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

[musicgold]

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

 

[hutchphd]

In an automobile typically the frame, body, and engine block are all connected stoutly to the negative battery terminal. Almost every electrical contrivance in the car uses the car itself for part of the conductive path.

 

[sophiecentaur]

Where could I connect the negative of battery C for this arrangement to work?

It's not so much 'where' but 'how well'. The ground connection of the starter motor will be good as it's straight on to the engine block. Your dying battery may be suffering from poor connecting leads to the starter and the engine block. If the connections are ok then why not connect your jump battery right across the old battery? If they aren't then unscrew, clean and re- connect all the terminals. Frayed or corroded cables should be replaced - but that may not be possible in a First Aid scenario.

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.

 

[hutchphd]

If the connections are ok then why not connect your jump battery right across the old battery?

Supposedly there is a chance of generating hydrogen gas during the process and making the consequential connections remote from the battery lessons the chance of an errant spark (or you) being near said gas. Flying sulfuric acid not good. Never seen it myself.

 

[Guineafowl]

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?

Current flows from the body of car A to the negative of battery A via the big earth/ground strap that connects them. The reason you make this last connection is so that the spark formed when the circuit completes is away from the flammable battery gases, You should disconnect in reverse order for the same reason.

Once car A is started, you should switch off car B before disconnecting anything. Car B’s alternator will be working at max output charging the A battery. If you disconnect while this is happening, the B alternator will be suddenly unloaded and likely to produce voltage spikes which damage the car’s electrics.

 

[sophiecentaur]

I never heard of a Hydrogen explosion out in the open with a jump starting process. Where could an explosive mixture build up?
I wouldn’t want to have my good battery left across a dead battery without my alternator helping it. After all, you can’t rely on the sick car alternator working. It could be the reason for the flat battery in the first place. Disconnecting with both engines idling seems favorite. Afair, that’s what I have always done.
It’s always amazed me that some owners allow themselves to be caught out this way several times in a row. People with manual transmission would park facing down a long slope so they could bump start with a tired battery.
Just putting off the evil day when you have to pay for a replacement.

 

[Guineafowl]

I never heard of a Hydrogen explosion out in the open with a jump starting process. Where could an explosive mixture build up?
I wouldn’t want to have my good battery left across a dead battery without my alternator helping it. After all, you can’t rely on the sick car alternator working. It could be the reason for the flat battery in the first place. Disconnecting with both engines idling seems favorite. Afair, that’s what I have always done.
It’s always amazed me that some owners allow themselves to be caught out this way several times in a row. People with manual transmission would park facing down a long slope so they could bump start with a tired battery.
Just putting off the evil day when you have to pay for a replacement.

I’ve heard of two batteries exploding in people’s faces - making the last jump connection directly to the battery post. Making the last connection to the engine block of the dead car also eliminates any earthing problems between starter and dead battery.

Disconnecting with both cars idling was fine before modern ECU-controlled cars and their output-modulated alternators, and array of sensors. “Load dump” of a suddenly disconnected modern alternator is well known. I think you’ll find the method I’ve described is now recommended by auto rescue organisations.

 

[hutchphd]

I never heard of a Hydrogen explosion out in the open with a jump starting process. Where could an explosive mixture build up?

I wonder if under certain conditions you can cause electrolysis creating hydrogen and oxygen internally in a bad cell. This would be a bad thing...(but entirely supposition on my part here!)

 

[256bits]

I wonder if under certain conditions you can cause electrolysis creating hydrogen and oxygen internally in a bad cell. This would be a bad thing...(but entirely supposition on my part here!)

Doesn't the explosion have to be internal for the battery to explode and spray the acid everywhere.
A pressure buildup within the battery has to overcome the casings stress limit.

An external combustion of the hydrogen produced by the battery would have to have the flame front travel into the battery through the caps and the little vent hole, into an oxygen/ hydrogen mixture within the battery of the correct composition to support combustion. That seems unlikely.

A defect within the battery seems to be more likely such as shorted plates in a cell allowing a large current to flow within the battery, vaporizing internal liquids and/or solids.
Afterall, when connecting the two batteries positive-positive and negative-negative, a charging circuit of the 'dead' battery is completed.
If the dead battery is still "functional" in that it would be able to be charged up, and dead, the initial charging current for this "functional" battery is minimal, since th internal resistance is quite high for a drained battery.
A large current would be able to flow within a defective battery if most cells are not completely discharged, so that the electrolyte is able to conduct, and a defective cell with a short.
From E=I R(internal ), not sure what R(internal ) would be say 0.1 ohms.
12 v-11v = I 0.1, giving I = 30 Amps through the dead battery. ( Using 11 v - 2v ( for the defective cell )for the dead battery voltage. )

Sticky Points
1. How dead should the dead battery be for an appreciable current to flow through it
2 . How much current is needed to vaporize internal components and liquid within the battery
3. What is the Internal resistance of a somewhat discharged battery.
4. Is a defective cell really be needed for this effect to occur.
5. Should one really be running the boost car when connecting, as this will increase the charging voltage, and thus the charging current. Does the boost car electronics limit the voltage/current being supplied to what is actually a short of the boost car alternator terminals in this situation.

 

[256bits]

I never heard of a Hydrogen explosion out in the open with a jump starting process. Where could an explosive mixture build up?

Hydrogen is light and should exit the exterior area quite quickly, instead of hanging around.
The volume under the cap - its already a gas, is there enough present in the space when ignited, if it ever does, to blow out acid from the confined volume. Seems suspect.

 

[256bits]

Disconnecting with both cars idling was fine before modern ECU-controlled cars and their output-modulated alternators, and array of sensors. “Load dump” of a suddenly disconnected modern alternator is well known. I think you’ll find the method I’ve described is now recommended by auto rescue organisations.

Would not a connect with the boost car running be also a load dump?

 

[chemisttree]

I never heard of a Hydrogen explosion out in the open with a jump starting process. Where could an explosive mixture build up?
I wouldn’t want to have my good battery left across a dead battery without my alternator helping it

It happened to me once. 70-something Chevy truck. I was working at Firestone as a GS at the time when a fella comes in and asks for a jump across the street at the K-Mart gas station. I go over and connect the cables just as I was taught, last connection being to his truck on the engine somewhere. No sparks and we’re good to go. I back away to catch his attention and signal him to try cranking it. When he does ... BOOM! The top of his battery blew off and launched 20 or so feet in the air. Acid everywhere. Luckily not on me. It sounded like a gunshot.

His battery must have had a contact failure at the terminal or the post inside the plastic case. Cranking it either shook it around or the draw somehow caused a spark. That was back when batteries had removeable filling caps and you had to top them off from time to time.

To this day I stand waaay back when someone is cranking the afflicted vehicle.

 

[chemisttree]

Hydrogen is light and should exit the exterior area quite quickly, instead of hanging around.
The volume under the cap - its already a gas, is there enough present in the space when ignited, if it ever does, to blow out acid from the confined volume. Seems suspect.

I always thought it was Brown’s gas being produced by the battery. There is a dead space above the acid level, about 1/2”, errr, 12.5mm high inside the case where the gas collects. It doesn’t float anywhere. As it is produced it exits via a vent nowadays. In the old days it exited through small vents in the removeable caps... right next to the questionable cable connections.

 

[Guineafowl]

Would not a connect with the boost car running be also a load dump?

I’d say that would be a voltage sag, followed by compensation by the charge monitor circuitry of the car. Mitigated, of course, by the reserve of the boost battery, and the inevitable voltage drop and current limiting of the jump cables and their connection points. It’s the disconnection that produces the spike, much like with inductors and flyback.

I’m quite surprised that EEs like yourselves are questioning battery explosions and load dump voltage spikes. It made me question what I know, which is a good thing.

I looked up a trusted brand of battery, and sought their safety info.
https://www.yuasa.co.uk/info/technical/health-safety/

I refer you all to sections D (gases) and E (jump start procedure). It confirms what I say, apart from suggesting the final jump connection could be directly to the -ve terminal, or to an earthed point. Every set of jump leads I have ever bought has said in the instructions “NEVER [capitals] make the final connection directly to the negative terminal”.

Another reference: https://www.theaa.com/breakdown-cover/advice/using-jump-leads
And another, for good measure: https://www.rac.co.uk/drive/advice/car-maintenance/how-to-jump-start-a-car/

Also note that modern smart chargers will not supply power until connection is made, often called ‘arc avoidance system’ or similar. Page two, under heading, ‘safe and easy to use’:

https://www.ctek.com/storage/ma/4e122ff2ca2147d89202d8ce2561386c/9549cddce9a64a7f8752932183162ee0/pdf/848FDD68BDE3402BA016C77F4B49B72A1C634710/MXS_5.0-productsheet-low-UK-EN.pdf

 

[256bits]

I always thought it was Brown’s gas being produced by the battery. There is a dead space above the acid level, about 1/2”, errr, 12.5mm high inside the case where the gas collects. It doesn’t float anywhere. As it is produced it exits via a vent nowadays. In the old days it exited through small vents in the removeable caps... right next to the questionable cable connections.

With overcharging oxygen and hydrogen can be produced.

With normal charge/ discharge the chemistry is such that water is produced or used up, and the battery shouldn't produce any gas. But real batteries do act under overcharge from the alternator.
A discharged battery you say would still have that gas in the space.

From this site,
http://giantbatteryco.com/GLOSSARY/Calculate.Industrial.Battery.Hydrogen.Gas.Emission.html
not very much hydrogen is produce during a charge for an industrial battery during charge - think forklift battery.

A typical lead acid motive power battery will develop approximately .01474 cubic feet of hydrogen per cell at standard temperature and pressure.

about 25 cubic inches

A car battery being about. at least 1/10th, or 1/ 20th the size or less, perhaps 2.5 cubic inches - 4 sugar cubes or so.
Ventilation of a room containing the charging batteries is recommended.
A car under the hood has adequate ventilation.

Another site
https://www.mathscinotes.com/2013/02/battery-outgassing-math/

... Maximum hydrogen evolution rate is 0.127 mL/s per charging ampere per cell at 25 °C and standard pressure (760 mmHg). The worst-case condition exists when forcing maximum current into a fully charged battery. ...

And goes into calculation mode, interesting.

But most car batteries these days are pretty much maintenance free, never having to add water due to H and O recombination technology within the cells while under use, rather than have the gases escape to the atmosphere.

 

[Dullard]

While it's true that disconnecting jumper cables with the car(s) running could cause a disturbance in the voltage regulation of either/both, I'm having a hard time believing that the disturbance would be any worse than a number of other 'load dumps' that occur regularly. I'm thinking of starter motor, headlights ON/OFF, electric seats/defrosters... I definitely wouldn't disconnect while cranking the other vehicle. As it happens, I had to perform this operation last Sunday (I guess I dodged several bullets) - If I'd have known how dangerous it was, I would have made my MIL stand closer to her car :).

 

[Guineafowl]

But most car batteries these days are pretty much maintenance free, never having to add water due to H and O recombination technology within the cells while under use, rather than have the gases escape to the atmosphere.

The explosions I heard about, anecdotal but no reason to doubt the sources (farmer friends of my uncle), would have been at least 20 years ago. Battery tech has moved on from simple vented tanks, but...

... You are working with stressed and faulty components. A dead battery could have an unknown internal fault, and will have been caned for several minutes in trying to start the car. In these situations, you can hear the battery off-gassing furiously, and prolonged abuse can warp the internal plates. Hence advice not to crank continuously for too long.

 

[Guineafowl]

While it's true that disconnecting jumper cables with the car(s) running could cause a disturbance in the voltage regulation of either/both, I'm having a hard time believing that the disturbance would be any worse than a number of other 'load dumps' that occur regularly. I'm thinking of starter motor, headlights ON/OFF, electric seats/defrosters... I definitely wouldn't disconnect while cranking the other vehicle. As it happens, I had to perform this operation last Sunday (I guess I dodged several bullets) - If I'd have known how dangerous it was, I would have made my MIL stand closer to her car :).

Headlights and heaters would be fairly minimal loads, and easily compensated by the charge monitor.

The starter IS a very heavy load, but while the engine is cranking, it’s turning very slowly compared to when running, and as far as I know a modern alternator kicks in only when the engine is started. In any case the alt output will be very far from maximum when the starter is disconnected, hence no voltage spike problems. Also, the battery will be depleted by the start, and better able to absorb surges.

What I’m referring to is the boost car alt, running at max output (by field control) against the almost dead short of a flat battery, suddenly being disconnected. Because its battery will be near full charge, there will be little buffering in the time taken to derate the alt. There’s also the inductance of the stator windings to consider.

 

[essenmein]

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!

 

[Guineafowl]

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!

That all sounds sensible, but can you explain why, as referenced in post #14, the two major auto rescue organisations in the UK (AA and RAC) and a major battery manufacturer recommend you shut down the ‘good’ car before disconnecting anything? You may think I’m wrong, but at least I have reasonable references behind what I say.

Doubtless, there’s transient protection circuitry installed, but surely it’s better not to challenge it unnecessarily?

 

[essenmein]

That all sounds sensible, but can you explain why, as referenced in post #14, the two major auto rescue organisations in the UK (AA and RAC) and a major battery manufacturer recommend you shut down the ‘good’ car before disconnecting anything? You may think I’m wrong, but at least I have reasonable references behind what I say.

Doubtless, there’s transient protection circuitry installed, but surely it’s better not to challenge it unnecessarily?

Unfortunately for me I don't generally listen to instructions that make no sense to me, so this line would make me question their guide lines significantly: (from AA link)

"Keep both engines off and wait for 3 minutes, then start the working car’s engine and let it run for 1 minute."

To me this makes no sense, if the dead battery is dead, you now have two large loads on the good battery, then waiting, esp depending on the fault in the bad battery could easily mean no dice starting the good car.

Maybe these guys are scared of a little arc when you connect or something, generally not a problem for the car even if the bat is completely gone because of all the accumulated capacitance on the B+ bus from all the connected ECU's.

Every bit of electronics in a modern car can hold the clamped load dump voltage of 40V. Almost all cars now use a central clamp to limit this voltage to 40V rather than older alternators which would hit nearly 100V. Generally this is done in alternator rectifier, the diodes are Zeners, so they limit the output voltage under load dump.

I'm sure there are many different ways of connecting the two cars to boost and an infinity of instructions in between.

I've always done this, I not going to claim it right or wrong, but had zero problems with everything from old diesels to modern luxury cars:
Good car idling
Connect positives on both sides (bat terminal to terminal)
Connect negative on both sides, (terminal to terminal) you should hear the running car take up a bit of load.
Rev good car to 2-3krpm for a bit
Crank car with dead bat
Disconnect with both running.

I suspect those instructions on AA are done in such a way that they cover their rear ends in case of anything ever going wrong anywhere and someone blaming their instructions.

 

[jack action]

I guess seeing is believing (It doesn't only happen when jump starting):

​

One of the most common reasons why automotive batteries explode is when the hydrogen gas that is produced during the charging cycle builds up inside the case and is ignited by a spark or a flame. Exploding car batteries send sulfuric acid and fragments of the battery case flying everywhere and these can cause injuries to anyone in the vicinity.

A lead-acid battery system produces hydrogen gas through the electrolysis of water when overcharged. Car batteries have vents on each battery cell to allow hydrogen to dissipate. However, the hydrogen gas may not be vented out properly for several reasons and these may include structural flaws in the battery, the vents are inadequate for proper dissipation or the battery may be extremely overcharged.

Sometimes, an automotive battery may not explode but the accumulation of hydrogen gas may cause the ends of the battery to become swollen and too hot to handle. This is also an sign of a malfunctioning charging system of the vehicle. The swollen ends are indicative of extreme overcharging of the batteries creating hydrogen in such large amounts that the ventilation system is no longer able to handle the gas build up. A hot and swollen battery is also dangerous.

Another potential cause of battery explosion is not related to hydrogen build up, but is caused by the build up of water vapor from a boiling electrolyte. The electrolyte may be accidentally heated when the terminals of the battery are short circuited.

Another explosive risk for older lead-acid batteries is sheer age. As lead-acid batteries age, they lose water, causing the lead plates in the batteries to warp. Sometimes, the energy requirement of the vehicle upon ignition causes the warped plates to flex and touch one another, which can lead to a sudden explosion.

Please, be safe.

 

[Guineafowl]

"Keep both engines off and wait for 3 minutes, then start the working car’s engine and let it run for 1 minute."

To me this makes no sense, if the dead battery is dead, you now have two large loads on the good battery, then waiting, esp depending on the fault in the bad battery could easily mean no dice starting the good car.

I must admit I don’t do this either. I guess its some attempt by a major (and potentially accountable, as you suggest) organisation to cover a wide range of potential faults, handled almost inevitably by non-qualified operators, using equipment of unspecified quality and condition.

This instruction looks to be aiming to allow a lower rate of charge to the dead battery, without the boost of the running alternator. This might mitigate arcing or possibly overheating of cables due to poor quality materials (eg at crimps) or poor connections. Many jump leads are kept in damp boot wells or even in the open in pick-ups.

However, as you say, this runs the risk of stranding the donor car as well, although I assume this is minimal. Also note the other two references don’t have this questionable step and still recommend shutting off the donor car. The Zener protection you mention is comforting, but why challenge it?

 

[essenmein]

I wouldn't be surprised if the "shut everything off" before disconnecting instruction is less about electricity and more about the risk of dropping cables or getting hands/clothing caught in rotating machines (eg clutch driven cooling fan)...

 

[Dullard]

On the subject of why the instructions are what they are (on automotive sites):

I have a similar issue with a similar audience. We (my company) build equipment - it always includes lots of sensors / electronics. On occasion, one of our welders needs to do a last-minute repair/modification. It's really easy to damage electronics with a TIG welder (if the ground connection(s) aren't properly managed. I spent several years trying to explain ground currents, etc. The standing instructions (now) are:

Disconnect every external wire.
Lift the unit on the forklift.
connect the ground lead as physically close to the weld location as possible.
If the ground is more than 4" from the weld, call me to look at it.
Weld.

Most of this is usually unnecessary. It works (every time).

My point: Not everyone has the basic knowledge required to custom-fit a situation. If you can come up with idiot-resistant rules, that's probably best.

 

[Guineafowl]

My point: Not everyone has the basic knowledge required to custom-fit a situation. If you can come up with idiot-resistant rules, that's pro

This is a good point. Even experienced car techs might not fully assess a fault before jumping the car, as well.

When I weld anything on a vehicle, all switches go off, the battery is disconnected, as is the alternator. A good close ground, too. I’ve certainly known cars damaged by improper welding prep. Crank sensors, alt regulators and even ECUs have been known to pop - Zeners or no.

 

[essenmein]

This is a good point. Even experienced car techs might not fully assess a fault before jumping the car, as well.

When I weld anything on a vehicle, all switches go off, the battery is disconnected, as is the alternator. A good close ground, too. I’ve certainly known cars damaged by improper welding prep. Crank sensors, alt regulators and even ECUs have been known to pop - Zeners or no.

Thats interesting, I've done precisely the opposite when welding on my trucks... lol

My rationale there was to leave the big capacitor/current sink connected to the system, as I figure this would be much better at preventing spikes than a bunch of higher impedance open circuit cables... Mind you all the welding I've done on vehicles was on the frame for off road armor etc, not body work, and always had my ground clamp close to where I was welding. Certainly seen some people put their ground clamp on for example a suspension link or something daft where the connection to the thing being welded was poor. You do have to put some thought into whether the 100+ A you are putting though the thing isn't also going to weld a ball joint or a bearing lol

 

[Guineafowl]

I wouldn't be surprised if the "shut everything off" before disconnecting instruction is less about electricity and more about the risk of dropping cables or getting hands/clothing caught in rotating machines (eg clutch driven cooling fan)...

Well, that’s got to be sensible advice for the general public. The Yuasa link specifies only the donor car, however.

 

[Guineafowl]

Thats interesting, I've done precisely the opposite when welding on my trucks... lol

My rationale there was to leave the big capacitor/current sink connected to the system, as I figure this would be much better at preventing spikes than a bunch of higher impedance open circuit cables... Mind you all the welding I've done on vehicles was on the frame for off road armor etc, not body work, and always had my ground clamp close to where I was welding. Certainly seen some people put their ground clamp on for example a suspension link or something daft where the connection to the thing being welded was poor. You do have to put some thought into whether the 100+ A you are putting though the thing isn't also going to weld a ball joint or a bearing lol

You renegade! Again, your explanation sounds plausible, but try searching ‘welding car precautions’ or similar. Every link will tell you to disconnect as much as possible.

 

[essenmein]

You renegade! Again, your explanation sounds plausible, but try searching ‘welding car precautions’ or similar. Every link will tell you to disconnect as much as possible.

Literally the first thing that comes up is:
https://forum.millerwelds.com/forum/welding-discussions/13080-precautions-mig-welding-on-my-truck

Some people say disconnect everything, then you get the mechs that say blah blah we never bother... lol

"I have welded on probably several hundred vehicles in my working life and have never disconnected either the battery or ECU and either I have been lucky or their is just a very minimal risk of damage. Dave "

"I have welded on dozens of cars and never disconnected anything. I also have never seen one muffler repair shop disconnect the battery, and their welding all over the exhaust systems everyday. "

"Autobody shops don't disconnect anything. "

 

[essenmein]

Keep in mind I'm not advocating for one way or the other here!

 

[essenmein]

Then our new (to us) car actually gets really upset if you disconnect the battery, if you do, you need to plug a computer into the other computer to tell the computer that everything is ok... Then the dash lights indicating its currently freaked out state go away.

I have not weld on this car (yet) lol.

 

[256bits]

I guess seeing is believing (It doesn't only happen when jump starting):

​

Please, be safe.

The writeup is pretty much my take on battery explosions.
I have mentioned the overcharge for hydrogen ( and oxygen ) production, defective battery, and vaporization of internal components ( boiling electrolyte ).
My contention is that the gist of 'be safe' sites lean towards an the aspect of an external spark to the terminal igniting hydrogen gas exterior to the terminal as being the root cause of an explosion. That is misleading. The explosions happen internally to blow the top off, not externally.
I question running the boost car alternator when connecting to the dead battery for certain reasons.
1. does the dead battery have an internal defect, such as a short in a cell.
2, will the dead battery accept high charging current if not defective.
In both cases, a high throughput current can vaporize internal parts and blow the battery apart.

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?

 

[essenmein]

IMO what it comes down to is that there are a variety of things that could be faulty that make a single answer here not valid. I think that's why the instructions are over cautious, because they have to cover all eventualities.

For example if you get into your car, turn on the ignition, does everything light up but you just don't have enough charge to turn the engine? Or do you turn the ignition and nothing happens? In the first case I wouldn't hesitate to jump start, but if the thing is completely dead I would have a bit of a look to see why say, zero volts, are on my battery. Or maybe your starter motor is dead, which can on the surface appear like a dead battery... and boosting won't do a damn thing.

(regularly sinking my truck in mud holes causes both the alternator and starter motor to fail far more often than I'd like!)

In one case my alternator died, field driver died, left the field winding permanently shorted to B+, alternator rectifier melted away, the only way to get home was to stop when the engine started miss firing so the tailing vehicle could top up my battery every half hr or so...

 

[Guineafowl]

Then our new (to us) car actually gets really upset if you disconnect the battery, if you do, you need to plug a computer into the other computer to tell the computer that everything is ok... Then the dash lights indicating its currently freaked out state go away.

I have not weld on this car (yet) lol.

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?

 

[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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