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Great post swduncan. We're all assuming (or I was, at least) that the battery is good because the engine failed to start from a jump, which would be a fair assessment if the jump was from another running vehicle. But we shouldn't assume the same when a jump pack is used because the battery in that might be bad, or just not have enough capacity to start the engine. I've certainly come across a few that were not fit for purpose.
 

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If it ran fine for a few days after a new battery then died, maybe it's a charging problem. The new battery had enough capacity for a few days, but was running at a loss, using more energy than putting back into storage. Have you checked the output of the alternator? I know you said you tried one of those jumpstarts, but cheap ones, may not be up to the task of assisting a really dead battery. Try fully charging the new battery with a proper charger, or jumping from a running car. If it does start, i would suspect a charging issue. Just a thought. good luck.
Disconnect the positive post on the battery while the car is running.
If the car continues to run then the alternator is charging and working as it should.
If the car stops when you pull the positive lead off of the battery you have a bad alternator not charging. It's as simple as that.

Try it and see what happenS and good luck. ?
 

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Disconnect the positive post on the battery while the car is running.
Disconnecting the battery while the engine is running isn't a great idea on a more modern car. Not to mention that he'd need to get the engine running in the first place.
Wouldn't it be better to just connect a voltmeter across the battery when the engine is running to check that the alternator is charging?
 

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I have done testing and the alternator will spike to around 20 volts when you remove a terminal when running. Not wise.
This really doesn't make sense to jolt up to 20 vdc.
If this were the case then where and how is the output of the alternator regulated?
Also you would be subjecting the battery to this same 20 volt output which again makes no sense from a supposed regulated output.
Anybody have an explanation for this?
I'd surely doubt newer cars are using the antiquated external regulator for the alternator.
Just curious to know the answer to this and always willing to learn something new.
Thanks.

Update.
According to what I've seen in replacement alternators for our cars they do have built in regulators in the alternator.
Again I highly doubt 20vdc is the regulated output but please let me know if this is incorrect. ?
 

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Alternators rely on battery voltage feedback either via a battery sensing or ECU control. When there is no battery to absorb the energy the regulator does not respond as it should and yes they spike. If you doubt what I am saying give it a try and lets see what damage you do. The effect is increased with engine rpm. Regulators have had built in regulators since the 1960's and 20 volts is uncontrolled output not regulated.
 

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Alternators rely on battery voltage feedback either via a battery sensing or ECU control. When there is no battery to absorb the energy the regulator does not respond as it should and yes they spike. If you doubt what I am saying give it a try and lets see what damage you do. The effect is increased with engine rpm. Regulators have had built in regulators since the 1960's and 20 volts is uncontrolled output not regulated.
Makes no sense what you say.
If the alternator internal voltage regulator is regulated at 14.55 volts output how can it possibly spike at 20volts?
Even if the ecu had some form of regulation or feedback to the alternator the alternator would still put out a maximum of 14.55 volts.

There is a temperature sensor used on the negative battery post that tells the ecu to further regulate voltage depending on physical battery temperature but the output would still be regulated to 14.55 volts max via the alternator voltage regulator.

I've removed the temperature sensor on my negative post and completely rewired my positive and negative terminals with heavier cables.
Ive monitored the output voltage via a obd2 scan gauge and notice a .2 - .5 increase in voltage output to the battery since removal of the temp sensor on the negative post.

14.5 is occasionally seen on the scan gauge but stays stable most of the time in the 14.1 to 14.3 volt range.

As to removing my positive post on my car to check alternator output I most certainly would if the need arose to do so.
I understand the complexities of the newer cars but what I'm seeing with our cars is that 14.55 will be the voltage output with or without a load from the alternator.

Please expound on this if you can as I'm always up for good technical conversations and if I'm wrong with this I'll surely be the first to say so. It wouldn't be the first time I've had to rethink things over. ??
 

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View attachment 442436

Here is the actual service manual stating the internal voltage regulator is governed to 14.55 volts +- .3volts.
Again if anyone knows otherwise please let me know. ?
If you look again ...
it shows that there are two ways for voltage regulation to happen
One is external mode; ECU controlled. The other is internal; voltage specified at around 14.55 volts
ECU does not list the voltage rating
The internal does list it at 14.55 plus or minus
So, if ECU controlled then voltage can vary greatly.
It would depend on what kind of field charging strength it is outputting to the alternator.

This is not to say you or CC is wrong.
Just that there are other things to consider
 

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The increase in output voltage has nothing to do with battery sensing or ECU control. It has to do with good old Ohm's Law and the reaction time of the voltage regulator. Wikipedia can explain it much better than I could...Alternator Load Dump
 

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Agree with that. Great explanation. The term is self inductance. Whenever I teach electrical principles I connect a very large coil to a 12 volt battery and demo that touching it has no effect. Then I challenge a student to disconnect it. If they don't get a big kick I get them to touch both parts when disconnecting. All the brave ones then line up and have a go. It's a fun way of demonstrating the effect of self inductance but maybe one day someone might have a heart attack.
 

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The increase in output voltage has nothing to do with battery sensing or ECU control. It has to do with good old Ohm's Law and the reaction time of the voltage regulator. Wikipedia can explain it much better than I could...Alternator Load Dump
Great article but looking at the source of the article (Texas Instrument) and what it implies (using the lm1588 processor as a over/ under voltage protection device which has no bearing on our conversation here) doesn't convince me of the generalities of its sourced materials what alternator was used, it's regulator etc.

I still would have no issue on disconnecting positive post from the battery to determine whether the alternator is working or not. ?

I will refrain from further conversation from this thread to keep the peace and keep the thread on topic.
Thanks for the response guys. ??
 

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The term is self inductance.
No, that's the a totally different physicist. The voltage spike isn't an induced voltage (Faraday's Law) it's caused by the significant increase in the circuit resistance when the battery is suddenly disconnected (Ohm's Law). At lease that's the way it was explained to me at AUTOSPARK school :)

Great article but looking at the source of the article
What happens when you disconnect the battery happens because of the laws of physics. AFAIK, Texas Instruments didn't write those.
 
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