Jump starting modern cars?

Forums › Transport (cars, bikes and boats) › Jump starting modern cars?

johno1234

2793 posts

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#306905 2-Sep-2023 16:52

One often hears that one should not jump start a modern car from a "donor" car or battery due to risk of transient voltage spikes blowing up one or the other's computers...

Is this true and can anyone say they have seen this happen?

Personally I don't see a battery to battery jump start being a risk at all. Batteries don't create voltage spikes. A running car also should not create a voltage spike and if it did, presumably that would risk blowing up its own computer.

The one risk I could see is connecting to another car and then starting or stopping the other car.

In short, I don't understand how a battery to battery or running car to battery jump start could cause damage. All this assumes not doing anything really dumb like connecting the cables the wrong way round, creating short circuits etc.

Anyone know?

Oh, and I had to replace the gigantic, expensive stop/start battery in my Touareg recently. The mechanic (or is he a technician these days?) said he had to recode the battery in the car's ECU. I can't follow that either. A battery, even these expensive AGM stop start monsters, are still just dumb devices. And if a keep-alive battery was paralleled across the car while the battery was replaced, the ECU should be none the wiser. Right?

Senecio

2710 posts

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#3123039 2-Sep-2023 17:32

When you hear of a technician coding a new battery in a car they are really just reseting the battery management software. The battery is dumb, but the car is smart and it is constantly monitoring a number of systems including the battery health. Once the battery health deteriorates to a certain point the car will disable some functions like stop/start etc... When they replace the battery they code it to the car to tell the car it has a new battery installed.

shk292

2853 posts

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#3123050 2-Sep-2023 18:54

VW dealer wanted to change my Passat stop-start battery the first time I had it serviced (4 years old), at a cost of $950. I declined. When I sold the car 2 years later, the battery was still doing just fine.

tweake

2391 posts

Uber Geek

#3123198 3-Sep-2023 09:57

johno1234:

One often hears that one should not jump start a modern car from a "donor" car or battery due to risk of transient voltage spikes blowing up one or the other's computers...

Is this true and can anyone say they have seen this happen?

Personally I don't see a battery to battery jump start being a risk at all. Batteries don't create voltage spikes. A running car also should not create a voltage spike and if it did, presumably that would risk blowing up its own computer.

The one risk I could see is connecting to another car and then starting or stopping the other car.

In short, I don't understand how a battery to battery or running car to battery jump start could cause damage. All this assumes not doing anything really dumb like connecting the cables the wrong way round, creating short circuits etc.

Anyone know?

Oh, and I had to replace the gigantic, expensive stop/start battery in my Touareg recently. The mechanic (or is he a technician these days?) said he had to recode the battery in the car's ECU. I can't follow that either. A battery, even these expensive AGM stop start monsters, are still just dumb devices. And if a keep-alive battery was paralleled across the car while the battery was replaced, the ECU should be none the wiser. Right?

most jumper cables have spike protectors built in. its the sparking from connecting them that can form spikes. i think these days the ecu's are a lot better than early ones.

with the batteries its not just telling the ecu the battery has been replaced, but telling the ecu what the battery specs are, so it knows how to manage it. its not just stop start cars either. a lot of vehicles run smart alternators (to help save fuel) and depending on system used the ecu needs to know the battery specs.

k1w1k1d

1519 posts

Uber Geek

#3123228 3-Sep-2023 11:16

The problem with jump starting is when you disconnect the cables after starting the car with a flat battery. The alternator can cause a high voltage spike (load dump transient) that may cause damage to electronic modules.

One possible solution is turning the headlights, heater fan, rear screen heater, etc on before disconnecting the cables. These will hopefully absorb any possible spikes.

My usual suggestion is to run the vehicle for 5-10 minutes to get a surface charge on the battery, turn the car off, disconnect the cables, and then try to start the car. If it won't start it is likely that the alternator is not working, or the battery has failed.

Previous posts have explained why new batteries have to be coded to some vehicles.

BlakJak

1275 posts

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#3124829 7-Sep-2023 22:32

I've jumped lots of cars and never had a spike cause any problems - even when only had crappy leads that didn't include spike arrestors.

However I picked up one of these about a year ago (now appears to have been replaced with this)

... I was nursing a pretty stuffed battery along in my car for about a year, used the jump box maybe five times in 7 or 8 months to start the car before I finally replaced the battery itself. I like the fact that you can completely isolate the power from the clips, get them properly seated, then switch power on... start vehicle, and reverse process. No sparks, no spikes, and quite a decent way to go.

No signature to see here, move along...

tweake

2391 posts

Uber Geek

#3125027 8-Sep-2023 15:50

BlakJak: However I picked up one of these about a year ago (now appears to have been replaced with this) ...

those look to use SLA batteries in them which tend to stuff out fairly quickly. loose their charge, dries out, or gets overcharged.

thats where the lithium ones are better.

BlakJak

1275 posts

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#3125030 8-Sep-2023 15:53

Given how infrequently I expect to use it now that i've bought a new car I can probably live with it until it dies.

When I was only propping up my nearly-dead battery it wasn't even really supporting a cold-crank... just helping it along. Sometimes i'd go to charge it afterward and the charger wouldn't even register that i'd incurred a drain on the fully-charged unit.

Still recommend one of these over using another car where possible.

I have used my jump box to help jump start a Police Captiva once. Far easier than trying to nose my car into place.

No signature to see here, move along...

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richms

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#3125041 8-Sep-2023 16:23

The issue is when the donor car is running and trying to charge and you crank the flat car and cause the charging system in the donor to go flat out at max output, then the cranking stops and it over volts things because it's being asked to do all that it can.

Richard rich.ms