Does a low-enough leakage-level battery discharge cause highly asymmetric cell discharge?

Question

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.

The Surprise

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.

Due Diligence

These are the questions that I have looked at before posting this:

Found somewhat relevant:

1. Why do batteries in consumer electronics get used unevenly?
2. Batteries in series discharging unevenly
3. Battery Pack Management: need to balance during discharge?
4. Inequal discharge across battery cell
5. Odd failure mode for AAA battery

Not sure if relevant:

1. Why is it only one of four batteries (Li Ion) is drained?
2. How to maintain equal discharging on batteries
3. 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.

Measurements

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

Verification

• 5.820 V - Measured in series
• 5.821 V - Mathematically summed via calculation

Observations

You can see that two of the cells were actually charged. This alone to me indicates that something out of the ordinary is occurring.

Circuit Details

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:

1. Normal (chemical reaction)
2. Semiconductor conduction (like a bipolar transistor)
3. 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.

Models?

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.

Finally

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.

---

Edit 1:

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.

---

Edit 2:

New evidence

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?".

Answer 1

Thanks for all the useful information to tell me your assumptions, predictions and measurements. Many students can learn from you.

There is no novel conduction method but the model may be new to you. It is still an R1-C2//R2-C2 simplified model.

You may have heard how difficult it is to have electrolytic caps with tolerances < 5%. Batteries are about a thousand times the C-value in capacitance in size as an e-cap. Tolerance deviations are no easier even with tight material and process controls but better in the same batch.

The simple battery model should clear up all your assumptions and explain why matching is so critical. It is rare that any set of cells will all decay or age at the same rate. There is a quadratic effect where the weakest cell drains faster and ages more when under or over voltage in rechargeables. The C and also effective series resistance both affect the short circuit current and discharge times. The ESR creates self-heating which improves the capacity from expansion and gap shrinkage and also accelerates the aging in most types. The greater the differences, the more rapid the differences expand. So that at end of life the weakest will be more than dead while the better ones are much less discharged due to effective capacitance.

Your load tends to be far greater than ESR so the lifespan of batteries depends on usually a 15% voltage range discharge rather than an RC=T 63% asymptotic mostly linear decay towards the target voltage of a capacitor after which it becomes exponential.

Read here for more details. ( sorry longish rambling, not edited yet)

TLDR;

This will show the memory model for a battery with 2 or more caps in parallel. How determinate parameters for thermal-model battery (Advanced question about battery modelling)

Answer 2

I think you're overthinking the problem.

You started with 4 cells in series giving 6 volts (more or less). You continued until the total battery voltage dropped to 3 volts.

So, what would you expect? Let's say, just to put numbers on it, your 4 cells had capacities of 1.1 A-hr, 1.1 A-hr, 1.05 A-hr, and 1.0 A-hr.

You discharged at 1 mA. After 1000 hrs, the weakest cell was discharged and effectively became inoperative. The battery voltage was now 4.5 volts

After another 50 hours, the second cell was discharged and became inoperative. The battery voltage now dropped to 3 volts, the LED went out, and you stopped the run.

Now, when you look at the cells, two show about 1.5 and the other two show about 1.4. This is not a voltage under load. If you had checked the cells when the LED went out but was still drawing current, I expect you'd have seen something like 1.5, 1.5, 0 and 0. The 1.4 volt levels show what happens when a discharged cell is allowed to relax with no current being drawn. For instance, if you try loading each cell with 1.5 kohm, you'll probably see the two good cells providing 1.5 volts, while the other two show much lower voltages.

Don't let the behavior of unloaded cells fool you about how they behave under load.

Answer 3

You are talking about corrosion.

This implies moisture. It may be from the environment and/or from the battery electrolyte.

The moisture can create parasitic current paths inside the battery.

The most probable parasitic current path (giving the assembly geometry) is between the middle point and one of the terminals.

There are a lot of factors (both random and deterministic) that may make one of the parasitic currents much more than the other. E.g. the positive terminal builds up oxide layer, it soaks up and retains more moisture. The electrolyte has different capilary behavior depending on the potential, etc, etc...

In short, the battery self-discharges unequally between its elements. The process may as well have started long before you installed the battery and may have unequally depleted the elements.

Answer 4

Carbon-zinc batteries are a very old technology that are cheap with poor performance and no guarantee. In China they are called Super Heavy Duty.

Energizer brand alkaline batteries are made in USA and have a 10 years shelf life and are guaranteed not to leak even if they have been fully used for 2 years. They have excellent performance.

I have some Copper top Duracell batteries leaking, rusting and dead in in a short time. Some Duracell batteries are made in China and have very poor specs.

I do not use Rayovac batteries.