LEAFspy says Bad Cells?! :(

[Mottyski82]

Long time lurker, first time creating a new topic/post.
I allowed my LEAF traction battery pack get super low (never have driven it to such a low a state of charge). I had used LEAFspy (not lite or pro) for the trip which got me down to 10 GIDs (3.6% GIDs). LEAFspy tells me .8 kWh remain when I had parked. The Point of this topic is that in the voltage histogram section, it showed two red lines, (said Bad Cells... #19, #40)!! Usually cell pair voltages register 9-14 mV diff.) Now, 109mV cell diff. Min: 3.428, Average: 3.503, Max: 3.537.

Batt stats: AHr=65.12, SOH=99%, Hx=99.48, 336.31V

I would post a screenshot or two if I knew how to do so, but alas... I know not how.

Has anyone EVer seen this note in red at the bottom of the Batt stats screen saying Bad Cells? If you have, what does it mean? Do you think these cells; #19 #40 could be replaced under warranty? Yikes!

I'm not fretting about it just yet. I LOVE my LEAF (EV)! Thanks in advance for the comments!!

 

[mctom987]

The red lines are shunts, not bad cells. All the shunt does is stop flow through that cell. Usually it's done because the cell is discharging faster than the others. If you continually see specific cells with an active shunt, it could point to an issue with that module, and it may need to be replaced.

That said, I have a screen shot with 94 active shunts.

 

[Nubo]

voltage differences between cells will be much more pronounced when they are nearly fully drained.

 

[mctom987]

voltage differences between cells will be much more pronounced when they are nearly fully drained. Charge to 100% and give it a few hours plugged in after that, to allow some cell balancing to take place.

Odd, I recalled having a rather high SOC. Looked up my screenshot, and it was 90.7%.

Edit: Image instead of text

 

[gbarry42]

If those cells are bad, you will also see a high differential while being charged, and the bad cells will show a higher voltage than any of the others. They'll be the first to reach full charge. You have to check while the charge is in progress.

 

[DaveEV]

Odd, I recalled having a rather high SOC. Looked up my screenshot, and it was 90.7%.

Yeah, seeing nearly all shunts on after a 100% charge is normal. a 16 mV spread between min/max is pretty typical, too when fully charged.

When you can only balance a pack by discharging cells, if you want to bring the voltage up of 1-2 cells that are low, that is your only option.

BTW, if you are wondering about the relatively low SOC (90.7%) despite having just charged to "100%", I saw the same thing recently after QCd the previous day - my SOC registered only 90.1% right after a 100% charge completed.

Back to the OPs question - IIRC LeafSpy can apply the formula for detecting cells that are low enough voltage compared to the rest that may qualify for replacement. This is only possible at low SOC. But that said, a 109 mV diff at 10 GIDs actually sounds pretty good. Last time I got my battery that low (3.9%) my diff was 333 mV. Even at 7.0% it was 222 mV.

 

[Mottyski82]

Thanks all for the great feedback so far! As it happens, I was expecting to QC last night @ the local Nissan dealership, only to find out that the thermostat filter was clogged and the CHADeMO was not operating because of the thermostat error. So anyway, cord to the level 2 which is a couple of parking spaces away was long enough to reach to the QC space, and so I ended up being able to balance the pack with a full charge... So maybe I was better off allowing for the 100% charge overnight.
I did come back to the car at about 63% SOC and took a reading with LEAFspy, and then again of course @ 8am when the cells had been balanced. It seems that the "Bad Cells" were probably just at the point where the battery management stuff shut down those cells to prevent over-discharge, because when I took the readings while charging and fully charged, the #19 and #40 cells did not seem to be much higher voltage than the others. During the charge check, the voltages of all the cells were dancing around like ripples on water, no cells were (REALLY) spiking high, and others seemed very similar to the ones in question. I will keep an eye on those warnings though, and probably track which cells are registering as "Bad Cells" on LEAFspy. If the fellow who created/produced LEAFspy is reading this, I sure would like to know his take on what "Bad Cells" is supposed to mean / what that was for in his mind...

Thanks again everyone!

 

[mctom987]

No problem. Thanks for making the font big and easy to read.

 

[TimLee]

... The Point of this topic is that in the voltage histogram section, it showed two red lines, (said Bad Cells... #19, #40)!! Usually cell pair voltages register 9-14 mV diff.) Now, 109mV cell diff. Min: 3.428, Average: 3.503, Max: 3.537.

Batt stats: AHr=65.12, SOH=99%, Hx=99.48, 336.31V
...
Has anyone EVer seen this note in red at the bottom of the Batt stats screen saying Bad Cells? If you have, what does it mean? Do you think these cells; #19 #40 could be replaced under warranty? Yikes!
...

Yes.
Jim had asked in the main thread for people to report if bad cells were detected.
Test is the Cell Voltage Loss Inspection (CVLI).
See http://www.mynissanleaf.com/viewtopic.php?f=44&t=12789&p=308501&hilit=cvli#p308501" onclick="window.open(this.href);return false;, pages 268 to 270 of 2011 manual.
I took time to run heater long enough to get down to voltage level couple weeks back. Took a long ways past LBW.
Had planned to report but didn't find time to review logs and graph the bad cells voltages yet.
I had eight showing bad at VLBW.
# were increasing as battery voltage went lower.
Might have been 12 to fourteen if I went to Turtle.

I'm not sure what Nissan response will be on replacing the cells. The test is for review of battery condition when fault codes are not present. And does say to replace modules containing cells that don't pass the criteria.

For me eight out of 96 is a lot. I will have to check the cell #s versus module location. But could be 6 to 8 out of the 48 modules.

 

[TimLee]

If you search MNL for CVLI there are about five pages of posts on the subject.
Some say the test is only done when there is a certain error code.
But the manual says "performed to identify cells that the cell voltage is on the decrease (although not judged as malfunction in DTC) and this may lead to a malfunction, if left stand."

 

[RegGuheert]

The Point of this topic is that in the voltage histogram section, it showed two red lines, (said Bad Cells... #19, #40)!! Usually cell pair voltages register 9-14 mV diff.) Now, 109mV cell diff. Min: 3.428, Average: 3.503, Max: 3.537.

The criteria for identifying a bad cell are as follows:

1) The lowest cell in the pack must be 3.712V or lower.
AND
2) Any cell which is below the average cell voltage by more than 150% of the difference between the maximum cell voltage and the average cell voltage is considered "bad".

In your case, the lowest cell in your pack meets both of these criteria:

1) It is at 3.428V, which is below 3.712V.
AND
2) It is also below 3.452V, which is Vavg - 1.5(Vmax - Vavg).

And, according to LeapSpy, there was one other cell which was also below 3.452V at that time.

So, should you be worried? I would say no. I think the test criteria are poorly worded, as I suspect that nearly every LEAF pack will show some failed cells by these criteria when the pack is as close to shutdown as yours was.

Instead, I think Nissan intended for this test to be conducted when the minimum cell voltage was right at or very slightly below 3.712V as the technician can get. In that case the test should give much more meaningful results than allowing it at *any* cell voltage below 3.712V..

I'm not fretting about it just yet. I LOVE my LEAF (EV)! Thanks in advance for the comments!!

If you want to be more sure, drive the car down to a minimum cell voltage of 3.712V or just below (around LBW) and see if any cells fail the test at that point. I can assure you that they will be fine in your case.

 

[DaveinOlyWA]

The red lines are shunts, not bad cells. All the shunt does is stop flow through that cell. Usually it's done because the cell is discharging faster than the others. If you continually see specific cells with an active shunt, it could point to an issue with that module, and it may need to be replaced.

That said, I have a screen shot with 94 active shunts.

shunts actually do the opposite. they allow flow from higher voltage cell to a lower voltage cell in an effort to get a better balance between the high and low cells

 

[RegGuheert]

All the shunt does is stop flow through that cell. Usually it's done because the cell is discharging faster than the others.

shunts actually do the opposite. they allow flow from higher voltage cell to a lower voltage cell in an effort to get a better balance between the high and low cells

While that would be the best type of shunting approach, it is likely too costly and unreliable to be used in practice with such a large number of cells, at least at this point in time.

In reality, the shunts in the LEAF merely allow ~1A of current to bypass the cell-pair which is shunted. As such, it has no direct effect upon non-shunted cell-pairs. How effective the 1A shunt is depends on how quickly the cell is being charged or disharged. At high current levels, a 1A shunt will have only a marginal effect, but at lower currents, they are very effective. That is why the tapering of the charger in the 2013s and beyond is to such a low current level: it allows the shunts to fully balance the pack in a single charge cycle to full.

 

[RegGuheert]

For another couple of data points, I went through my records from LeafSpy to find the data points where the minimum voltage was close to 3.712V, but still below it:

- One point occurred on May 26, 2014. Here is the data:

Min: 3.705, Average: 3.717, Max: 3.733.

1) The minimum voltage is below 3.712.
BUT
2) The minimum voltage is NOT below 3.693, which is Vavg - 1.5(Vmax - Vavg).

- But the next data point I have where the minimum voltage is a bit lower was taken about two weeks later, on June 10, 2014, looks like this:

Min: 3.610, Average: 3.679, Max: 3.714.

1) The minimum voltage is below 3.712.
AND
2) The minimum voltage is below 3.626, which is Vavg - 1.5(Vmax - Vavg).

As such, this points to a "bad" cell, according to the test as laid out by Nissan.

There are additional data points at lower minimum voltages where the same cell shows up as even farther below the threshold.

But before I would try to make a claim against the warranty, I would want to run this test with a fully-balanced pack (in our 2011, multiple days in a row charging to 100%) and having discharged slowly to just below the 3.712V threshold and see how things look.

BTW, one thing I will point out about this particular test: If a cell (or cells) fail this test in a fully-balanced pack, I expect it will be covered under the original 8-year, 100,000-mile warranty that came with our LEAFs. As such, this may be a way to try to address premature battery degradation with Nissan by showing an actual cell "failure". If they get enough of these claims, it may cost them enough money that they will want to replace the entire pack. We'll see...

 

[Mottyski82]

Wow, great explanation RegGuheert!
I rally appreciate your own data points in relation to my own.
So let me ask if I understand you correctly... Are you saying that you suggest that before I bring the car in for the next battery check, I charge the car a couple / few cycles using the 100% charge option to better balance the pack. Then, when I have them run the annual battery checkup, I opt for not leaving it there overnight for a free charge, and instead, try to deliver it to them with a SOC where voltages are closer to 3.7v or even 3.6v?
I try not to allow the pack to sit for hours @ either a low voltage like that or @ a high voltage as a full charge.

In my opinion, this should be exactly what the annual battery checkups should be for / about. Nissan claims that the battery checks are to spot a problem (in the way you are using the car) before it ruins the battery. I suppose, it doesn't exactly sound like they want to identify cells which could be replaced even though they did fail during the warranty period, because that's money out of their pocket. But, replacing those bad cells earlier rather than later could have a major effect on the longevity of the battery pack as a whole! What are EVerybody's ideas on how to go about trying to get these bad cells replaced? After all, that is one thing of beauty about the design of the LEAF battery pack... The ability to fix the battery before the bad cells start to "pull down" the performance of the "whole" pack!!

Thanks again EVeryone! Great responses!

 

[RegGuheert]

So let me ask if I understand you correctly... Are you saying that you suggest that before I bring the car in for the next battery check, I charge the car a couple / few cycles using the 100% charge option to better balance the pack. Then, when I have them run the annual battery checkup, I opt for not leaving it there overnight for a free charge, and instead, try to deliver it to them with a SOC where voltages are closer to 3.7v or even 3.6v?

No. As you correctly stated, the annual battery checkup does not include performing the Cell-Voltage Level Inspection (CVLI) test which I have discussed. That test is time-consuming and they will only do it if you report a weak cell. Insteady, the annual battery checkup simply reads data from the vehicle's computer to let them know the condition of the battery and whether or not the customer has been abusing it. It includes things like how many times a charge was initiated above 80% SOC level and how many times the battery temperature got up to seven bars, etc. There is a post around here somewhere in which Ingineer details what the car reports.

I try not to allow the pack to sit for hours @ either a low voltage like that or @ a high voltage as a full charge.

Me, too. Our LEAF spends most of its life between 25% and 40% SOC. The downside of this approach is that the pack can get quite out of balance after a few weeks or months of this type of charging. A MY2011/2012 LEAF with an out-of-balance pack can take several charge cycles to 100% to achieve a good balance. MY2013 and beyond can apparently balance in a single charge to 100%. Which model year do you have?

In my opinion, this should be exactly what the annual battery checkups should be for / about. Nissan claims that the battery checks are to spot a problem (in the way you are using the car) before it ruins the battery.

I don't disagree, but as I mentioned, the CVLI test likely is a bit too much for a routine test.

I suppose, it doesn't exactly sound like they want to identify cells which could be replaced even though they did fail during the warranty period, because that's money out of their pocket. But, replacing those bad cells earlier rather than later could have a major effect on the longevity of the battery pack as a whole! What are EVerybody's ideas on how to go about trying to get these bad cells replaced? After all, that is one thing of beauty about the design of the LEAF battery pack... The ability to fix the battery before the bad cells start to "pull down" the performance of the "whole" pack!!

While I agree that the bad cells do "pull down" the performance of the full pack, I do not agree that bad cells will adversely affect the longevity of the pack as a whole. Rather, I believe that the presence of a "bad" or weak cell or cells tends to *reduce* reduce the stain on the other cells in the pack, causing them to experience lower maximum voltages and higher minimum voltages. This is due to the way the battery management system restricts cell voltages: it limits the maximum and minimum of the highest or lowest cell. Thus the other cells will not reach that level unless the pack is fully balanced. The downside is that the capacity of the pack tends to look like the capacity of the lowest cell-pair times 96, so range is impacted.

All that said, I do not think replacing a weak cell is a panacea. While such a fix can improve the overall capacity of the pack, it will not result in a balanced pack. Rather, the two cell-pairs contained within a module will have more capacity than the rest of the pack and their shunts will work hard to try to keep them in balance with the rest.

To me, the bigger issue is that I do not wish to have the dealer opening our factory-sealed battery pack unless it is really, really necessary. Frankly, what we have seen with our LEAF is mostly due to cell imbalance rather than a weak cell. I do believe cell-pair #37 is a bit weaker than the rest, but that didn't stop me from driving over 69 miles last Friday and arriving home with 34% charge remaining. So while the car's instrumentation is reporting a loss of 16% of the original capacity and a "Health" of only 70%, I'm achieving best-ever (by quite a bit) trip performance out of the vehicle. (And, yes, I understand there are many other factors involved!)

My plan is to continue to drive the car as much as possible and keep an eye on the battery balance and individual cell condition. If any cells get particularly bad, I suspect that will result in rapid loss of capacity bars and the possibility to claim a new battery under the battery capacity warranty. So far I see no obvious signs this will happen. Otherwise, I intend to see how long we can use the vehicle with the original battery and then try to claim a failed cell or cells close to the end of the 8 years/100,000 miles if there are cells which clearly fail the CVLI tests.

As it stands now, I'd prefer to keep our LEAF out of the hands of the dealership if possible. The good news is that with LeafSpy we can easily run the CVLI test ourselves, so we are not fully at the mercy of the our dealerships.

BTW, a single cell which is much weaker than the rest is likely to look similar to the image linked from this post. Did you case look somewhat like that, or did you have a much bigger spread with the rest of the pack?

 

[Mottyski82]

Very well said again, RegGuheert.

Yes, I have a 2013. I have been allowing for a less balanced pack by rarely (maybe 20% of the time) charging to full for the balancing at the end of the charge. I figured that lower state of charge, but slightly less balanced, would be better for the battery to survive the "Vermont heat". The scorching 80 - 85 degree heat! All 10 days of them. HEH. Now that the second summer is winding down, I will probably mostly charge to 100% from now on... Or at least 80% charges.

Your last comment was very helpful for me to understand clearly. I have to agree with you that it does make sense for the "bad cell(s)" to mostly just be unbalanced rather than bad, per se. Also, that it makes sense that all it would effect is slightly less range, and actually, better (lower) state of charge for more of the time it's sitting charged! Oddly enough, it seems almost beneficial to the longevity of the original cells.
I too am hoping to get as much use out of the original battery pack... Replacing a module possibly to prolong the useful life of the thing... It's much more environmentally friendly to keep something which has already been manufactured going than to just buy a new one when the old one was working just fine. The reverse is called: *Gasp* Consumerism! YIKES! Or maybe just wasteful consumerism anyway.

A couple more comments here: Yes, I agree... Keep the sealed battery pack sealed. Only have the local dealership open it to save someone's life! HA! Nope, I took a look at the image you linked... My two "bad cell(s)" were nowhere near as low as that one was in relation to all the others. After all this, I really do think my first encounter with this "Bad Cell" stuff is mostly just imbalanced cells looking like they dressed up for Halloween!

 

[Mottyski82]

Hey EVeryone,

I was wondering if anyone has a diagram showing which cell pairs are in which modules...? I want to get an idea as to how many modules (and which modules) would be affected by low voltages within cell pairings. In other words, which modules to replace based on our own individual results of the CVLI test.

Thanks!

 

[JonO]

The cell pair schematics are in the same document that the CV** test is found in. They are readily available on line with a quick search. Nissan Leaf EVB manual.

 

[TomT]

Nissan will not accept data from anything other than their own test methodology so you would have to have them run a test before any warranty cell replacements... Odds are, yours would pass their test based on your reported data...

In other words, which modules to replace based on our own individual results of the CVLI test.