On April 8, 2017 at 14:19, Ernie Gilman said...
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.
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.
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.
That force is simply the voltage differential. Take that away and there is no loop for electricity to complete and flow stops.
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.
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.
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?
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.
I told you it was a picky question!
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.
