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

[Guineafowl]

The boost car alternator - is it designed to supply 200 cranking amps when the attempt to start the dead battery car.
At one time years ago it was not recommended to run the boost car or damage the diodes, and now it is.
What changed?

In a normal jump start, the donor battery would surely supply most of the power. And modern alternators are many times more powerful than those of old. Perhaps that?

 

[essenmein]

So which German manufacturer made your car? ;)

I have an automotive welding book that says disconnect everything. Perhaps if you don’t, then you generally get away with it as long as the Earth is good. Also, the advice may change due to the shift from big, heavy AC machines to DC inverter types. Who knows?

Ahem, starts with B... and has roundels... and the battery is in the boot on the passenger side for "balance".

 

[sophiecentaur]

'Explosions' require a mixture ratio of Hydrogen to Oxygen (18 - 60%). I could believe that Oxygen could be present in the space over the battery liquid and that the mixture could be 'just right (inside) But how would this be ignited by a spark outside the battery? What sort of flame would you get to form and travel from a sparking jump lead to the breather hole and then propagate into the cavity? How much H gas would there be in the region of the spark after a minute or two of jump charging? An old town gas gas ring could be lit, only by holding a match quite near the ring and there has to be gas coming off with an audible 'hiss'. H gas is not released by electrolysis at that sort of rate when charging at a few Amps.
Imo, the best explanation of a battery blowing its side off would, to me, be that a heavy current causes a high resistance internal connection to get red hot and the contained explosive mixture is relatively easy to ignite.

Edit. I had a friend who blew the side off a battery and repaired it by sticking the side back on with Evostick. He claimed he used it in his car for some while afterwards. I suspect he gave the join some extra support to keep it on but 10/10 for effort and money saving - he was a poor student at the time.

 

[musicgold]

Just to clear things up, the two batteries should be in parallel. Every ground connection will effectively be to a common node and so should all the + leads. The jump battery should, ideally, be in a car with the engine running. Charge should flow into the dead battery and its volts should soon reach a proper charging value. After connecting the jump battery, wait a minute or so for the running alternator to perk up the 'dead ' battery (give the engine a few revs above idle) and keep the jump engine at that speed whilst you try to start the casualty.

I was under the impression that the reason we are never supposed to connect the two batteries in parallel is that it would cause a large surge of current from the boost battery to the dead battery.

I believed that connecting just the positive of the boost battery to the positive of the dead battery provides a separate voltage source (and isolates the dead battery) to start the alternator and once the alternator is on, it slowly charges up the dead battery. Am I missing something?

 

[hutchphd]

All methods wire the two batteries in parallel. The only question is exactly how the wires are arranged, and where you are standing when the final connection is made..

 

[sophiecentaur]

a large surge of current from the boost battery

Do you think that "surge" would be more than the hundreds of Amps that flow through an initially stationary starter motor? The volts across a rapidly charging flat battery very soon reach more than 12V. Compare that with the battery Volts across a nearly stalled starter motor on a cold morning with thick oil in the engine. Just watch the headlamps dim and almost go out!

 

[sophiecentaur]

At one time years ago it was not recommended to run the boost car or damage the diodes, and now it is.
What changed?

Diodes, along with a lot of other semiconductors, have advanced a lot in the last decade or so. (Early days they were just not suitable for automotive alternators.)

I believed that connecting just the positive of the boost battery to the positive of the dead battery provides a separate voltage source (and isolates the dead battery) to start the alternator and once the alternator is on, it slowly charges up the dead battery. Am I missing something?

You are missing the fact that there is no 'extra' resistance or other component deliberately added in any charging arrangement. The Crock leads fit happily over the battery terminals and, where else on the vehicle is there anywhere that's not electrically on the same circuit node? As has been stated several times above, it's a simple parallel connection. We are talking very basic DC circuit theory here.

 

[Guineafowl]

'Explosions' require a mixture ratio of Hydrogen to Oxygen (18 - 60%).

What sort of flame would you get to form and travel from a sparking jump lead to the breather hole and then propagate into the cavity?

Whether those two farmers experienced a genuine explosion or not is unclear. Certainly what happened was, they made the final connection and immediately there was a flash and the battery blew its top, showering them with acid. Whether the force was enough to split the casing or just blow out the fill caps is also unclear. Whatever happened, it’s made me very careful around sparks and car batteries...

H gas is not released by electrolysis at that sort of rate when charging at a few Amps.

There are several responses like this, and I can’t emphasise too much - we’re not talking charging at a few amps, or anything else ‘normal’. We’re talking a faulty battery, probably with high internal resistance, that has been electrically beaten, and will be off-gassing considerably, partly due to heat, partly electrolysis, perhaps.

Here’s how I look at it - there are lots of examples of things that should be safe, but become unsafe because of a fault, a deviation from normal. In theory, it should be safe to touch the neutral terminal of a lamp, because it’s close to equipotential with you, but of course you don’t.

You could extend this to: “I’ve touched 100 neutral terminals and have always been fine”. Cf. “I’ve always jump started by connecting the batteries directly”. Safety rules have to encompass a wide variety of possibilities. Those farmers had probably been doing that all their lives.

 

[sophiecentaur]

Whether those two farmers experienced a genuine explosion or not is unclear. Certainly what happened was, they made the final connection and immediately there was a flash and the battery blew its top, showering them with acid. Whether the force was enough to split the casing or just blow out the fill caps is also unclear. Whatever happened, it’s made me very careful around sparks and car batteries...There are several responses like this, and I can’t emphasise too much - we’re not talking charging at a few amps, or anything else ‘normal’. We’re talking a faulty battery, probably with high internal resistance, that has been electrically beaten, and will be off-gassing considerably, partly due to heat, partly electrolysis, perhaps.

Here’s how I look at it - there are lots of examples of things that should be safe, but become unsafe because of a fault, a deviation from normal. In theory, it should be safe to touch the neutral terminal of a lamp, because it’s close to equipotential with you, but of course you don’t.

You could extend this to: “I’ve touched 100 neutral terminals and have always been fine”. Cf. “I’ve always jump started by connecting the batteries directly”. Safety rules have to encompass a wide variety of possibilities. Those farmers had probably been doing that all their lives.

That's full of good points and you are absolutely right about 'safety first'. However, my biggest personal point of interest in not so much that bad things happen - they do- it's the real cause and I think that batteries exploding is probably not due to an external spark directly igniting the (possibly) explosive mixture in the battery cavity. Is there a possible 'flame path' for this? The vent holes in the battery caps are very small diameter and wouldn't the Davey Miners' Lamp principle actually prevent a flame front from getting down the hole?

You mention internal resistance and that could be a very localised series load inside one cell (?). A surge charging power of a few kW (easy for the donor battery to supply that) could actually boil a small volume of liquid somewhere inside the battery and that could produce a few Bar of excess pressure, which could distort the flat side of the case or, as suggested, blow out one of the filler caps. The Energy supplied would be in the region of, perhaps a couple of kJ (2kW for 1s) I found this Wiki article which has a list of 'events' which correspond to various amounts of Energy in kJ. We're talking in terms of the energy of an M16 rifle bullet.

The problem with all the witnessed events is that they only happen once per person and they are over in such a short time.

I just thought of another possibility. Oxygen and hydrogen would be produced as well as steam and it could be at a rate of more than a hundred times the rate under normal high charging (say 40A max). The sheer volume of gas produced could beat the breather holes (which could be blocked in an old battery) and just cause a mechanical explosion, which wouldn't need ignition. The Bang! would be the first thing and then there could be ignition of the hydrogen as it leaves the scene. The same amount of Energy would be involved as the 'boiling' scenario above.

 

[hutchphd]

Has no one included the possibility of an internal spark inside a defective cell? If that cell has a high resistance, and there is inductance across the parallel combination of batteries, removal of the "charging" battery will create a rapid diminution of current and consequent back emf at the defective cell. If it contains hydrogen and oxygen they will ignite from within, assuming the (likely deformed) plate geometry promotes a spark...?

 

[sophiecentaur]

consequent back emf at the defective cell

What significant inductance is present in the two wires, connecting the two batteries?

 

[hutchphd]

I'm thinking that when you disconnect the charging battery is when the real danger occurs. The target car is often running so there can be relays and field coils and lots of stuff attached to the possibly damaged battery.

 

[Torbert]

I've only had one battery blow up on me, back when young and inexperienced. As the new guy in the shop I had the job of starting the dead cars. Alaska in the 1960's had lots of vehicles with dead batteries towed to the shop.
Restarts involved a 100 amp charger, hooking it up, turning it on, then waiting several seconds before cranking the vehicle.
The car starts, then charger is turned of and disconnected from the battery terminals. A direct connection was used as the charger is off during the connection phase.

This time I flipped the switch off and grabbed the negative clamp as someone yelled and everything disappeared.
"Its not off" was the yell that did not finish. There were battery parts in the ceiling as well as one piece that had to be pulled out of my forehead. Lucky I had my glasses on, lost eyebrows, hair, and my insulated vest.

Knowing the exact path or procedure used for the gasses to ignite doesn't concern me near as much as just knowing it happens.

 

[Guineafowl]

I think that batteries exploding is probably not due to an external spark directly igniting the (possibly) explosive mixture in the battery cavity. Is there a possible 'flame path' for this? The vent holes in the battery caps are very small diameter and wouldn't the Davey Miners' Lamp principle actually prevent a flame front from getting down the hole?

Isn’t the principle behind a Davy lamp that the metal wire mesh acts as a heat sink and robs any gas outside of its activation energy?
A battery vent has neither metal wire nor mesh. It’s conceivable that a mixture of H and O can funnel a flame front into the battery cavity. After all, flashbacks, where a flame front travels down relatively narrow gas nozzles and hoses, are well known to occur in oxyacetylene welding. They are fitted with flashback arrestors for this reason.

Knowing the exact path or procedure used for the gasses to ignite doesn't concern me near as much as just knowing it happens

Even if you can’t envisage the mechanism, I think it’s worth hooking to the engine lifting eye, not the -ve terminal, last. A small change that can make a big difference. You also obviate any problems with the dead car’s -ve strap.

 

[musicgold]

As has been stated several times above, it's a simple parallel connection. We are talking very basic DC circuit theory here.

I am still confused. If it is a parallel connection, then why not connect the negative of the boost battery directly to the negative of the dead battery? Why do we have to connect it to the body of the disabled car? It would be helpful if you could explain your idea with a circuit diagram.

Here is what I have seen on the net.

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[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.