldered in parallel, on each battery. These are the same devices that are added to expensive jumper leads.
AnswerID: 34986

Reply By: Auto - Friday, Oct 24, 2003 at 16:42

Friday, Oct 24, 2003 at 16:42
True that you can prevent damage when jump starting and disconnecting the battery (first at ground) but advisable to put the key on your pocket. Some cars close the doors up when the battery is separeted.
AnswerID: 34996

Reply By: Mickl - Friday, Oct 24, 2003 at 18:28

Friday, Oct 24, 2003 at 18:28
Yes they do prevent damage, invest in a good set or make your own (as already posted), also read your owners manual, it will tell you exactly how to jump start your particular vehicle.

$150 sounds a lot.. here in SA (RAA) it's only around $60 for standard membership and for us "Travellers" the "Plus" option is around $90.
AnswerID: 35006

Reply By: ray91 - Friday, Oct 24, 2003 at 22:25

Friday, Oct 24, 2003 at 22:25
When you jump start any car with jump leads ,and the car starts, the car with the flat/dead battery charging system is picking up the 12 volts through the jump leads from the good battery so the alternator does very little (hardly charging thinks battery is fully charged).The second you remove the jump leads the alternator gets a low voltage signal and charges flat out, HUGH surge ,the regultor then drops the volt/amps down but not always before frying the electrics. This is what destroys the computer.
AnswerID: 35039

Reply By: Eric Experience. - Friday, Oct 24, 2003 at 22:32

Friday, Oct 24, 2003 at 22:32
There is a lot of missinformation on this subject, when you connect two 12 volt bateries together the correct way around there is no surge or harmonics. A lot of dealers who dont understand electrical systems tell these sort of stories so they can blame someone else when they damage your car. There is a risk of over voltage when using a cheap battery charger on a dead flat battery, but if you have the ignition turned of the computor is safe. as stated before if you switch your park lights on while connecting the battery you will well and truely control any electrostatic effects. Eric.
AnswerID: 35040

Follow Up By: David N. - Saturday, Oct 25, 2003 at 07:40

Saturday, Oct 25, 2003 at 07:40
Yep- agree with the above.
I have jump started many modern cars with fancy electronics, using care and common sense. Never ever had a problem- nor have I heard of any one else either. If you are connecting one 12v battery to another just where are you going to get 100v from? The most I've ever managed to get out of ANY 12v battery is about 15v max when on charge. Beats me where you'd get more....... Urban myth.
Like I said- care and common sense- don't connect reverse polarity or make a dumb mistake and you'll be safe.
FollowupID: 25339

Follow Up By: crfan - Saturday, Oct 25, 2003 at 10:35

Saturday, Oct 25, 2003 at 10:35
David N Petrol cars get around 80 000volts out of there coils from a 12v battery.
And the word is that you may get away with jump stating a car 90% of the time with no effects but you only need to do it once and damage a $200-$4000 computor.
also you may be doing damage that does not show up straight away.
Best to play it safe and buy a set of surge safe leads (around $60 for a set)and read the manual for there procedure.
FollowupID: 25345

Follow Up By: joc45 - Saturday, Oct 25, 2003 at 18:40

Saturday, Oct 25, 2003 at 18:40
Agree with Eric and David. Tho given the horror stories that perpetuate on the perils of jump starting, it amazes me why manufacturers haven't already built in the surge suppressors and reverse polarity protection; they're certainly cheap enough (one hefty 16v zener diode will do both).

crfan, the 80,000 volts from the ignition system is only a problem if the HT wiring can cause a spark-over to the primary 12v wiring; ie, nothing to do with jump starting. Most vehicles have the HT wiring well separated from the 12v wiring.
FollowupID: 25361

Reply By: Russell Terrano - Saturday, Oct 25, 2003 at 20:31

Saturday, Oct 25, 2003 at 20:31
My understanding is the damage is caused by the power surge caused by the alternator surging to fill the gap left by the newly disconnected battery/vehicle. The NRMA guy said this is overcome by turning the headlights on before disconnecting the charging battery/vehicle. Makes sense to me.

I've also been told most modern cars are protected form these surges now but how would you know.
AnswerID: 35104

Reply By: Member - Neil & Lynne(Bunbury) - Monday, Oct 27, 2003 at 12:59

Monday, Oct 27, 2003 at 12:59
Firstly forget turning on headlights...this is so much cr&p...

OK now for a absolutely safe way to jump start ANY car from ANY car, re read Ray91's post above, then read it again. He is spot on with what happens.

The damage that can happen will happen at the moment of disconnection of the leads as Ray91's past says.

To prevent this all that needs happen is to do it this way exactly.......

1. Switch off the motor of the jumper car...ensure ALL electrics, ignition, dome light etc are turned off in the jumpee car....
2. Connect red / pos leads to pos terminals of both batterys...
3. Connect black / neg lead to neg terminal of good battery...
4. Connect other end of neg lead to a good earth as far from the dead battery as possible, preferably on the motor. (this prevents the risk of exploding the battery if there is a small spark and there is hydrogen present.....a VERY real possibillity).
5. Start the jumper car....after a few moments start the jumpee car.
6. Now the most important part....leave the connections as is for 5 minutes or so, then turn off the jumpee car....NOW disconnect the leads in the reverse order you connected them.
7 The "dead' battery will now start the car by itself with no risk of damage.

This method removes the damaging sparking hazard of disconnecting differing state of charge batteries.

As a Towing contractor I am frequently called to jump start vehicles and have never had a problem with this method.
Life's too short to say .. "I can't do that !"
AnswerID: 35232

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