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RegGuheert
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Leaf Number: 5926
Location: Northern VA

Re: LEAFspy says Bad Cells?! :(

Mon Aug 18, 2014 5:02 am

Mottyski82 wrote: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..
Mottyski82 wrote: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.
RegGuheert
2011 Leaf SL Demo vehicle
10K miles: Apr 14, 2013, 20K miles (55.7Ah): Aug 7, 2014, 30K miles (52.0Ah): Dec 30, 2015, 40K miles (49.8Ah): Feb 8, 2017, 50K miles (47.2Ah): Dec 7, 2017.
Enphase Inverter Measured MTBF: M190, M215, M250, S280

DaveinOlyWA
Posts: 12978
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Leaf Number: 314199
Location: Olympia, WA
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Re: LEAFspy says Bad Cells?! :(

Mon Aug 18, 2014 5:21 am

mctom987 wrote: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
2011 SL; 44,598 miles. 2013 S; 44,840 miles.2016 S30 deceased. 29,413 miles. 2018 S40; 5775 miles, 488 GIDs, 38 kwh 113.37 Ahr available, SOH 98.21, Hx 115.75
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RegGuheert
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Re: LEAFspy says Bad Cells?! :(

Mon Aug 18, 2014 8:21 am

DaveinOlyWA wrote:
mctom987 wrote: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
2011 Leaf SL Demo vehicle
10K miles: Apr 14, 2013, 20K miles (55.7Ah): Aug 7, 2014, 30K miles (52.0Ah): Dec 30, 2015, 40K miles (49.8Ah): Feb 8, 2017, 50K miles (47.2Ah): Dec 7, 2017.
Enphase Inverter Measured MTBF: M190, M215, M250, S280

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RegGuheert
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Re: LEAFspy says Bad Cells?! :(

Mon Aug 18, 2014 8:49 am

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...
RegGuheert
2011 Leaf SL Demo vehicle
10K miles: Apr 14, 2013, 20K miles (55.7Ah): Aug 7, 2014, 30K miles (52.0Ah): Dec 30, 2015, 40K miles (49.8Ah): Feb 8, 2017, 50K miles (47.2Ah): Dec 7, 2017.
Enphase Inverter Measured MTBF: M190, M215, M250, S280

Mottyski82
Posts: 69
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Leaf Number: 405969
Location: Hinesburg, VT

Re: LEAFspy says Bad Cells?! :(

Mon Aug 18, 2014 9:22 pm

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! :)

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RegGuheert
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Re: LEAFspy says Bad Cells?! :(

Tue Aug 19, 2014 5:08 am

Mottyski82 wrote: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.
Mottyski82 wrote: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?
Mottyski82 wrote: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.
Mottyski82 wrote: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?
RegGuheert
2011 Leaf SL Demo vehicle
10K miles: Apr 14, 2013, 20K miles (55.7Ah): Aug 7, 2014, 30K miles (52.0Ah): Dec 30, 2015, 40K miles (49.8Ah): Feb 8, 2017, 50K miles (47.2Ah): Dec 7, 2017.
Enphase Inverter Measured MTBF: M190, M215, M250, S280

Mottyski82
Posts: 69
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Location: Hinesburg, VT

Re: LEAFspy says Bad Cells?! :(

Tue Aug 19, 2014 8:44 pm

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
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Re: LEAFspy says Bad Cells?! :(

Tue Oct 28, 2014 4:06 am

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
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Leaf Number: 003530
Location: Manitoba, Canada

Re: LEAFspy says Bad Cells?! :(

Mon Apr 06, 2015 9:39 am

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.
2011 Leaf SL
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12,000 kms, minty!
Using LeafSpy Pro since the launch of the iPhone version (Mar 2015)

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TomT
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Re: LEAFspy says Bad Cells?! :(

Mon Apr 06, 2015 1:44 pm

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...

Mottyski82 wrote:In other words, which modules to replace based on our own individual results of the CVLI test.
59,991 miles/12 bars/289 Gids/68.54 AHr/101% SOH/101.64% Hx 7May15 w/ new Lizard (barely made the warranty).
71,770 miles/12 bars/256 Gids/59.04 AHr/88% SOH/87.92% Hx 3Mar16 at lease return.

Now driving a 2016 Volt Premier. Model 3 reserved.

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