Can you really pull a battery cable and the engine still runs?

[harrisonm]

I have heard that you can pull a cable off the battery while the engine is running and it will continue running. I don’t remember which cable you pull. I’ve never tried it, but I don’t see how that could work. I seem to remember that it was a way to test your alternator. What do you folks think?

 

[RustyRatRod]

Well sure, but it's not good for the alternator.

 

[Stihl011]

I'm not sure you can, at least with an alternator with a field coil. The alternator needs a DC voltage source to run a current through a coil that create a magnetic field inside the alternator so that the rotation of the alternator around that magnetic field creates current. That DC magnetic field comes from the field coil, and the voltage source for the field coil comes from the battery itself.

One thing you don't want to do is start it with a battery and disconnect the battery. If you do that, the alternator has been dumping current into the battery, and when you disconnect the battery, that current has nowhere to go, and ends up with a voltage spike that can destroy your vehicles electronics if they are not protected. This is called a load dump, you can google it, I'm not making it up.

I could be wrong...I know electronics, cars maybe not so much. I could be missing something about how a particular car works, like an alternator with a permanent magnet or something.

 

[RustyRatRod]

Well I am sure you can. I've done it hundreds of times. But as I said, it's not good for the alternator.

 

[harrisonm]

I'm not sure you can, at least with an alternator with a field coil. The alternator needs a DC voltage source to run a current through a coil that create a magnetic field inside the alternator so that the rotation of the alternator around that magnetic field creates current. That DC magnetic field comes from the field coil, and the voltage source for the field coil comes from the battery itself.

One thing you don't want to do is start it with a battery and disconnect the battery. If you do that, the alternator has been dumping current into the battery, and when you disconnect the battery, that current has nowhere to go, and ends up with a voltage spike that can destroy your vehicles electronics if they are not protected. This is called a load dump, you can google it, I'm not making it up.

I could be wrong...I know electronics, cars maybe not so much. I could be missing something about how a particular car works, like an alternator with a permanent magnet or something.

Well I am sure you can. I've done it hundreds of times. But as I said, it's not good for the alternator.

You guys just pointed out why I have never tried it. It's kind of like, "Why would I want to do that?"

 

[Stihl011]

I assume you had to start with the battery connected then disconnect it? I guess the alternator probably powers the field coil at that point. I didn't think of that. Still, pretty sure bad for alternator and electronics.

 

[RustyRatRod]

I assume you had to start with the battery connected then disconnect it? I guess the alternator probably powers the field coil at that point. I didn't think of that. Still, pretty sure bad for alternator and electronics.

Absolutely. It can pop a diode right quick like. I've been lucky and never done that. Only reason I've ever done it at all was because I was in the field or I did not have access to a meter or charging system tester.

 

[1WildRT]

What it's really bad for is anything that doesn't like higher voltage... It doesn't hurt the alternator any... But if you decide to try it hook up a volt meter and watch the number climb, quickly.... I've seen upwards of 70 volts... If you're just doing a brief off & on to see if the charging system is working it's pretty low risk, nut leave it off for a minute & bad things can happen...

Back in the late 80's auto manufacturers were playing with windshield De-ice systems... Laminated conductive coatings on the glass & circuits to isolate the alternator & direct the power straight to the glass... An iced over windshield would be ice free in less than a minute.... But when the circuity failed that full voltage would destroy all sorts of stuff....

 

[Professor Fate]

Laminated conductive coatings on the glass & circuits to isolate the alternator & direct the power straight to the glass... An iced over windshield would be ice free in less than a minute....

And during the summer you could cook pancakes on it! Yumm!

 

[1WildRT]

And during the summer you could cook pancakes on it! Yumm!

Only if it wasn't working properly... They had sensors to prevent the system from turning on if ambient temperature was above 32 degrees... But for burgers maybe we car re-wire it....

 

[RAT ROD AL]

That's was a quick way to check if your alt was charging, pull hot cable off battery, if still runs alt is charging.

 

[oldkimmer]

U may want to see how Del reacts to this. @67Dart273 Hopefully he is back on here soon. Kim

 

[67Dart273]

U may want to see how Del reacts to this. @67Dart273 Hopefully he is back on here soon. Kim

Hi Kim. Very sporadic, for me, for now.
This so called test is an old wives tale, and proves nothing, and may harm "stuff" because on many systems, especially if the alternator is charging (a low battery) "hard" it will cause a VERY high spike.

This can damage modern electronics, your stereo, etc

Why does it NOT prove anything?

Two situations. Let's say the charging system is just fine, and you think it isn't, and you yank a cable and it STOPS. This might only mean, that it is idling too low, and won't charge enough at that RPM to cover the loads

Situation two: You pull the cable and it STAYS running, but the alternator actually has at least one diode In this case the output will be "dirty" and just one diode can turn (estimate) a 37--40A alternator into about a 15-20A alternator. This is especially true if the bad diode or stator section is shorted instead of open

You will NEVER find this "test procedure" in any mechanics book anywhere.

 

[oldkimmer]

Thanks for the great tutorial Del. Kim

 

[Lord Sparky]

If your battery is dead and you jump your battery to start the car then disconnect the second battery isn’t the car basically running on the alternator until charged? Which is common?

 

[sabre67]

 

[YY1]

Yes, you can and I've never had it hurt anything doing it dozens of times.

The battery is there for starting.

The rest of the car should run just fine with only the alternator and regulator.

Putting the cable back on is potentially more of an issue.

 

[Dartswinger70]

Yes you can do it on an old car but its not recommended on a newer car.

 

[cudamark]

If your battery is dead and you jump your battery to start the car then disconnect the second battery isn’t the car basically running on the alternator until charged? Which is common?

Yes, but, it still has the battery in the circuit to help take any spike hits and/or excess voltage.

 

[YY1]

The regulator takes care of spikes and excess voltage.

 

[cudamark]

If that were totally true, pulling battery terminals off and back on wouldn't be an issue....but it is. The voltage spikes can pop diodes. The regulator won't prevent that. A battery in the system does.

 

[Dartswinger70]

I recall the voltage spikes are why this should never be done on a computerized car...

 

[MadScientistMat]

What it's really bad for is anything that doesn't like higher voltage... It doesn't hurt the alternator any... But if you decide to try it hook up a volt meter and watch the number climb, quickly.... I've seen upwards of 70 volts... If you're just doing a brief off & on to see if the charging system is working it's pretty low risk, nut leave it off for a minute & bad things can happen...

Back in the late 80's auto manufacturers were playing with windshield De-ice systems... Laminated conductive coatings on the glass & circuits to isolate the alternator & direct the power straight to the glass... An iced over windshield would be ice free in less than a minute.... But when the circuity failed that full voltage would destroy all sorts of stuff....

This. An alternator won't stop on a dime - it will briefly still be putting out enough current to charge the battery for a split second. You'll not just test the alternator, but also test if the other electronics in your car can survive a high voltage spike.

If you want to check if the alternator is working, use a volt meter.

 

[Mattax]

and the voltage source for the field coil comes from the battery itself.

That's where your veering off course.
Power comes from the alternator, as long as its the highest voltage power source.

That's was a quick way to check if your alt was charging, pull hot cable off battery, if still runs alt is charging.

For the poor guys with brand X that didn't have ammeter. Otehrwise just look at the ammeter. If the battery is discharging then the alternator isn't supply power, or enough power.

If your battery is dead and you jump your battery to start the car then disconnect the second battery isn’t the car basically running on the alternator until charged? Which is common?

Correct. And its still running on alternator after the battery is charged. That's why the ammeter shows no current flowing out of the battery.

If that were totally true, pulling battery terminals off and back on wouldn't be an issue....but it is. The voltage spikes can pop diodes. The regulator won't prevent that. A battery in the system does.

Yes.
Lets explain this a little more if we can.
The voltage regulator controls current flowing to the windings that create the electromagnetic field. When the regulator cuts current, the electromagnetic field begins to break down. When current is allowed to flow, the electro-magnetic field builds up.

The alternator output on these older cars and trucks is only dampened with a capacitor. The battery connection is on branch at the main splice, so it helps dampen some noise, but not as much as on systems where the alternator connects more directly to the battery.

 

[67Dart273]

If your battery is dead and you jump your battery to start the car then disconnect the second battery isn’t the car basically running on the alternator until charged? Which is common?

A dead battery is a very low impedance and will help prevent spiking