Alternator running unconnected.

[ongolo]

Running an alternator open circuit may or may not cause problems. As has been said a residual magnetism in the laminates (not the coils themselvs) could result in a voltage being generated.

If the diodes get blown, then not from any current since there is no load, but from an overvoltage developing with no load and no curent flowing, resulting in a braekdown of the P-N junction of the diode.

All diodes have a certain operationg range after which the reverse voltage (in an alternator the current pulses forward and backward) will destroy the diode and be either short circuit or more often open circuit.

Residual magnetism is in fact required when you run an induction motor as a generator. Yes almost any electric motor can be used as a generator!! but that requires the addition of a certain size capacitor , size depending an the expected load and required phase shift.

I should have said the following earlier. A different picture exist when the alternator is connected and running, the field coils are excited and then you open the lead charging the battery, depending on the revs, the voltage could go sky high and the diodes will break down in every case.

I suppose, one could fit 600 Volt diodes and would then be safe, but I am not sure of this one. I have been considering for sometime to mount a diode board outside . I dont think there is a reason other than compactness to have them inside the alternator. Maybe one day.


happy new year

ongolo

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[MainlySteam]

If you are responding to me Andy, if you read my posts you will see that I am not talking about flat batteries but batteries which have reached the end of their lifetimes when little active material exists in the plates ie the active area of the plates is small because most of the plate material has been permanently converted. I do agree with Alex however, that under charge a "flat" battery has a low internal resistance (otherwise the charge cycle we observe would be the other way around ie max current would flow as the battery reached peak charge, which is, of course, not what happens.)

I am not talking about batteries in reasonable condition where the active plate area is still large. The relevance is to the original question in the thread is that when there is no active plate material (or very little) at the complete end of a battery's life and if that means that the internal resistance of the battery is very high (I am not saying it is, I am only asking) then alternators can be run on an engine that is being tested with no battery connected (ie against an open circuit) with no problem because they are expected in normal applications to be able to run against an expired end of life battery.

To take another approach to what I am saying is, if one tipped all the electrolyte out of a battery which was connected to an alternator (or boiled it all out, or it leaked out through some fault) so that the internal resistance of the battery was essentially infinite and one then ran the alternator into it, would the alternator be damaged. Good sense would indicate to me that it would not otherwise this would be a common killer of alternators in cars. Again, I am not saying this is so, I am only seeking clarification from someone who knows.

John



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[ongolo]

Hi John,

You are quite right, but somebody uses a little weird logic on the example given.

A flat or empty battery has certainly low internal resistance while excepting a charge. And a fully charged battery has low internal resitance while being discharged. Low resistance means nothing more but able to handle large currents which ever way, either in or out.

When a battery dies, it is either slow sulfation of all cells, or one cell goes dis or is extremely sulfated. Then such a sick battery is definately high internal resistance, it will simply not except charge anymore.

On the other hand, a fully charged battery is low resistance when discharging which is normally accepted, but nothing prevents you from charging more into the already fully charged battery. It has still low internal resistance nohting changes to high resistance, you will just boil it to death.

I hope I have gotten all the terms correct, I am not english speaking.

Hope this helps, I always enjoy your post John.

regards ongolo

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[ongolo]

Hi John,

Me again, when a battery is dying or dead and does not accept charge anymore under normal conditions of a bout 14 Volt, cranking the voltage up to say 40 Volt will result of some current flow into or through the battery, without the battery being charged. It is no more than a resistor and this resistor would prevent the voltage going excessive and prevent damage.

Having absolutely no electrolyte in a battery would be open circuit. After all, there must be a reason why diodes sometimes die in normal operation.

regards ongolo

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[MainlySteam]

Thanks Winfried, that is what I imagined would be the case.

If that is so then one would have to assume that, in response to the original poster's question, there is no problem running an alternator into an open circuit as I would assume that operating an alternator into an end of life battery of very high internal resistance (ie essentially an open circuit) causes alternators no harm (it must be a reasonably common occurance for that to happen in actual service and I have not heard of that damaging alternators).

Does that sound sensible or a load of bollocks?

John

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[ongolo]

John

You are right. Another unexplaind myth is, that one should not weld on a car with the battery leads connected. I could never see a reason behind this and have welded all my life while batery leads were connected. One must just make sure that the ground and the weld is elctrically direct in good contact.

Happy new year


from 22S and 14 and a bit east

ongolo

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[halcyon]

Technically it will be ok, if the field warning light is not connected it will just spin, alternators rely on a feed to the rotor coil to start charging. Be safer though to put a tempery feed from a battery to alternator, remember the neg feed as well.
To charge the alternator takes a feed from the warning light, causing a magnetic field in the rotor, this generates a current in the stator which feeds three sets of diodes, a pos and neg set for main output, and a feild set which generate a supply to the rotor to keep charging, and a back feed to the warning light to put it out. A low battery has a low internal resistance, the alternator simplistically generates watts. As the internal resistance of the battery increases with charge, the voltage to push (the old emf) current throught rises, as there is a limit to output due to total watts available, the current will fall. This carries on till the alternator reaches it's it regulation voltage, at this point it shorts out the feed to the rotor, and thus stops output current, thus voltage at battery falls and feed to rotor is restored. Now that the voltage has reached a peak, and battery resistance carries on increasing current will fall to a very low level, thus stopping over charge. The alternator does not control amps, only volts, using the internal battery resistance to reduce amps, and prevent overcharge.

Brian



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[andyball]

I have known alternators start to charge with the warning light unconnected ( & many more that didn't, as you describe)....all "ordinary" types with the usual charge indicator & diode trio set-up.

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[andyball]

Fitting diodes outside the case

Electrodyne (US firm) will do just this if you ask , or so a newsgroup message goes : the poster reckoned that after years of hassle with blowing diodes on his boat (huge battery bank, huge alternator,better with electrodyne than any others, but still sometimes troublesome) he got them to make one with a remote fan-cooled diode box (more room in the alternator for windings too, he suggested) & has had no trouble since.

I presume that a cheap/easy source of cooling air from the fan also makes most alternator manufacturers choose the internal route.

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[brianrunyard]

You're going to have a battery connected to start the engine, so why not connect the output terminal to the battery positive to be on the safe side, as you don't require it to charge no need to connect the IND terminal.
This way you won't have to worry if the alternator starts charging due to residual magnetism.

<hr width=100% size=1>Brian
http://homepage.ntlworld.com/brunyard

 

[ongolo]

To Halicon or Brian

You like so many others go about the internal resistance of a battery and you are confusing mainly steam who asked how things really work when charging a battery.

To present a picture of "increasing internal resistance" is completely wrong.

According to you the charging current increases until 14 Volts are reached and then the internal resistance prevents any further current flow.

This is how it works in principle the exact values depending on battery and alternator size:

If you have an empty battery that measures 11V and apply 14V you will charge with 30A, when the Voltage reaches 12V you will cahrge with 20A at 13V with 10A and when reaching 14V zero Amps. This would indicate that we now have a battery with infinite resistance and any higher voltage no longer causes a current to flow. If this were so, we would not need regulators.

The pitfall most people do not understand is first of all you dont apply 14 Volt to the battery but 14V - 11V = 3V to start with since the battery already has 11V by itself. Now when you reach 14 V the internal resistance has not changed, but since the battery voltage is now equal to the 14V charging voltage, and since there is no longer a potential difference current stops flowing.

If you no apply 14V battery voltage at this stage and add 3V more you again will have a current of 30A flowing because the electrolyte in the battery is still conductive. However, now we no longer charge the battery, but we split water into 2 x H and 1 x O. If anybody does not believe me, try 17V on a full battery, watch the bubbles come out of the electrolyte, keep open flames away. It might go bang.

So there is no increase in resistance to speak of - only a reduction of the difference between battery voltage and applied charging voltage. And that is the true story.

Happy new year to all.


ongolo


P.S. I wish I knew as much about making money as I know of electronics. ))

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[JSB]

What happens when the battery is fully charged, does the excess energy from the alternator go to earth?
JSB

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[andyball]

Re: Normal alternator running

No: most boat & car alternators have a coil of wire with a DC current flowing through it to provide the magnetic field necessary to make it work : called the field coil, very often it's a rotor within the output coils (stator). If you take away the voltage energising this "field coil", alternator output drops to zero (or as near as makes no odds).

So, when the regulator decides (note, it depends on regulator voltage setting- not specifically that the battery is "charged"), it stops the Volts to the field, output drops away...& when it drops below a certain level.....it switches back on again......& so-on etc.

Many motorcycle & outboard alternators used rotating "permanent" magnets - often with these, the excess power goes to earth, as you thought.

Some alternators have non-rotating field coils AND output coils - the movement of magnetic field being accomplished by a rotor that has no wires in it or from it : in these, & permanent magnet alternators, there are no carbon brushes to wear out.

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[MainlySteam]

For the apparant contradiction of a charging battery having an increasing to high resistance from the point of view of the charging circuit during charge but a low resistance when charged from the point of view of its discharge, the following is it.

To the charging circuits (the alternator and the regulator) the battery is just a black box to which Ohms Law (V=IR) applies, they do not have the intellegence to say "Hey, I am looking into a battery". So following Ohms Law when the battery is under charge, what we find is that if the charge voltage is kept constant (say an internal regulator at 13.6 v) then we all know from experience that as the battery approaches full charge the charge current reduces. So if one looks at Ohms Law we have a constant voltage but a reducing current and the only way this can be is if the resistance of the black box (the battery) is increasing. The charging circuit tries to be a constant voltage supply (ie it tries to keep the voltage at 13.6 v no matter what the resistance of the black box it is looking into).

When the battery is very flat, the charging circuit may not be able to supply all the current that would normally flow because the battery's resistance is very low (eg 60 amp alternator, 13.6 v => battery resistance approx 0.2 ohm) so given a single step regulator the voltage of the charging circuit will drop. A stepped regulator simply endeavours to replicate a simple constant current source ie as the resistance of the battery increases during charge, the charging circuits voltage increases to maintain the current - it cannot be allowed to be a complete constant current source as we know that high currents at the end of charge will damage the battery.

As the battery comes up to charge, the charge current may drop to 5 amps say, in which case its resistance has increased more than a hundredfold over the 60 amp example above. It is important to realise that these resistances are still very low (about 3 ohms for 5 amp charge at 13.6v) but the fact remains that as a battery is charged, from the point of view of the charging circuit the resistance is increasing.

In the case of discharging the battery into a circuit, a light or whatever, the battery sees the resistance of the circuit it is looking into and Ohms Law again applies. Again we know batteries under discharge are able to supply very high currents and that current is only limited by the total resistance of the circuit and the battery's ability to maintain its voltage (it tries to be a constant voltage source). Again, looking at Ohms Law one can see that this must mean that the battery has a very low internal resistance, noting that the internal resistance can lowered by increasing plate area either by bigger battery or by paralleling batteries.

However, we know that as a battery discharges the current it produces into a constant load reduces, so again by Ohms Law as the battery flattens its internal resistance is increasing. As explained above, connect a charger to the flat battery and it sees things as being the other way around.

Hard to explain, but trust that may help clarify my previous use of terms.

As Ongolo says, this is all due to the changing voltage of the battery, but from a design and description point of view the above applies, whether the resistance changes when looking into a black box from its voltage changing or because there is a switch which opens and closes inside it does not matter from the external viewpoint.

John

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[claudio]

Ongolo,

You are correct in your description.

In order to visualise the process, imagine connecting two water containers together with a pipe at the base. One container is empty, the other container is full and is fed with a supply with an overflow.
The empty container will fill up quickly at first, seeking the level of water in the full container.
This is equivalent to charging a battery, the container is an energy store, it does not disipate energy like a resistance.

When the container, which was once empty is filled it is equivalent to a fully charged battery and has capacity to do work.

I hope I explained things and haven't added to the confusion on this matter.

Claudio

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[MainlySteam]

<<<This is equivalent to charging a battery, the container is an energy store, it does not disipate energy like a resistance>>>

Unfortunately the laws of physics have not changed to make this so, otherwise many of our battery problems would be over. The battery does dissipate energy like a resistance as does your tank and water experiment (consider, for example, the loss of the velocity energy [KE=0.5mv^^2] from the water when it enters the second tank and is decelerated to a stop due to friction with the water in the tank - that energy is converted to heat just like in a battery or any other resistance).

John

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[claudio]

<<<<<<<< Unfortunately the laws of physics have not changed to make this so, otherwise many of our battery problems would be over. >>>>>>


Nobody says the laws of physics have changed, I agree a battery has internal resistance, but this is NOT what reduces the charging current, it is the increase in the level of energy stored in the battery. Ongolo is correct, when the potential difference between the charger ( a regulated supply) is equal to the battery voltage, charging current will cease.

There are resistive losses in a battery as there are self discharge paths, this is why you have to put more Amp hours into the battery than what is given back out.

With a 'good' battery these losses are minimal.

Are you saying the 'black box' be it a resistance or a battery are identical ?

If this is so, why doesn't a lamp illuminate when connected to a resistance (resistor / rheostat) ?

Why can't a pure resistance store energy ?

How can a battery 'decide for itself' when to change from a high resistance (when fully charged) to an extremely low resistance (when the charger is removed and a load connected)

This is not physics, it's 'magic'.

Any elementary electronics book or course will describe 'potential difference' and a battery being an 'energy store'

May I suggest that anybody who wants to know about this subject, buy or borrow a book and get the facts.

Claudio

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[MainlySteam]

<<<Are you saying the 'black box' be it a resistance or a battery are identical >>>

Exactly!

Who analyses the internal workings of any electronic/electrical component when professionally designing something? No one. All you regard it as is a black box with knowledge of its interface. For example, if you design a charging circuit do you dismember the external regulator down into internal details. No, all you are interested is how it reacts/interfaces to the outside world (the alternator and battery). A battery reacts as a black box having changing resistance and voltage on its terminals.

Sounds like you think you are talking to an amateur on this. So happy for you to ignore what other pay me to tell them, my last post, moving on, have fun.

John

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[ongolo]

John,

This is getting ridiculous, at first you posted that you dont know much about the subject and now you get paid for knowing this.

A battery is NOT a resistance, the electrical process is actually not only a function of the laws of physics, but also an electro-chemical problem which converst chemical energy into electrical energy by simply adding acid to a new battery.

Do you know of any other "resistance" that has such properties?

And the losses you mentioned from the friction of the water etc is farfetched hair splitting like saying you cannot walk uphill because the air resistance at walking speed is too much.

I think, there are some people here who know exactly what is going on and there is just as much misconception as it is about the speed of the electrical current which most people get to believe is equal to the speed of light, but a better understanding reveals the an elctron moves only at about 1/ms.

regards ongolo

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[claudio]

John,

Your model of a battery being like a resistance only exists when the charging voltage is equal to the battery terminal voltage.

A model that exists only for one set of pre-determined conditions is nonsense.

Back to the water example it's like saying that water will always find a level of 1m because the containers are 1m tall.

I've been in electronics for over 30 years and what your saying is new to me. Any 15 year old taking an electronic GCSE course will be aware of 'potential difference' and 'energy store'

I'm sorry if I sound like I'm talking to an amateur, I have read your other threads and they have provided much informed information.

I think that what you are saying in this thread is really confusing and misleading to some of the other readers.

Can you give me an explanation to my questions in my previous email ?



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