Preprint Article Version 1 This version is not peer-reviewed

Accelerated Reported Battery Capacity Loss in 30 kWh Variants of the Nissan Leaf

Version 1 : Received: 15 March 2018 / Approved: 15 March 2018 / Online: 15 March 2018 (07:19:52 CET)

How to cite: Myall, D.; Ivanov, D.; Larason, W.; Nixon, M.; Moller, H. Accelerated Reported Battery Capacity Loss in 30 kWh Variants of the Nissan Leaf. Preprints 2018, 2018030122 (doi: 10.20944/preprints201803.0122.v1). Myall, D.; Ivanov, D.; Larason, W.; Nixon, M.; Moller, H. Accelerated Reported Battery Capacity Loss in 30 kWh Variants of the Nissan Leaf. Preprints 2018, 2018030122 (doi: 10.20944/preprints201803.0122.v1).

Abstract

Analysis of 1382 measures of battery State of Health (SoH) from 283 Nissan Leafs (“Leaf/s”), manufactured between 2011 and 2017, has detected a faster rate of decline in this measure of energy-holding capacity for 30 kWh variants. At two years of age, the mean rate of decline of SoH of 30 kWh Leafs was 9.9% per annum (95% uncertainty interval of 8.7% to 11.1%; n = 82). This was around three times the rate of decline of 24 kWh Leafs which at two years averaged 3.1% per annum (95% uncertainty interval of 2.9% to 3.3%; n = 201). For both variants there was evidence for an increasing rate of decline as they aged, although this was much more pronounced in the 30 kWh Leafs. Higher use of rapid DC charging was associated with a small decrease in SoH. Additionally, while 24 kWh cars with greater distances travelled showed a higher SoH, in 30 kWh cars there was a reduction in SoH observed in cars that had travelled further. The 30 kWh Leafs sourced from United Kingdom showed slower initial decline than those from Japan, but the rate of decline was similar at two years of age. Improvements in the battery health diagnostics, continuous monitoring of battery temperatures and state of charge, and verification of a fundamental model of battery health are needed before causes and remedies for the observed decline can be pinpointed. If the high rate of decline in battery capacity that we observed in the first 2.3 years of a 30 kWh Leaf’s lifetime were to continue, the financial and environmental benefits of this model may be significantly eroded. Despite 30 kWh Leafs accounting for only 14% of all light battery electric vehicles registered for use on New Zealand roads at the end of February 2018, there is also the potential for the relatively poor performance of this specific model to undermine electric vehicle uptake more generally unless remedies can be found.

Subject Areas

electric vehicle; Nissan Leaf; lithium-ion battery; capacity loss; battery degradation

Readers' Comments and Ratings (17)

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Comment 1
Received: 16 March 2018
Commenter: Ken Morison
Commenter's Conflict of Interests: I own a 24 kWh Leaf.
Comment: The data speak for themselves. The affiliations of these authors shows they will be all keen to promote EVs. They will given a great deal of thought into the impact of publishing this data. Their discussion raises a number of uncertainties in the data and its interpretation, but there would have to be significant systematic errors for the trends to be incorrect. It is not clear that the 2nd order polynomial are correct, but at no point do they extrapolate them, and the indicated trends are consistent with past results (as found with a Google search).
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Comment 2
Received: 20 March 2018
Commenter: Kirk Summerwill
The commenter has declared there is no conflict of interests.
Comment: A few points for the author editor to consider:

The reference to a facebook group for information (NAZ EV Owners Group (2018) doesn't allow readers to find that evidence, nor is any critical evaluation or description of that evidence given - without better referencing I would mention that it is anecdotal rather than an online poll or other activity (if true). Caution is clearly required as cultural narratives may exist in USA audiences that may not be present elsewhere.

The study notes that the sample are self selecting and convenience sample, but no critical appraisal of the issues this may bring or the actual method and process of selection/recruitment is given so readers cannot determine the extent to which this is a methodological influence.

Overall though a compelling study. Greater control for user behavioural factors would be an advantage also.
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Response 1 to Comment 2
Received: 21 March 2018
Commenter: Daniel Myall
The commenter has declared there is no conflict of interests.
Comment: Thank you very much for your feedback. We agree that the reference to the NZ EV Owners Group on Facebook in the introduction has limitations and that mentioning anecdotal evidence instead would be a better approach. We'll make this revision in the next version of the report.

In the discussion (lines 294 to 302) we cover some potential issues around the convenience sample and why we don't believe sampling bias could explain the results. In a nutshell (i) the difference between the Leaf variants is huge, so any representativeness bias would have to be huge to explain the result; (ii) many people joined long before any suggestion of an issue with the battery was being aired (indeed most signup soon after buying their vehicle), (iii) many didn’t scan at the start but did so when we asked them to check. But more fundamentally, (iv) any bias would presumably be operating in both 24 and 30 kWh subsamples, so bias itself is most unlikely to be a sufficient explanation for the group difference. Now that the news of the 30 kWh battery capacity loss is being openly discussed, there is much more risk that those with unusually degraded batteries are more likely to sign-up – so we will work hard to test for such bias in the data received from now on.

We agree that it would be beneficial to provide more details on how users were recruited and we'll include this in the next version.

Many thanks for your critique.
Comment 3
Received: 21 March 2018
Commenter's Conflict of Interests: I own a 2 year old Leaf 30 kWh with 45.000 km.
Comment: To my understanding are the numbers publish based on the SOH value reported by LeafSpy.
I believe this value is an undocumented value and it is not clear what it stands for.
It must be a different value than Nissan is using, otherwise my Leaf shall not show a full health scale, bur0t LeafSpy shows SOH 81% at my 30 kWh Leaf.
Either SOH or the value shown by Nissan are wrong.
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Response 1 to Comment 3
Received: 22 March 2018
Commenter: Daniel Myall
The commenter has declared there is no conflict of interests.
Comment: Thank you for your critique.

Yes, we have based our main analysis on the State of Health (SoH) metric that is reported by LeafSpy. We are not sure what you mean by Nissan’s ‘health scale’ but expect that it might mean the number of ‘bars’ displayed on the dashboard to the extreme right of the state of charge bars. So, when you refer to your 81% SoH battery pack showing “full health scale”, do you mean that you are showing 12 out of 12 bars? Please confirm so that we can fully evaluate your concern.
In the meantime, we see no reason to discount the use of SoH as a broad scale metric, provided it is used within safe limits of inference. i.e.
  1. As we say at lines 60-63 of our paper, “SoH is a value officially referred to by Nissan as 'LBSOH' and is generated by the car's battery management system and outputted by the Nissan Consult 3 tool (My Nissan Leaf, 2012)”. We agree that the exact details of how that metric is measured is not declared by Nissan, but the SoH metric is in common use and interpreted as the energy holding capacity of the battery as of now compared to when the car was first manufactured.
  2. Our data show a broad scale association between SoH and the number of bars on the dashboard (see new figure at end) which Nissan use for warranty triggers and which is in common use in the market place. There is considerable variation between individual vehicles and overlap between models, but overall a strong correlation between the two measures.
  3. Assuming that we have interpreted your comment correctly, you having 81% SoH is entirely as expected for your ‘12 bar’ 30 kWh Leaf. The data show that on average a 30 kWh Leaf loses its first bar at the latest by 76% SoH, and on average at a later stage of battery degradation than a 24 kWh Leaf. If the rates of decline reported in Figs 1 and 3 of our paper continue at their current rate, we expect many New Zealand 30 kWh owners to lose their first bar in the next six months (the average for the oldest 30 kWh Leafs in New Zealand is around 83% and our statistical model shows that it is falling at around 10% per year)
  4. We observe temporal fluctuation (up and down) in SoH related to recent driving and charging behaviour (See Fig 2, and discussion at Lines 276-278 of the paper), but this ‘nuisance’ or ‘disruptive’ variation is insufficient to obscure the large difference observed between 30 kWh and 24 kWh Leaf variants.
  5. Unless the instrumentation was changed in some drastic way between manufacture of the 24 kWh and 30 kWh variants (we hope that Nissan can confirm either way), the very large difference observed between the models cannot possibly be explained by measurement bias. Any such difference would have had to affect both bars and SoH, judging from the correlation in the figure above, so we do not expect SoH alone to be misleading our broader interpretation.
  6. Social media reports from owners of 30 kWh Leafs in the hot climate of southern USA have reported strong and proportionate declines in actual range of their vehicle that are commensurate with loss of bars. The correlation between SoH and maximum range will become all the more evident once bars are lost, but the influence of short term fluctuations in range because of terrain, driving conditions and temperature will make this hard to detect in the early stages of battery degradation.
  7. We have already completed preliminary testing on a range of 30kWh Leaf to compare SoH and energy stored in the battery as measured under controlled conditions. We are still refining our test procedure before conducting testing on a much larger number of vehicles. Details on our exact methodology will be made publicly available to allow others to conduct their own testing and compare to our results. Until this next stage of testing has been completed, we will continue to refer to ‘reported battery capacity’ rather than just 'battery capacity’ to make it formally clear that we have not yet directly measured energy-holding capacity.

In conclusion, we are confident that SoH is a reliable and scientifically-based metric of battery energy holding capacity that has sufficient repeatability to signal evidence of accelerated degradation of 30 kWh batteries in their first 2.3 years of life. Your concerns, and similar scepticism expressed by contributors to the EV owners social networks in recent days, suggests that we should now include the new Figure below in version 2 of the paper.

Your comments have helped us clarify the messages even if though we don’t agree with your critique. Thanks!


Comment 4
Received: 22 March 2018
Commenter: Matt W
The commenter has declared there is no conflict of interests.
Comment: Are you certain that Leafs are shipped at 0 years of age with the internal system reporting 100% SOH?

Also, even if Leafs do report 100% SOH at 0 years, are you sure that that the systems doesn't for example calculate some parameter SOH' and then simply apply a cut-off of 100% to SOH' to get SOH. The battery firmware could presumably do any kind of non-linear tricks it likes.

If either of those situations is correct then the curvature of the model may be less useful as a predictor of a future trend.
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Response 1 to Comment 4
Received: 22 March 2018
Commenter: Daniel Myall
The commenter has declared there is no conflict of interests.
Comment: Thank you for your feedback. It is our understanding that a new or reset Leaf battery management system (BMS) reports a SoH of 100% and the BMS then updates this value as the battery is cycled. We have seen cases of SoH at 101% or 102% so the BMS doesn't appear to clamp SoH to 100%. We will make this clearer in version 2 of the paper.

We agree that extreme caution should be applied when predicting future decline. As discussed in the paragraph starting on line 341, there is a need to test and refine an underlying model of battery SoH decline, and it is uncertain at this stage whether decline will progress in the fashion detected so far.

As we gain further longitudinal data in the 30 kWh cars over the next months the uncertainty around SoH's trajectory in the first year will have less influence on the model's shape. In the meantime we have refrained from forward extrapolation, partly because if the paucity of data in the first year and partly because the underlying model of battery degradation and its drivers are not certain. We need for more of the batteries' life cycle to elapse before we could make plausible projections of what to expect next.

Thanks again for your critique.
Comment 5
Received: 23 March 2018
Commenter: Roberto San José Mendiluce ( Brubaker.Nissan )
The commenter has declared there is no conflict of interests.
Comment: Hola me llamo Roberto San José Mendiluce y soy el Primer Taxi ​100 % ​ Eléctrico ​ ​​en España.

​​Conduzco un Nissan LEAF desde Octubre del 2011 ​. Pertenece a la primera generación ​que se fabricó en Oppama (Japón) y está equipado con una batería de litio de 24 kWh. He recorrido más de ​321.400​ kilómetros con Zero Emission y Zero averias.

​Mi objetivo es claro, transmitir las ventajas medioambientales y económicas que ofrece una Movilidad 100 %​ Eléctrica. Quiero demostrar la fiabilidad y el mínimo gasto en mantenimiento de este tipo de Vehículos. Para ello me he propuesto alcanzar las 200.000 millas recorridas con un Taxi 100 % Eléctrico, conservando la batería "original de fábrica" y sin haber efectuado ningún cambio de pastillas de freno.Publico toda la información que puedo sobre ​los Vehículo ​s​ Eléctrico ​s​ y las Energías Renovables y en especial lo relacionado con el Nissan LEAF.
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Response 1 to Comment 5
Received: 26 March 2018
Commenter: Daniel Myall
The commenter has declared there is no conflict of interests.
Comment: English translations of the original comment and our reply are included below

Gracias por su comentario. Qué fantástico oír sobre tan largas distancias recorridas sin fallos mecánicos en el Leaf de 24kWh. También estamos muy interesados en publicar sobre los beneficios medioambientales y económicos asociados con el uso de vehículos 100% eléctricos. En éste momento, nuestro artículo ilustra la única amenaza seria a los beneficios económicos y medioambientales de los vehículos eléctricos. Coincidimos con usted, como tan bien ha relatado, en que las baterías de 24 kWh tienen una longevidad excelente. Esperamos que a nuestras observaciones sobre las baterías de 30 kWh les siga un mayor entendimiento científico de la causa y su solución.

Original comment: Hello my name is Roberto San José Mendiluce and I am the 1st 100% electric taxi in Spain.

I have driven a Nissan LEAF since October 2011. It belongs to the 1st generation built in Oppama (Japan), equipped with a 24 kWh lithium battery. I have logged more than 321,400 km with zero emissions and zero mechanical issues.

My goal is clear, to transmit the environmental and economic advantages from a 100% electric transport medium. I want to demonstrate the reliability and minimal maintenance in these types of vehicles. For this, I plan to drive 200,000 miles with a 100% electric taxi, with the original factory battery and without having had to change the brake pads. I publish all the information that I can about electric vehicles and renewable energies. In particular, all related to Nissan LEAF.

Our reply: Thank your for your comment and it is great to see such large distances traveled in a 24 kWh Leaf with no maintenance issues. We are also very keen to publish about the environmental and economic benefits of electric vehicles. This is the only serious threat that we have seen so far to the environmental and economic benefits of electric vehicles, and illustrated so dramatically by your experience. We agree that the 24 kWh batteries are lasting extremely well. We hope that our observations of 30 kWh batteries will be followed by greater scientific understanding of the underlying cause so that solutions to be developed.
Comment 6
Received: 25 March 2018
Commenter: Andras Soltesz
The commenter has declared there is no conflict of interests.
Comment: It would be really great to see a separate diagram for the 24 kWh "Lizzard" batteries (2014+) because I suspect, they would show much better values than the older ones.
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Response 1 to Comment 6
Received: 6 April 2018
Commenter: Henrik Moller
The commenter has declared there is no conflict of interests.
Comment: Thanks for the suggestion Andras. A preliminary analysis could find no evidence of a difference in battery degradation in the two groups. However we have been sent many more records in the past three weeks and will be analysing these shortly. A formal test for differences in the two types of battery will be included in the next revision of the manuscript.
Comment 7
Received: 29 March 2018
Commenter: Kenneth Kar
Commenter's Conflict of Interests: I am an owner of Nissan Leaf
Comment: This paper uses age as the independent variable for battery degradation. Another important factor to consider is mileage. For a given age, some EVs have been driven over longer distance, and so their batteries may have gone through more cycles of charging and discharging. Battery capacity decreases with the number of cycling. The finding that 30kWh battery degrades at a higher rate may be partially explained by the fact that people drive further with a 30kWh Leaf, therefore the battery is subjected to more charging cycles . It would be interesting to see the analysis of SoH vs. mileage.
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Response 1 to Comment 7
Received: 6 April 2018
Commenter: Henrik Moller
The commenter has declared there is no conflict of interests.
Comment: Thanks for your comment Kenneth. See lines 170 to 174 of the paper. There are indeed associations of State of Health with distance travelled in the data, but these were quantitatively weak effects. The dominant associations are 'calendar age' of the Leaf and whether it is a 30 kWh or 24 kWh variant. It is interesting that the association with distance went in opposite directions for the two variants, which we take as another clue that the batteries are responding quite differently from each other.
Comment 8
Received: 31 March 2018
Commenter's Conflict of Interests: I lease a Nissan Leaf
Comment: Just wanted to give you some feedback from a 2017 Leaf driver in Hawaii’s warm climate. At 12 months and about 13,000 miles the 1st bar disappeared (11 bars showing). Nissan dealer “said” this is normal. Our salesman said his 2016 Leaf 30 kWh battery is loosing bars faster than his 2013 Leaf 24 kWh battery. He also said he is getting similar feedback from other Leaf owners he has sold Leafs too.

Our typical commute is about 35 miles round trip plus town errands under 5 miles round trip (air conditioner always on). At home it’s in a garage so it’s only in the sun when out and about. 90+% of driving is flat land and under 45 mph due to the low speed limits and all the traffic lights. It’s usually plugged in to charge (level one charging) daily as soon as we get home with the battery typically around 60% or more in charge and by morning it’s 100% or close to 100%. So maybe keeping the battery at a higher state of charge is not good. Also I watched a video somewhere of someone reading from the 2018 Leaf manual which recommended waiting an hour after driving to allow the battery to cool down a little before plunging it in to charge. So maybe it’s similar for a 2017 Leaf. We ha e never done any level 3 charging and only occasionally use level 2 charging for a short time while at a store.
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Response 1 to Comment 8
Received: 6 April 2018
Commenter: Henrik Moller
The commenter has declared there is no conflict of interests.
Comment: Thanks for your observations. Loss of the first bar indicates that your 30 kWh Leaf probably has a State of Health between 72% and 83% (see our graph at Comment #3 above). This is a very high rate of loss in 12 months. Social media networks have reported many similar examples, but of course we most need a large and representative sample before we can quantify the fleet averages in different regions and climates, and the effects of travel distance and charging patterns.

Your saleperson's comparison of their own 30 kWh and 24 kWh Leafs is allows a more tightly "paired comparison" because presumably both are operating in the same terrain and climate and being charged in similar ways. Another family that owns both a 30 kWh and 24 kWh Leaf has also reported (on social media) that the 30 kWh Leaf is losing battery capacity faster.

We hope that our paper will spur collation of data in systematic and robust ways using a 'citizen science' approach that learns from the experience of early adopters of electric vehicles of all makes and models.
Comment 9
Received: 12 June 2018
Commenter: Matt W
The commenter has declared there is no conflict of interests.
Comment: Following up on my observation that the curves in Figure 1. appear to have been constrained to be 100% state of health at 0 years of life, even though there are no actual data at or very near 0 years.

As I said, curves fitted without this constraint would be a better predictor of the actual trends in the data and perhaps avoid implying a curvature not supported by the data.

As a further improvement, I would also remove from the curve fit all the data points throughout the time series that are at or very near 100%. I think we can agree that the state of health measure plotted appears cut off at 100% and a possibly significant number of data points appear to be sitting just at or below 100%.

The basic conclusion of the paper is clear directly from the data (accelerated decline in the 30 kWh model compared with the 24 kWh one(s)) and doesn’t rely on any curve fitting. I think we agree that curve fitting can be useful in predicting future performance but that such predictions are characterised by uncertainty and need to be viewed with caution. The inference that "both battery models showed increasing decline with age" is clear from the data but any conclusion (as the fitted models imply) of increasing rate of decline with age might not be so clear at all if the year 0 constraint and the 100% state of health data were not considered in the fit.
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Comment 10
Received: 16 June 2018
Commenter: Rory Boreham
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