Hey guys,

My battery died and I had to jump the car today, and it got me thinking....just a couple random questions since I'm a curious person. I tried google but couldn't come up with much.

1) why do you need to turn the 'live' car off first before connecting cables? Since the battery is good, isn't the battery still live even if you have the car off?

2) ^what happens if you don't turn the live car off and just connect the cables?

3) when a car is running, the electrical power all comes from the alternator right? so theoretically a car can run (barring voltage spikes) without a battery once its started?

4) when jumping the dead car, is the power coming from the live cars battery or alternator?

5) ^is that power charging the dead cars battery or is the power going straight to the starter of the dead car?

6) with the cables connected, are the batteries now in parallel or series? what about alternator and battery?

thanks in advance! just really curious what is actually going on

 

You dont

Nothing, not sure where you read that you have to turn off the parent car

The battery completes the circuit, so technically no, you cannot run without the battery connected, it also acts as a filter when the car is running. Meaning it filters the alternators power to cleanly sustain a voltage to your electrical components. Without it you would see electrical noise all over the place.

From the entire circuit. You are basically connecting a 12v circuit (in actuality 12v-14v) in parallel with your own car's for the purpose of powering the starter.

It would charge it if you left it on there for quite some time, but the purpose isn't to charge the car, it is to jump the starter

In parallel. If in series you would be pumping 12v + the voltage of your 'dead' battery (typically anywhere from 0v - 9v). Not something you want to do

 

In addition, jump starting a car is not recommended and should only be used as a last resort.

 

why not? is it because of the potential risks - explosion, frying the electronics? what other potential harmful effects are there?

what would be the first resort then? just a new battery straight up?

 

its stated in the manual under jump starting, step 5. that's why i was wondering! i didnt think it was necessary either

[QUOTE=paimon.soror;4656424]You dont



Nothing, not sure where you read that you have to turn off the parent car

 

Different situations warrant different things...

What I tell my kids (and yes, I like my kids) if there is no other option than jumper cables:

1) why do you need to turn the 'live' car off first before connecting cables? Since the battery is good, isn't the battery still live even if you have the car off?
depending on how dead the battery is, it has the opportunity to suck a lot of power out of the good car in a short period of time. This can lead to 2 cars that do not start. if the donor car is running, at least it is running. In some cases when you hook it up, you will hear the idle drop significantly and the alternator whine. This means the dead one is pulling all the power the battery can provide, as well as a lot from the alternator.

2) ^what happens if you don't turn the live car off and just connect the cables?
you risk 2 dead batteries, depending on the health of the donor car battery and how long you leave them connected

3) when a car is running, the electrical power all comes from the alternator right? so theoretically a car can run (barring voltage spikes) without a battery once its started?
theoretically works for a school test, others replied better.

4) when jumping the dead car, is the power coming from the live cars battery or alternator?
both, if the live car is running

5) ^is that power charging the dead cars battery or is the power going straight to the starter of the dead car?
again, both. the dead battery will fight with the starter for power. that is why I like to send as much as possible,

6) with the cables connected, are the batteries now in parallel or series? what about alternator and battery?
all in parallel

I have actually started a car with the dead negative battery connector disconnected, to keep it from sucking down the donor. This was a pickup getting started by a datsun, if I remember. Once the pickup was running, we connected the battery, then disconnected the jumper cables. Not recommended, but is the only way to do it unless we gave the datsun an hour to charge the truck battery. This is not recommended, but is an option when you are hours from civilization.

On this same note, if at first the dead one does not start, I tell them to leave the live one running for a few minutes. This will bring up the dead battery enough to drop what it is drawing, and have more for the starter. This is a necessity for cheap, thin, jumper cables.

Again, this is what I tell my kids. And, I have AAA for my kids.

That help?

 

Turning off the live car is a pretty common instruction when jumping.

The idea is that the car being started will draw more current than the alternator can supply, possibly damaging the live car's charging system.

I always leave the live car running, and never had a problem. If you turn it off, you have the possibility of draining the good car's battery, leaving you with two dead cars instead of one.

There are a lot of tips and procedures out there that were written by magazine writers who don't know squat about cars, but after enough people read them in Parade or Good Housekeeping they take on a life as if they were right.

That's an important point. Sometimes the dead car's battery can be so far down that it sucks down what the donor car is providing. I've also had the positive result of "jump" starting a car using hookup wire instead of jumper cables. Just wired things up and let the donor car run for 15 minutes or so to charge the dead battery.

Ken

 

correct me if i am wrong, but i think you are saying:
1) leave good car running
2) connect cables
3) start dead car

whereas everywhere i seem to read says:
1) turn off good car
2) connect cables
3) start good car
4) start dead car

i feel like you can just leave the good car running, hence my question why everywhere says to turn it off first? or else like you said, couldnt you end up with 2 dead batteries??

 

yea its definitely stated in the manual to turn off the good car.

that explanation about drawing too much current makes sense. thats what i was looking for!

i always have left the good car running, so i didn't know if i've just been lucky nothing has went wrong or if it was unnecessary. seems like everyone here is suggesting its unnecessary

 

Overall just the risk of damaging electronics by doing it incorrectly or the risk of damaging the car that is trying to jump start the car with the dead battery. The best bet is to remove the battery and a take it to get tested and charged.

When I was 21 or so, I had a 280ZX Turbo, I tried to jump start my friends huge Dodge RAM Power wagon and we both ended up stranded, lol. My Z was running but when we tried to jump the Dodge it killed my Z. It fried the ECU, ended up costing me like $1,000.00 to get it replaced. Since then I have read the owners manual of every car I have owned and many have stated that jump starting is not recommended.

Unless the lights were left on or something then you have bigger issues that need to be addressed so it needs to be fixed properly rather than jumping a dying battery or jumping a car with defective or corroded terminals/wiring. If you flooded the car then I can see having a proper quality (built in safeguards) booster on the car so the battery doesn't die while attempting the factory deflood procedure but that would be the only time IMO.