A Tale of Two Batteries
This is concerning two different 6-volt lantern batteries that have behaved the same exact way (I believe.) Here is a datasheet for the type of battery, a Rayovac # 941 General Purpose Carbon Zinc , but I have seen this pattern in alkalines as well.
Discharge has been with a constant current circuit running a white LED at 1mA, starting with a full battery until the LED went out, so from 6V down to about 3V. (Not sure how many months this took.) Then I did it again with another battery of the same kind.
I expected the cells to have evenly discharged, meaning that we went from 6 volts down to 3 volts, so 3/4 = 0.75 volts is what I expected each cell to be showing.
I opened up each battery and was surprised to see that the discharge was highly asymmetric, leaving two cells at zero, and the other two cells mostly charged. What is the explanation for this? Does this effect have a name? The most surprising thing was when almost the same exact thing happened with a second battery.
These are the questions that I have looked at before posting this:
Found somewhat relevant:
- Why do batteries in consumer electronics get used unevenly?
- Batteries in series discharging unevenly
- Battery Pack Management: need to balance during discharge?
- Inequal discharge across battery cell
- Odd failure mode for AAA battery
Not sure if relevant:
- Why is it only one of four batteries (Li Ion) is drained?
- How to maintain equal discharging on batteries
- Why is only one out of 2 batteries is discharging when connected in series?
Building a New Battery
As a result of this, which ends up being fortunate, I took the 4 good cells from the two batteries and made up another battery to run the LED for another few months. This has now been running for about a day now.
The constant current measures at 1.039mA
During the 1.039mA constant current discharge, the cells measure:
- 1.541 V
- 1.540 V
- 1.367 V
- 1.373 V
- 5.820 V - Measured in series
- 5.821 V - Mathematically summed via calculation
You can see that two of the cells were actually charged. This alone to me indicates that something out of the ordinary is occurring.
Here is the model of the circuit that I am using: (I use the pulse voltage source to show how the circuit responds to voltage change)
It should not matter, but:
- R1 measures 097.2 K-ohm.
- R2 measures 975.0 K-ohm.
How many ways can current pass through a cell?
These are the possibilities in my mind:
- Normal (chemical reaction)
- Semiconductor conduction (like a bipolar transistor)
- Static electricity (enhanced, through electrolyte)
This is pure imagination and speculation on my part, but based upon what I see as strong evidence of something happening which I don't understand, but seems to me to be obviously present. Whatever is happening is happening inside the battery, along side what we normally experience when a cell discharges. Because it can't be seen, it's easy to speculate it away to non-existence with excuses.
(On a possibly related note, I just took a bunch of corroded cells out of a pack of alkaline batteries that were all new, not hooked up to anything, and yet corroded. Why? The dead cells I removed were also corroded. I have no idea if the corrosion is related to the uneven discharge in the two cases I observed, but I don't think it started out that way.)
If the current passing is low enough, my hypothesis is that for the stronger cells, the current passes through the cell causing normal cell chemical reactions, whatever that is (this effect seems to happen with carbon-zinc, alkaline, and lithium-ion, perhaps lead-acid). For the weaker cells, my hypothesis (continuing on with the same proposed example) is that the current gets through the cell by another mechanism whereby the chemical reaction is suppressed, but the electrolyte is conducting the current without the normal chemical reaction taking place. In other words, conduction occurs through all cells equally (for a 4s-1p for instance) but the chemical reactions occur preferentially, happening in the strongest cells, and suppressed in the weaker cells. What makes a cell stronger or weaker is the result of a wide variety of factors.
Does anybody know of any existing (or speculated) models of this?
How can any of this be proven or disproven?
You never know what the practical implications for a new understanding might be. We can certainly use better batteries. And I for one would appreciate my devices not being destroyed by corrosion. For either of those to be affected is a long shot, but you never know.
My new direction
I have decided not to use the cells as a night light, but rather to discharge them and record how much life each one of them have remaining at this point.
The reason this is important is because of speculation in one of the due-diligence links that even though the voltage shows as being high, there's really not much life left in the cells. So I want to measure the actual life left to prove whatever is the truth of the situation.
I will report back with the results. If you have a better idea, or any information to help, I would appreciate it if you would please let me know.
I know it's a lot of information, and I probably asked a lot of questions, but I am really only asking the one question:
How do I prove the existence of a novel conduction path in weaker battery cells?
I have taken numerous Li-ion battery packs apart, and they seem to exhibit a similar pathology. That is why I think that this is a multi-chemistry effect.
The best answer so far says "The voltage alone doesn’t tell the whole story" but the voltages that I reported were under 1mA constant current load.
To strengthen my argument, one multimeter that I have has a battery tester built in, and reports the mA resulting from what must be a 360 ohm resistor inside. For a 1.5mA cell 4.0mA represents fully-charged, and for a 9V battery, 25mA represents fully charged, according to the face plate label. I report the voltage for all the cells under this load:
With the 360-ohm resistor, the cells measure:
- 4.2mA at 1.491V
- 4.2mA at 1.488V
- 3.7mA at 1.328V
- 3.7mA at 1.340V
And altogether 15.6mA at 5.574V.
This should furnish a decent voltage under load. Also, it should be apparent that two of the cells were actually charged instead of discharged (or at the very least did not discharge), and the other two are close to fully charged. (These are the 4 good cells out of the 8 total cells).
I'm working on discharging the cells at a higher rate, to prove how many mAh they have. This may take a week or so.
Do our models have to change?
I don't think I've ever heard of a battery being discharged to fifty percent the original voltage, and have two completely dead cells, and two completely charged cells. Would somebody with the "tomes" about this, and the knowledge about this, tell me -- is this to be expected, or is this a new discovery? In particular, I want to know if our model(s) have to change.
I just found a multimeter after having misplaced it a while ago, and the 9v battery was dead. I just opened up the battery (copper-top) and it has 6 x AAAA cells in it, and there is no corrosion visible, but one cell is dead, and the others are 2.9, 3.3, 3.3, 3.4, and 3.7 mA (out of 4.0mA). One was zero, the others were 75% to 92% (tested with a 360 ohm resistor, 4mA = full). This is additional evidence that a leakage-level discharge causes highly asymmetric discharge.
My new hypothesis is this: Like the base-emitter current enables a larger current collector-to-emitter in a bipolar transistor, so a very small leakage current allows the cells to rebalance themselves, and that to be the primary dynamic.
I encourage you... Find an old device, long forgotten, with a dead 9V battery in it, and open up your own slowly-discharged 9V to see what I'm talking about. I'm sure this is widespread and not rare. But it is not widely known or widely understood. Why does it do this? So...
The title for this question used to be "How to prove novel conduction path in weaker battery cells in two 4s-1p carbon zinc batteries discharged at 1mA from 6V to 3V?".