30kWh Leaf - Weak Cell - How Bad Is This?

[TCA]

Could someone please give me a steer as to how bad a problem I've got with a single weak cell and how to manage it.

I've attached 2 LeafSpy screenshots from 1st June (when I decided to run the charge down for a look) and from 30th June (today). I don't have any error codes, a full charge shows around 120 miles on the GOM but at 50%-60% SOC and below the range decreases at a rapid rate.

https://imgbox.com/z7cwCvar

https://imgbox.com/TTcds82R

Here's a few readings (in date order) which I hope give an idea:

1st June: Dash=7% SOC; LeafSpy: 30.1% SOC ( 19 GIDs 5.2%, 1.5kWh left), 173mV, weak cell=3.456V
2nd June: Dash=100% SOC; LeafSpy: 97.5% SOC (323 GIDs 88.5%, 25.0kWh left), 62mV, weak cell=4.057V
3rd June: Dash=14% SOC; LeafSpy: 35.5% SOC ( 45 GIDs 12.3%, 3.5kWh left), 132mV, weak cell=3.533V
13th June: Dash=35% SOC; LeafSpy: 51.3% SOC (113 GIDs 31.0%, 8.8kWh left), 139mV, weak cell=3.652V
21st June: Dash=81% SOC; LeafSpy: 76.7% SOC (260 GIDs 71.2%, 20.2kWh left), 89mV, weak cell=3.866V
29th June: Dash=59% SOC; LeafSpy: 61.4% SOC (190 GIDs 52.1%, 14.7kWh left), 125mV, weak cell=3.741V
30th June: Dash=83% SOC; LeafSpy: 78.7% SOC (268 GIDs 73.4%, 20.8kWh left), 131mV, weak cell=3.879V
30th June: Dash=100% SOC; LeafSpy: 97.6% SOC (324 GIDs 88.8%, 25.1kWh left), 80mV, weak cell=4.039V

The cell difference at full charge has gone from 62mV to 80mV in a month. I'm now getting over 130mV difference at 83% SOC, compared to 89mV at 81% SOC just 10 days ago. I haven't dared run it below 35% SOC given the 139mV reading on 13th June but don't really know how bad this is.

I assume cell replacement is required but I'm in the UK and Nissan seem only interested in degraded batteries for the 8-year warranty. It appears a few people have had successful warranty claims for cell failure in years gone by under the 5-year EV components warranty, but that has now expired. Pretty disgraceful from Nissan that they won't cover this.

Anyway, that aside, for the time being how do I manage to prevent things getting bad very quickly? And how bad can it actually get?

I had phone advice from an EV technician at Nissan advising me to keep the cell difference below 150mV (when I was getting 139mV at 35% SOC), but now I'm getting that difference at 83% SOC. He reckoned I'd have 2 months until the cell packed in completely. He also said exceeding 200mV for 3 full drive cycles would result in turtle mode and at around 500mV you'd be unable to start the EV.

I just wanted to see how that advice tallied with the real life experiences of those here who have experienced this?

How much damage am I doing by driving at high cell differences? This looks to be getting worse quite quickly. If I stay below 150mV I might be charging from 80%-100% every 20-30 miles. Any advice for the short term?

Thanks in advance for any replies.

 

[LeftieBiker]

Welcome. The 8 year warranty should cover defective cells as well as degradation. What the Tech said about imminent cell failure doesn't seem to me to jibe with the experiences of Leaf drivers over here (it seems a bit extreme), but I can't swear to that. I think the Tech may have been trying covertly to let you know that if you can get the car to Turtle, then it should register an error code that will get you a new pack - or at least a new cell.

The images are not linked properly. Some browsers like Firefox will open them in separate tabs, but other browsers won't be able to view them at all.

 

[TCA]

Thanks for the reply.

It was actually the Nissan tech who explained that the warranty would be refused. Hadn't dawned on me that the very few successful warranty claims in the UK were a few years back and only because the vehicles were within the 5-year warranty. It's a nonsense but the 8-year warranty is very specific, albeit disingenuous. The battery warranty doesn't cover a failed cell despite having as much an effect as a degraded battery. That's the line in the UK anyway. Plenty have tried and failed.

I agree what the tech said re failure did sound extreme and it's certainly made me very nervous about driving the car. Especially as I see the LeafSpy readings rapidly declining. Having seen mentions of EVs with really high cell differences mentioned on the forum, I just wanted to get a handle on what I could expect from here and what damage I'm doing.

(I edited my first post with actual links for the images as couldn't get it to work with the

 

[TCA]

In terms of what is experienced while driving with a weak cell, will it just mean the low battery warning comes on much quicker, swiftly followed by turtle mode?

The low battery warning on my test a month ago was at 17% SOC but guessing that'll be much higher now. Given the above LeafSpy readings, can anyone advise a SOC to stay above to stop things getting much worse much quicker?

Just don't want to get to the state where it's undrivable before I can get it attended to.

 

[Stanton]

In terms of what is experienced while driving with a weak cell, will it just mean the low battery warning comes on much quicker, swiftly followed by turtle mode?

That's basically true, but the problem is: it can vary/change rapidly depending on driving/environmental conditions (e.g. rapid acceleration on a cold day) so no SoC may be safe. You definitely have a weak cell, and in my experience anything >150mV differential will definitely cause problems. The good news is: it's only 1 cell/module (less parts/less labor).

I wish I could tell you that Nissan will gladly replace your weak/bad module...but that's just not the case. I have been through this myself, and you have 2 hurdles to climb:
1) finding a (good) replacement module (there are 4 cell pairs in a single module)
2) finding a shop that will do it

Nissan is no good on both counts, because:
1) they don't have any modules available anyway (you need to find one on eBay), and
2) they will quote you an outrageous price to do it...because they really don't want to do it (they want to sell you a new pack...which are also in short supply).

Good luck: my module replacement cost me ~$2k (all in...parts and labor).

 

[TCA]

In terms of what is experienced while driving with a weak cell, will it just mean the low battery warning comes on much quicker, swiftly followed by turtle mode?

That's basically true, but the problem is: it can vary/change rapidly depending on driving/environmental conditions (e.g. rapid acceleration on a cold day) so no SoC may be safe. You definitely have a weak cell, and in my experience anything >150mV differential will definitely cause problems.

Thanks. I just witnessed exactly that. After driving a 24 mile round trip from 100% SOC with driving in parts at 40-50mph, LeafSpy SOC finished at 78% with the cell difference at 133mV. After the next round trip of 9 miles, at slower speeds, LeafSpy SOC finished at 71% with the cell difference at 106mV. After a further (same route) round trip of 9 miles, at the same slower speeds, LeafSpy SOC finished at 64% with the cell difference at 120mV.

I previously thought it would just generally get worse at all states of charge, without anything like the above happening.

 

[TCA]

A couple of quick questions for those more than knowledgeable than me.

I'm frequently AC charging to keep the SOC high, in the hope this keeps the weak cell topped up and limits any damage so I can continue using the car before I can get it fixed.

Am I doing more damage to the weak cell by frequently charging (AC not DC) or better to let the voltage run down more?

I'm now seeing 165mV cell difference at 90% SOC, so very wary of doing anything too extreme.

Is there a safe maximum cell voltage (for that weak cell) that I can aim to stay above, irrespective of the mV cell difference?

e.g. if that individual cell is above say 3.75V, will the car continue to run properly without warnings and turtle mode, despite really high mV cell difference?

Thanks for your help.

 

[goldbrick]

The car's battery management system will keep all the cells within a safe range. As you know, turtle mode is engaged when the lowest cell reaches the lower limit so keeping the SOC higher will help prevent that.

In general, keeping the SOC high all the time will accelerate the wear on all the cells, including any weak ones. I'd say the difference will be small in the short term but that's just how Li batteries are. Since the weak cell is lower than the rest it will see less time at high voltage so it should be affected less than the other cells but again, I'd say the effect is small and I don't know how a 'weak' cell would behave relative to a 'normal' cell in these conditions. I'd guess it wouldn't matter much, so I'd just try to prevent turtle mode since that is a certain problem if it happens.

 

[TCA]

Thanks goldbrick.

Is the low battery warning also triggered based on low cell voltage? So can I safely drive without doing more damage until that appears, regardless of the high mV cell difference?

If so, does anyone know at what voltage the lbw appears?

I think I read turtle mode triggers at 3V but wouldn't want to go near that.

 

[Stanton]

I'm now seeing 165mV cell difference at 90% SOC, so very wary of doing anything too extreme.

As expected, your voltage delta (unfortunately) continues to get worse. Unfortunately, there's really nothing you can do slow/stop the progression...including SoC levels. Keeping the pack "topped off" will mask some of the extreme SoC variance, but you still have to be mindful of (long term) damage to all of the "good" cells/modules at pro-longed high SoCs.
While turtle is based on a single cell, LBW/VLBW are based on GIDs, so I'm not sure how you relate that to an overall pack voltage. The truth is, I'm not really sure it matters because that bad cell(s) is going to "run the show" anyway.

 

[TCA]

Thanks Stanton.

Keeping the battery topped up would only be in the very short term.

What I'm trying to work out (with my limited knowledge) is how best to run the car without precipitating further damage until I can get this issue sorted.

Avoiding turtle mode and DC charging would seem obvious, but do I have to constantly monitor the voltage delta or can I slow the advance to total failure in the short term by just keeping the voltage of the weak cell above a certain level?

I appreciate it's only going one way but any advice so I don't make matters worse more quickly would help. Or just futile?

 

[Stanton]

Avoiding turtle mode and DC charging would seem obvious, but do I have to constantly monitor the voltage delta or can I slow the advance to total failure in the short term by just keeping the voltage of the weak cell above a certain level?

This would be a good approach, but I guess I failed in my main point: it doesn't really matter what you do at this point (short of leaving the pack @100%--which is very bad) because the bad/weak cell/module will dominate the results. For example, my replacement module is slightly higher in voltage (by design) than the rest of my pack...so any charging I do always terminates when that cell/module reaches the target SoC. While that robs me of some capacity at high SoC, it has eliminated any unpredictable behavior at low SoC. So it's like I have a 38 kWh (or somesuch) pack instead of a 40 kWh pack (which is still a huge improvement over 24 kWh).

 

[TCA]

Thanks again Stanton.

I understood what you were saying. I was just looking for a way to avert total failure in the very short term. I know it's inevitable.

Is turtle mode the worst thing that can happen in the first instance?

Can I avoid turtle as long as I keep that weak cell over 3V, even if the voltage delta is in the hundreds of mV?

I appreciate that this will ultimately prove impossible and become more and more difficult but any small thing I can do to buy some time right now would be a great help.

 

[goldbrick]

I would say turtle mode is the worst that could happen (meaning getting stuck somewhere....) since the battery controller should shut things down before any physical damage occurs.

As far as I know, keeping the weak cell above the voltage threshold for turtle should allow the car to operate normally. Turtle mode is triggered by a single cell only so the voltage of the rest of the cells doesn't matter when the first cell reaches the turtle threshold.

 

[TCA]

Thank you goldbrick. That clarifies things.

It's a bit scary seeing these ever-increasing mV cell differences, so it's good to know that nothing untoward is likely to happen until the single weak cell hits the turtle threshold.

 

[TCA]

I've just read a post elsewhere where an EV technician said the maximum deviation allowed before turtle is 200mV but that has to be consistently exceeded for a number of drive cycles. His manual says 3 full drive cycles, but apparently it can vary.

I'm not sure what a drive cycle actually means.

The tech also said around 500mV deviation would result in the car being unable to restart.

Any thoughts on the above? What does a drive cycle mean?

 

[goldbrick]

I'd assume a drive cycle means the car was powered off then on, ie everytime you start it is one drive cycle.

That behavior is certainly possible but it all depends on how the BMS/LBC is programmed. It seems odd to me to force turtle mode like that without throwing a fault of some kind first and I've never heard of this happening to anyone, but as they say, anything's possible with code.

 

[MikeinPA]

I've just read a post elsewhere where an EV technician said the maximum deviation allowed before turtle is 200mV but that has to be consistently exceeded for a number of drive cycles. His manual says 3 full drive cycles, but apparently it can vary.

The tech also said around 500mV deviation would result in the car being unable to restart.

This is essentially what I observed with my 5/2013 Leaf. I had about threes consecutive cycles where the delta was greater than about 200 mv--I don't recall precisely but that is close. At this point I got a permanent turtle and the P33E6 DTC was set. To be clear, I drove the car and when I got home, the weak cell was much lower than the rest. Charge, repeat 2x (about, this was several years ago) and then the car is permanently in reduced power mode even when "fully" charged. LS would clear the DTC, and the turtle would go away, and the car would drive normally (for some values of normal). Lived with that for a while--work has been busy and I have had a very hard time finding replacement cells. Last fall however, one cell was so low that a) the car would not drive and b) car would not charge because all the other cells were topped off. This would have been about 500 mv. At this point I opened up the battery and discharged the higher cells to good storage values.

 

[TCA]

Thanks Mike. Very useful to know what actually happens in practice and the fact you could manage it up to a point.

Just so I'm clear, are we basically saying that once the cell difference is at 200mV or thereabouts, then just driving another twice (or so) without recharging will cause turtle?

Did you get any warnings on the dashboard before the turtle and DTC kicked in?

 

[MikeinPA]

Here is my report at the time https://www.mynissanleaf.com/viewtopic.php?p=582167#p582167. I don't recall any other prior notifications from the car. All of my information was coming from LS up to that point. However, I did come to believe that the
SOC % provided on the dash was behaving as follows: beginning at full charge, the SOC is based on an average pack voltage, and will decline normally with respect to miles driven, so that at say 8 miles driven the SOC will read 90%; but at some point the SOC switches over to tracking just the weakest cell and can drop very rapidly ( 10 to 20% on a single hill). I have seen this same behavior reported elsewhere many times. Hope this helps.