Giving a jump start to a car - how does the circuit become complete?

[jack action]

In the second image, you can see that the negative pole of the battery is connected to the engine block (or the green cable in the third image; which is also connected to the body). That would be the cable labeled "TO GROUND" for the stalled vehicle battery in the first image.

So the whole car (frame, body and engine) is just an extension of the negative pole of the battery.

 

[Guineafowl]

Connection no. 4 in the lower diagram is effectively the same as the dead battery negative. The upper and lower diagrams are electrically the same. The only reason for connecting as per lower diagram (subject of much debate on this thread) is so that the spark created when making no. 4 is physically away from the battery and potentially explosive gases.

 

[jack action]

On my social media, the following funny showed up! I thought it would be most relevant to share it here:

 

[sophiecentaur]

That’s a nonsense diagram because it doesn’t show both negative connections. Unless there is a load of high resistance in a rusty connection, - joins to - through the body.

 

[chirhone]

Every time I've jump started a car, I've always connected the jumper leads battery terminal to battery terminal, I've never used the chassis as negative, maybe because all the bits are usually coated in thick oily goop we put all over everything to stop them instantly rusting here with all the salt on the roads. The boosting car should be running before you make the connection unless you want to end up with two stranded vehicles. Once connected its help full to run the boosting vehicle up to a couple k rpm so the alternator is actually delivering some power, they are not the best at idle. My old diesel truck could not be jump started over a cable, the boosting vehicle had to get enough charge into my battery before the old donk would even turn over (something something high compression ratio something).

Then load dump. This is a very specific thing: the battery disconnecting from the charging circuit under heavy charging load. The battery acts like a large zener diode, so unless your battery is so dead its not there at all electrically, you will not be creating load dumps in your car when disconnecting the boosting vehicle. Then, all the electronics in a modern vehicle are designed to with stand a load dump, no OEM in their right mind will put some bit of electronics on that that has not passed those tests!

Yesterday I jumped start a friend's car. I couldn't park in front of the car head to head because of some pipes. So we had to put in series two sets of wires with the alligators clips biting in each other (imagine 4 pcs of connecting wires and extending each pair of red and black). Initially I couldn't start the other car. Then I had to rev my source car to some rpm. I don't know if it is the reving or the series connections causing some resistance? What you think?

 

[davenn]

I don't know if it is the reving or the series connections causing some resistance? What you think?

The power losses (voltage drop) with a long lead length will be very high when attempting to draw
400+ amps of starting current.

 

[Guineafowl]

Yesterday I jumped start a friend's car. I couldn't park in front of the car head to head because of some pipes. So we had to put in series two sets of wires with the alligators clips biting in each other (imagine 4 pcs of connecting wires and extending each pair of red and black). Initially I couldn't start the other car. Then I had to rev my source car to some rpm. I don't know if it is the reving or the series connections causing some resistance? What you think?

The revving would have increased your car’s working voltage, overcoming some of the voltage drop from the long cables. You can just leave one car charging the other through the leads for a few minutes in these situations.

Also, most jump leads are junk - too much insulation, too little copper.

Here’s a tip - cable tie or tape the two leads together at suitable intervals along their length, leaving some play at the ends. They will be much easier to handle and store.

 

[Chris S]

Car A has a dead battery and car B gives a jump to it. I connect the positive of battery B to the positive of battery A and the negative of battery B to the body of car A.

1. Am I essentially creating a separate circuit by choosing a different ground than that of battery A? How is the current flowing back to battery B?

2. How many such separate circuits are theoretically possible ? For example, let's say , battery B is not strong enough, so I leave the connections as they are and bring battery C. I connect the positive of battery C to that of battery A (and of B). Where could I connect the negative of battery C for this arrangement to work?

Thanks

The two cars become one circuit because the grounding portion of the two cars are connected when the two ground terminals of the two batteries are connected by the jumper cable. Connecting the grounding circuits of the two cars is not necessary to jump start car A. You could also use the battery in car B to start car A by removing the battery from car B, then connect the jumper cables, thus the battery in car B is connected to the circuit in car A.

Incidentally, after you jump start a car, the longer you let both engines run, or let car B run on its own, the more the alternator(s) will be charging the battery in car A, so it's a good idea to let both cars run for approximately 10 minutes while the cables are connected, then drive car A for 30 minutes or more.

 

[essenmein]

On my social media, the following funny showed up! I thought it would be most relevant to share it here:

View attachment 257547​

I bet they got home!

 

[essenmein]

Yesterday I jumped start a friend's car. I couldn't park in front of the car head to head because of some pipes. So we had to put in series two sets of wires with the alligators clips biting in each other (imagine 4 pcs of connecting wires and extending each pair of red and black). Initially I couldn't start the other car. Then I had to rev my source car to some rpm. I don't know if it is the reving or the series connections causing some resistance? What you think?

The higher resistance will have limited the current from the source car to the bad car.

Basically jump starting a car is actually doing two things in parallel, its charging the dead battery and trying to provide starting current for the dead cars starter motor. If the cable resistance is too high for the starter motor current, either because you have a poor cable, or long cable, or the starting motor current is huge (eg my old diesel), then you wait and charge the dead battery a little, then it will be able to start the car.

Probably the most extreme of this I've done, and its less "jump starting" and more "remote charging", no jumper cables around, only some old 14/2 house wire in the back... The lady whos car died looked a bit perplexed as some crazy dude striped some wires, warped it around the posts and left this mess of a thing sitting with everything running, but after a few minutes of charging her car started no problem.

(don't try this at home... or something)

 

[pikpobedy]

Shut off the good car before boosting.

 

[256bits]

(don't try this at home... or something)

I've done it.