The charger voltage needs to be higher than the battery for current to flow from the charger to the battery. Otherwise the reverse will happen.

How much higher determines the amount of current that flows. Divide the voltage differential by the resistance of the wires+internal impedance of the batteries and you get the current that flows.

The current flow is regulated by charger usually so the above sets the maximum it could ever be, not actual.

The charge voltage also determines how high the battery voltage can climb. If set too high relative to battery chemistry, it can permanently damage it. Smart monitoring of the charge voltage helps avoid this problem.

While there are useful information there, the person who wrote it does not have proper experience in the field. He runs a company that makes battery monitoring hardware which he promotes in there from time to time. Everything he says is stuff he has learned which unfortunately in some cases is wrong (especially in the case of Lithium cells).

All that you wrote is true and in fact it's stuff I know. The original question is about a subtle detail.


My question comes down to this: how much higher must the charger voltage be for current to flow? Everybody looks at this and says exactly what you just said, but everybody just skips right over what happens between no current flow and some current flow.


Since this is a physics question, we get to imagine perfect parts. I want to know the voltage difference that, with zero theoretical resistance, is required to force the electron to flow. It's not possible that it will flow with no force on it, and force in electricity is represented by voltage.

Now, to what I was asking about: The idea of connecting two batteries, plus to plus and minus to minus, introduces something that doesn't happen with a charger: As the battery with more charge (A) transfers charge to the other battery (B), A goes down in voltage while B increases in voltage.

The question was, will the two batteries come to exact equilibrium, or is there some minimum value of voltage differential that must exist for that last electron to go from A to B? This would leave A with a slightly higher voltage than B. It could be millivolts; it could be nothing. I've not seen a definitive answer to that.

Consider a water metaphor. You've got a dish filled with water. There's a small section of the dish that's lower than the rest of it, and that section is dead level. What happens as we slowly add water to the dish?

The water level comes up to the level of that section, and then rises above it, forming a meniscus, until the force from the resulting head of water is large enough to overcome the surface tension of the water. Water then spills out of the dish. Incidentally, if you stop filling the dish at this point, the water will flow until it's below the height at which it started flowing, but a smaller meniscus will remain.

I'm asking in this battery question if there's something similar going on with the batteries as I've posed the question. Is there some teeny tiny amount of voltage that A must be higher than B for electrons to flow? If not, why not?

Another water metaphor: We construct a bucket shaped so that when you pour water into it, when there's 100 pounds of water, it tips, spilling out the water; the container is shaped so that when it's empty, it returns to its upright position.

Charging a battery is like tipping this bucket over and over, electron by electron, until the battery is charged. Is is not possible (with the double battery connection) that as charge is transferred, the end position has some water in the bucket, maybe even 95 pounds? The last electron has been pushed into B, but A still has the tiniest bit of voltage differential... at this point it gets foggy even for me.

I told you it was a picky question!

I gave that answer :).

Here is more detail. For current to flow, the voltage as seen by the target battery needs to be higher than its voltage. Once that happens, the current will flow. It is as simple as that. It has to be otherwise the most basics rules of electricity flow are violated.


That force is simply the voltage differential. Take that away and there is no loop for electricity to complete and flow stops.


If you leave them together long enough, they will achieve equilibrium (no electron transfer). Note however that the "tail" of this equilibrium may be very, very long. It may take weeks or months to get there. But eventually it will as the two cell voltages equalize.

As they sit and age, that flow can occur again in one or the other direction.


For B to increase its voltage, you need to overcome its internal losses (i.e. heat generated due to internal impedance/chemistry). Either way though, electrons will travel from A to B. It is just that at very small differentials, there is not enough to overcome the losses and result in the charge of B. A will deplete its charge and hence voltage and eventually the electron flow will stop.

In general, the micro-behavior of batteries is extremely complex and actually beyond science understanding! It may be shocking but for example, we don't really know or can predict the exact way Lithium batteries charge and discharge.

At macro level we can simplify things and that works 99% of time. But answering things like dynamic impedance of a battery is amazingly complex chemical process that depends on so many factors from temp to exact condition of material and chemistry. The water analogy by contrast is dead simple.

At macro level, cell to cell equalization happens all the time in battery banks and from personal experience, the cells will equalize over a few weeks. But then as soon as you draw power from them, they lose that and the process repeats again.

Erie,

We think of a body of water as being flat, but it is not. Gravity is also in the dynamic and the surface on a large body of water follows the gravity field (which might not be uniform).

Again on our body of water, Brownian Motion will sometimes throw a water molecule toward the surface, breaking surface tension, and be lost. Conversely, a nearby water molecule may rejoin the pool. And, at a high temperature (boiling) all of the molecules will want to leave the pool. We can look at this on a more precise level by examining "partial pressure", sometimes equated with "vapor pressure". If you place a liquid or solid substance in a sealed, evacuated space, this evacuated space will eventually become filled with evaporated substance at a pressure such that there is equilibrium between departing molecules and arriving molecules. This is the vapor pressure of that substance at a specific temperature. When a liquid boils, its partial pressure equals the surrounding atmospheric pressure and the molecules leave the liquid state.

In our battery, there will be similar thresholds. I've never seen any charts giving the minimum voltage differential below which one cannot strip or force an electron into the battery chemistry. You could crudely measure this with a battery, variable voltage source, and an ammeter.

In the charging schemes that I've seen, charging is regulated by current control. There is a maximum safe voltage to be applied in order to avoid secondary breakdown mechanisms. Cell voltage can be used to sense the endpoint of the charge.

Out of curiosity, what prompted this discussion?

You know, anyone could have ended this interminable attempt to get an answer by saying this to begin with:


If you don't know really know that, how can you know that batteries connected as I describe will come to an equilibrium of ZERO volts difference?

Anyway, back to the slog:


No, you didn't. I asked HOW MUCH VOLTAGE. Your response is "more." I agree but it's not the entire answer.


HOW MUCH? 1 millivolt? one microvolt? 2.73 microvolts? .04 femtovolts? 42? And... what about the situation causes it to be this value? Avogadro's number is a number that defines something... what's the number that defines the minimum amount that a supply must exceed the battery voltage for current to flow? It can't be zero, so it has to be something. How much is it?


Agreed, except for things at, perhaps, a quantum level (I don't know if that's the right idea to invoke). Perhaps the "voltage" caused by one excess electron is what's required to make that last push to equilibrium. (What if there's an odd number of electrons, so that last electron charge has nothing to push?) Perhaps less than one electron's charge might be present as a voltage, meaning it would be possible to not push any more electrons, yet not be at equilibrium.


Okay, let's try this. Take away half that voltage differential -- there will be flow. Then take away half of that -- there will be flow. Then continue to do this a thousand times. This seems to lead to a condition where a femtoscopic (much smaller than microscopic) voltage differential is present, yet no electrons move.

It simply seems impossible for us to say that a voltage infinitesimally larger than zero will cause current to flow, but no current will flow with zero difference. We seem to be inept at calculations involving infinity or the inverse of infinity.


That's a revealing sentence. You're stating that equilibrium is no electron transfer but in the discussions I've had, including this one, people in general have been stating that equilibrium is exactly equal voltages. I'm saying it's possible for the two things to be different and I'm asking by how much might it have to be before current flows.


Time is not an issue in this search.

Upon what do you base your conclusion that there will be zero voltage difference between the two cells? The fact that the question seems not to have been asked, and I've never heard it as a topic, doesn't mean you can ignore it.


Yet nobody is willing to say "we don't know."


And you have experience or peer-reviewed reportage that this equilibrium is ZERO volts difference, not femtovolts?


That makes sense because this is indeed the macro level compared to what I'm asking about.

We don't know what happens at quantum/chemistry level at all times why a battery does what it does. Ohm's law however, doesn't give a darn about that. As long as we know the voltages at two ends, we know current flows and the direction of it. I have given you the method by which we can compute all of this.

Put a lightbulb on a battery and see it go out eventually. You can measure that time without again, knowing a darn thing about what is exactly going on from microsecond to microsecond in the battery. These are macro effects we care about as users of power.

Now if you want to design better batteries, then you sign up to know more. But even there as I mentioned, there are significant barriers to what we know and as a result, simple experimentation is made by for example introducing rare items from periodic table to see if longevity, charging ability, charge availability, voltage, etc. is improved or not.

For your part you seem to have invented an issue, a barrier to flow of electricity, which you say you don't know what it is. Yet insist on someone explaining it to you. I don't know how to explain concepts that you have conceived. I can only explain how in real life the batteries equalize each other as this is fundamental to building battery packs (two of which I have built to power my EV Quad and RV). And here, all we care about is practical aspects of how the system works. No one cares if there are femtovolt differences causing femtoamp current flow. Maybe you care about .01 degree temp in your room, but I don't. :)

I am afraid of explaining more as there seems to be this barrage of disagreement and disapproval in return. Hopefully someone else can provide a satisfactory response.

You need to look at this on a smaller level- if a difference in charge exists between two bodies and they're separated by some amount of space, it may not move unless that space is extremely small. If the bodies are conductive and in contact or in an electron cloud, it will move. If the charge is one electron, the electron may be shared at the point where the bodies make contact (or they're equally conductive and positively biased in said cloud, assuming simple attraction is the goal) but if the difference is two electrons, the body with higher charge will lose one and they will achieve equilibrium. Once they reach equilibrium, no current will flow.

Time is always involved. Nothing happens without some time passing.

Yes. Do electrons do this onto and off of wires? I'm looking for the reason for these facts here.


Yes.

I'll bet it's too tiny to measure with anything crude.


Amir used a helpful term, macro. This is all macro stuff. The stuff I'm asking about is at a micro level.


Somebody somewhere asked whether two batteries, one charged and one discharged, would come to the same voltage if connected plus to plus and minus to minus. I was wondering if there might be some difference in potential too small to move any other electrons. Of course, this would not be measurable AT ALL as long as the batteries were connected. The lower voltage would be on the wires with any possible voltage difference sort of "teeming at the gates" of the supply battery.

Arcing only happens when the voltage is sufficient to cross the distance, depending on the dielectric coefficient of the medium. To answer the question, yes, it can happen but as I understand your original question, you're not concerned with large voltages, you're asking about current at a very small level, which is how I have been addressing my comments.

If two batteries are connected and one has been depleted, they shouldn't be lead-acid because the byproduct of repeated or rapid discharge prevent the plates from recharging as well as virgin lead and acid. To your question about a difference that's too small, I don't think so- everything I was taught and have read says that as long as a potential difference exists, even if it's one electron, it wants to move. Take away the net charge and you'll have no current. However, if you make/break contact or move the wires/bodies, the charge can change because of Triboelectric effect. This can be heard in audio cables with a center conductor that can move inside of the dielectric when walking on or handling the cable.

If you allow the batteries to remain connected, then no, you can't easily measure current but you could connect a small resistor in series and use an extremely sensitive device to measure the voltage drop across the resistor. However, at the level of charge in the one or two electron range, it's unlikely that we have anything outside of a major lab to check this.

The lower area in the dish is lower because of a theoretical barrier, right? You could level it by removing this barrier- it would do this automatically, but it's because of gravity, not mutual attraction.

Using the convex meniscus as an analog of voltage, you would need more pressure to force the water over the edge than it would take for electrons to flow.

Here it is-

[Link: en.wikipedia.org]

Use the formula in the Coulomb's Law section.

At the micro level there will be a certain potential difference required before electrons can be ripped off or forced into molecules. Don't push this analogy too far, but you could think along the lines of the forward drop across a semiconductor junction. There is also Brownian Movement that will throw electrons across even when the potential is slightly below the threshold and there will be a bias to migrate from the high charge battery to low.

You'll also have some secondary effects that will play out over time. After the batteries are separated, the voltages will drift. You can see this with a single cell and a charger. Immediately after terminating the charge cycle the measured battery voltage will be higher than the voltage measured an hour from now. This is why some simple charging schemes might under charge a cell by misinterpreting this high voltage as the "done" signal. Even with a charged capacitor you can short the capacitor and measure zero volts immediately after the short is removed, but sometime later a voltage will appear. The is due to "dielectric absorption" and a reason why certain capacitors are a bad choice for use in high speed circuits.

On the macro level I think that you are safe to say that identical batteries in identical condition, differing only by the initial charge level before the head to head connection is established will tend to equalize over time. Realize that simple problems, such as a temperature gradient in the room, will tinker with the final results.

Who the hell brought up arcing? I'm asking about something that happens with a connected connection. I can't see how anything having to do with arcing can address the question.


Now that you've brought up arcing, which is outside the range of what I was asking about, I have to ask you to state the question that you think you're answering in this paragraph.


Sigh.
Since you've referred me to an article named "Coulomb's Law," I have to ask which section is the section you refer to as the Coulomb's Law section.

And since the question I proposed has to do with connections made via wire, I have to ask you how attraction and repulsion in free space relate to the question of whether two batteries connected plus to plus and minus to minus, that is, not a free space condition, when they come to "equilbrium," are at an "equilibrium" meaning no current flows but there might be the teensiest difference in voltage, or an "equilibrium" meaning they are at exactly the same voltage.

I totally agree that this might only be measurable by lab equipment.

Speaking of which, did I ever tell about the physics researcher who connected a galvanometer to a test device using several short pieces of telephone wire draped across equipment? He got no results! So he measured the galvanometer with an ohmmeter and discovered the galvanometer was open. So he checked out another, then another, galvanometer, and found every one of them was open!

(Soon but too late he learned that the amount of current coming out of an ohmmeter can blow a galvanometer.)

"connected connection", eh? A bit repetitively redundant I think, although at a microscopic level, is the contact area clean enough to truly be a perfect connection?

You asked "Do electrons do this onto and off of wires?"- what would you call that, if not 'arcing'? In open air or at a microscopic level, electrons leaving one conductor and moving toward another is still called 'arcing' if it happens because of electrical charge.



Did you even scroll down in the link" There's a section titled 'The Law'. It might be in there, but you'll have to decide.


You're telling me that I'm wrong but you don't seem to understand that what I/we gave you answers the question. You wrote "at an "equilibrium" meaning no current flows but there might be the teensiest difference in voltage"- by definition, that can't happen if they are truly in equilibrium.

If you had mentioned the type of Ohmmeter, it would have been helpful. If the researcher used something like an old Simpson analog meter, it shows that he clearly didn't know that it can illuminate an incandescent bulb while the leads are connected to it.

I'll assume you included this wonderful anecdote because I wrote that it would be difficult to measure the tiny charge with common test equipment- I didn't think anyone would want to bother with that but it would be a good way to go about it.

You still haven't shown the specific application for your question. Writing that you want to know what happens when two batteries are connected and about the point where no electrons move isn't specific enough, really- it doesn't include the reason for wanting to know this. If you're trying to develop some kind of product to maintain battery charge level or something else and you want to patent whatever it is that you're doing, I understand the vagueness but this info is out there- you just need to know how to ask the questions that will yield the answers you NEED, which we have already given. The problem is that we apparently haven't given the answers you want.

I placed that right after amir's description of Brownian motion, during which some molecules of water (in his example) fly off of the surface and others join the water. I was asking if electrons do this, that is, some flying off of wires into the air while others come from the air onto the wires. This is not arcing as arcing is a phenomenon that occurs when one makes and breaks wire connections. There IS a parallel, which is corona discharge, but you won't get that from a couple of batteries connected plus to plus... once again I'm trying to bring the discussion back to the original AND ONLY situation I wanted to discuss.


If it's possible for current flow to stop because the difference in voltage between the two bodies is too small, even at the molecular level, to make any more current flow, then they could be at equilibrium as seen from ten feet away while not at equilibrium at the molecular level. Both cases might be called "truly in equilibrium" while different from one another.


If you take an electronics class, you will be finding out things while having no specific application in mind. Lack of a specific application does not mean the information is not worth gathering, nor the journey not worth taking. One of my favorite books is Feynman's "The Joy Of Finding Things Out." THAT is my specific application -- to find things out. If I ever use the information, cool, and I'd be ahead of other people who never thought to ask.


Sure it does. I want to know. If you want to say that I didn't give the reason that I want to understand, then okay, but I'm not going to go build something when we get this worked out. Sorry if that's not practical enough!

I want the answers that describe the situation. People have said that electron flow stops; that the batteries are at equilibrium; that things equalize; but nobody has said yes or no as to whether there's some tiny voltage differential required to get the electrons started.

I don't want to confuse these batteries with diodes, but when you start in electronics you learn (A) that diodes only conduct in one direction. Later you learn that (B) when they conduct, there's a forward voltage drop across them. I'd say with regard to the battery problem, we are all insistent that we know what happens and we're talking about an (A) level of things, while I'm wondering if there's a (B) level of things, much more subtle, that we simply don't bother to ask about.