Optimal Portion of Battery to Use for Longest Battery Life?

[RegGuheert]

OK, I have about 72F average ambient for Phoenix, AZ, and I believe that typically 70F is used. Hopefully, you are willing to agree that 7F above ambient for the entire life of the vehicle is pure speculation? If the car was parked outside, I believe that it would closer - perhaps 1 or 2F above ambient. The Leaf develops very little operational heat, and spends the vast majority of time just sitting. Not charging and not driving. Let's have the intellectual honesty to admit that these numbers are guesses. They cannot be used to project the life of the pack, and the projections based on them should not be used as reference.

Guesses are used for engineering all the time. I'm sorry, but I think a guess of 7F above ambient is entirely reasonable and that 2F is likely way low. Simply put, if the battery team at Nissan came back to project engineering with a passively-cooled design and said it had an average lifetime temperature rise of 1C over ambient for the typical Phoenix commute of 50 miles they would be sent back telling them it was too heavy and too expensive and to shoot for 3C to 6C instead. We'll know more once we have some LEAFScans in our cars with which to take some battery temperature data. But really, we are arguing about a very small difference in projected life based on the curves given, perhaps less than one year.

 

[surfingslovak]

Guesses are used for engineering all the time. I'm sorry, but I think a guess of 7F above ambient is entirely reasonable and that 2F is likely way low. Simply put, if the battery team at Nissan came back to project engineering with a passively-cooled design and said it had an average lifetime temperature rise of 1C over ambient for the typical Phoenix commute of 50 miles they would be sent back telling them it was too heavy and too expensive and to shoot for 3C to 6C instead.

I'm sorry, but we'll have agree to disagree then. While I appreciate the interest and energy you brought to the forum, I disagree and I'm disappointed with both the tone and the intent of your posts around battery degradation. We don't know what we don't know, and none of us here was involved in the design of Leaf's battery pack. I'm certain that it's incorrect to use 85F for Phoenix on the chart Charles has provided. Likewise it's wrong and premature to project 5 or 6 years of life on for the batteries in Phoenix based on the little data that's available. That said, I'm going to have less time to monitor the forum, and I would hope that you will not continue on this trajectory. It's argumentative at best, and not helping anyone or anything.

 

[Stoaty]

Average annual temperatures for various cities: http://www.cityrating.com/averagetemperature.asp" onclick="window.open(this.href);return false;

Phoenix - 72.6
Los Angeles - 63

 

[KJD]

Matthew Simmons - Author Twilight in the Desert

The problem is you need to look at the data 3 times and then go back and look again at the data and then and only then come to a conclusion. Most people come to a conclusion and then look around for some data to support that conclusion

 

[abasile]

As it appears that battery degradation is non-linear with respect to pack temperature, i.e., that going from 100 F to 120 F is much more significant than going from 80 F to 100 F, I suspect that knowing the average temperature of a given area is only moderately useful. It's the high temperature extremes that we probably ought to care about most. For instance, the average temperature in Honolulu is 77 F, actually 5 degrees higher than Phoenix, yet I doubt we'll see anywhere near the same level of degradation among Hawaiian LEAFs.

 

[Stoaty]

As it appears that battery degradation is non-linear with respect to pack temperature, i.e., that going from 100 F to 120 F is much more significant than going from 80 F to 100 F, I suspect that knowing the average temperature of a given area is only moderately useful. It's the high temperature extremes that we probably ought to care about most.

Could very well be true. One thing I have noticed is that people in Phoenix often report seeing 7 temperature bars. While I park in the fairly hot San Fernando Valley in the summer, I have never seen more than 6 temperature bars.

 

[RegGuheert]

I'm sorry, but we'll have agree to disagree then. While I appreciate the interest and energy you brought to the forum, I disagree and I'm disappointed with both the tone and the intent of your posts around battery degradation. We don't know what we don't know, and none of us here was involved in the design of Leaf's battery pack. I'm certain that it's incorrect to use 85F for Phoenix on the chart Charles has provided. Likewise it's wrong and premature to project 5 or 6 years of life on for the batteries in Phoenix based on the little data that's available. That said, I'm going to have less time to monitor the forum, and I would hope that you will not continue on this trajectory.

I'm sorry you see it that way. It seems to me that many here are extremely uncomfortable to discuss the possibility that this will turn out badly for some of the LEAF owners in hot climates. (Note that it's O.K. around here to suggest that this might be only an instrumentation problem or just a seasonal variation.) I'm sorry, but a rosy outcome is not the only possibility here and I have been trying to collect the most accurate data I can find and interpret it in a reasonable way. I am NOT trying to bias any analysis in a negative way, but rather I am trying to uncover the most likely outcome. I'm not interested in basing my estimates of LEAF battery life on statements from Nissan marketing VPs since those do not correlate well with what I see as good data others have collected here.

Frankly, I consider Charles' chart to represent a best-case scenario for all of us since it only covers calendar life. If we each plug in the average temperature where we live (assuming NO temperature rise) we can get an idea what the absolute longest time we can expect to drive our LEAFs before we are down 30% in battery capacity. Is the chart accurate? I don't know, but it comes from a credible source, IMO. It's certainly more data than Nissan provided to any of us when they asked us to sign the long disclaimer.

It's argumentative at best, and not helping anyone or anything.

Sure it is. It is helpful to anyone who is reading this that is trying to evaluate whether an EV is right for them or not. We should be saying: "Take this chart. Plug in the average temperature for your location, you can find that at XYZ website. You should expect that BEFORE the number of years shown on that plot for your local temperature your LEAF battery will be down in capacity by 30%. It's not perfectly accurate, but since we ALL desire to have SOME estimate of the what to expect, it's the best data we can come up with. YMMV." Many people are considering purchasing an EV for the very first time and they do not know what to expect. Most people are not battery engineers and they want some guidance. Tony's chart is a great help! And he has even added a -15% capacity version to go along with it. But when will -15% happen? That information is important, too.

As I've said elsewhere, this information should be coming from Nissan. But it's not. And just like Tony's data, we will not always agree, but let's not make certain topics off-limits because they are uncomfortable or might turn someone away from the Nissan LEAF. That's not a bad thing if it happens, IMO. The last thing the EV movement needs are a bunch of people buying BEVs for the wrong reasons and becoming very unhappy in a few years.

 

[GaslessInSeattle]

based on things I've read over time and watching what the situation in hot climates, I have concluded that the Leaf battery is not all that different from other Li-ion batteries. Cycling the battery more towards the middle of the pack appears to be worth doing. Until firm numbers are available, I have decided to limit SOC below 80%, aiming for 50% for overnight and long periods during the day when convenient. This means topping off the battery right before I use it, roughly estimating how much I need for it to reach 50% before it sits for many hours either at my destination or back at home. I do not plan on avoiding 80% or 100% charging just avoiding leaving the battery at high soc for prolonged periods. I'm thinking of it as limiting total life hours of the battery at high soc. The car spends most of it's life parked, and it's during those times where the total life hours at high soc can really add up day and night. Cycling the battery in the middle range will be made much easier when we have a good QC network in the metro area so that leaving the car around 50% won't result in me being left in a pinch, unable to do what I need to do. As for QC'ing, I'm planning on using a QC whenever needed. It's anecdotal for now, but I have noticed that the challenge with QC'ing isn't just the heat from the quick charge but but also that a Quick Charge allows you to get back on the highway at high speeds quickly, combining residual heat from continuous high speed driving with multiple QC'ing. continuous hard driving and multiple fast charging is a bad combination. The temptation is to try and make up for time lost at the charging stations by driving fast and QC's really open up the option of being less mindful of driving efficiently. I have so far concluded that if I am going to drive fast that I should not plan on doing more than 2 QC's close together, watching the temp gauge and trying to avoid going over 7 bars. continuous QC's will likely require much slower driving to avoid discharge heat combining with charging heat, leading to damaging high heat spikes. ... very much looking forward to the Leafscan to begin nailing down some benchmarks. right now we are a bit in the dark as to what the battery temp actually is. Of course, as summer heat kicks in in our relatively moderate Northwest climate, we will get a better sense of what the dynamics and thresholds are for multiple fast charging, driving speed and ambient temperature.

for now, I think it's fair to say that people in hot climates would be wize to limit SOC if convenient, crack windows while parked in direct sunlight and generally avoid prolonged exposure to extreme heat... I sure do wish Nissan would be more upfront about what they know about all this! It really doesn't help that folks are getting surprised with what pretty clearly seems to be early capacity loss in areas of the country with extreme summer heat.

 

[RegGuheert]

This fits with the information from http://www.nrel.gov/vehiclesandfuels/energystorage/pdfs/53470.pdf" onclick="window.open(this.href);return false;

Thanks again for this link! As discussed, this presentation is for a different chemistry, but I think it is still informative since it includes most of the aging effects in the simulations that are done.

I have been looking at the curves shown in the graph at the bottom right of page 8. Can we draw any conclusions from thes pe curves? I think we can. Here are my conclusions:

- A battery which loses about 20% of its capacity in 8 years loses about 5% during the first year.
- A battery which loses 2% or less capacity in the first year MAY be able to retain more than 80% of its capacity after 15 years.
- The amount of capacity loss in year two is just slightly lower than the amount of loss in year one (at least for batteries which have a life of 8+ years).

So, do these conclusions apply to the batteries in our LEAFs? I think they do and they could potentially be used to estimate what our batteries might do in the future. Unfortunately, we would need to know *accurately* how much capacity loss our batteries had in the first year to be able to make any extrapolations.

But I don't think even those of us with LEAFs manufactured one year ago have enough information to know how much battery capacity has been lost. GIDs and capacity bars may be showing a certain amount of capacity loss, but we still do not know the *exact* defiinition of a GID and whether it is all battery capacity loss or if there are othe factors at work. Perhaps it is all battery capacity loss we are seeing, but I doubt it since TicTocks GIDs went up last fall.

While I would love to see a set of cuves provided by Nissan like the ones in the graph with all of the assumptions given, I do not see it happening. So I guess we'll have to continue to try to figure out what the losses really are based on whatever data we can collect.

 

[Herm]

The NREL study on battery longevity uses 82.4° F as the worst case constant temperature for Phoenix... it does not seem so bad but apparently it makes a big difference in battery life.

 

[surfingslovak]

The NREL study on battery longevity uses 82.4° F as the worst case constant temperature for Phoenix... it does not seem so bad but apparently it makes a big difference in battery life.

Yes, but the cells that form the basis of all these reports are quite different. It's a different chemistry (NCA), different form factor (cylindrical) and different thermal profile (see below). We cannot take a model and the associated parameters derived from different cells and apply them verbatim to the Leaf. That would be foolish. Even if we did some comparative analysis, which we didn't, it would be a long-stretch. Don't get me wrong, I find their reports interesting and useful. They obviously spent a lot of time and effort testing these cells. That does not change the fact though, that they are different from what the Leaf is using, and we have to see the model critically.

The cylindrical NCA cells run a lot hotter than the AESC cells. Partially because of their form factor, and partially because the US06 test cycle results in a 10C discharge. Look at the 1C discharge by comparison, it's only about 4F above ambient. The US06 test cycle would result in about 1C load in the Leaf, because 20 or 24 kW is enough to sustain freeway speeds. And since the cells in the Leaf have better thermal properties, they would likely run cooler than what the 1C scenario below projects.

The 10C discharge significantly accelerates aging and capacity fade. If claims are made that the NCA chemistry was inherently better than what the AESC cells have to offer, it's data like this that you shouldn't ignore. The hybrid battery runs a lot harsher cycle than what the cells in the Leaf will see. Even if you used a better chemistry, the aging results could be comparable as a result.

 

[Herm]

Yes, but the cells that form the basis of all these reports are quite different. It's a different chemistry (NCA), different form factor (cylindrical) and different thermal profile (see below). We cannot take a model and the associated parameters derived from different cells and apply them verbatim to the Leaf.

The only useful number from that report for the Leaf is the 82.4° F constant temperature for Phoenix. Thats all I meant. BTW, the NMC cells in the Volt are laminated, not cylindrical.

 

[surfingslovak]

The only useful number from that report for the Leaf is the 82.4° F constant temperature for Phoenix. Thats all I meant. BTW, the NMC cells in the Volt are laminated, not cylindrical.

Of course, understood. I'm not sure why we are talking about Vot's cells, perhaps I missed that. From what I can tell, all of the NREL studies used cylindrical NCA exclusively. They did a lab simulation of the temperature profiles of several cities, and compared the results to isothermal conditions. They found that the cells aged about the same in a 28C isothermal (that's your 82.5F) environment as they did in the simulated Phoenix environment.

Unfortunately, the specifics of the aging process are dependent not only on the ambient temperature, but also on the cells themselves, their thermal profile and operating temperature. That's why I said last weekend that these results most likely did not apply to AESC cells. If you ran the same experiment, and emulated the typical operating conditions found in the Leaf, I can virtually guarantee you that the isothermal temperature will be different. Will it be higher than the 72F average ambient found in Phoenix? Yes, very likely. Will it be the same 82.5F NREL found to be working for their particular NCA hybrid pack? Perhaps, but what are the odds? Personally, I believe that it would be less than that, but that's just a pure guess. It could be higher too, depending on how heat resistant the AESC cells were. This is the reason why I cautioned you and Reg not to take the NREL reports literally.

 

[Herm]

I'm not sure why we are talking about Vot's cells, perhaps I missed that. From what I can tell, all of the NREL studies used cylindrical NCA exclusively. They did a lab simulation of the temperature profiles of several cities, and compared the results to isothermal conditions. They found that the cells aged about the same in a 28C isothermal (that's your 82.5F) environment as they did in the simulated Phoenix environment.

I concede your point..

 

[surfingslovak]

Herm, perhaps you know this already, but if you wanted to have a look at all related NREL studies: here is an index. I looked at many of them, and that's when it dawned on me that they apparently used cylindrical NCA cells throughout their test program. I found this reference to be quite interesting, and that's where I pulled the charts from.

There are quite a few knowledgeable people on this forum, and I'm hopeful that sooner or later we'll come up with a model that would help us predict the aging process a bit better. Something we could all live with, and that would be supported by data from the field. I'm really concerned about using Gids to determine capacity fade BTW. The readings are simply too inconsistent. We may have to wait for Phil's Leafscan.

 

[gschettl]

Is there any way to activate these battery heaters to achieve the battery performance I experience when it's above ~50°F? When I've got 3 bars or less battery temperature, it's really tough to reach routine appointments with a 50 mile round trip. We had 50 days below zero this year in MN and it was a struggle for months to manage my normal driving routine. It would be ideal to have a battery pre-heating option to trick the car into a warmer season.

 

[JPWhite]

Is there any way to activate these battery heaters to achieve the battery performance I experience when it's above ~50°F? When I've got 3 bars or less battery temperature, it's really tough to reach routine appointments with a 50 mile round trip. We had 50 days below zero this year in MN and it was a struggle for months to manage my normal driving routine. It would be ideal to have a battery pre-heating option to trick the car into a warmer season.

This an obvious option, suprised the 2013's didnt sport it. Hopefully they will add such an option along with the lizard battery. Pre warming the battery will impact overall efficiency which may be the reason Nissan havn't implmented yet.

 

[thankyouOB]

is short-term heat -- less than 15 hours -- but at around 110-120 bad for the 2011 battery?

 

[DaveEV]

is short-term heat -- less than 15 hours -- but at around 110-120 bad for the 2011 battery?

How bad heat is is directly proportional to the temperature and time spent at that temperature thanks to Arrhenius' equation.

Every 10C rise in temperature doubles rate of capacity loss. 1 hour at 45C (113F) is similar to 4 hours at 25C (77F).

Catastrophic battery failure doesn't happen until temps get over 130F or something like that, presumably that's when the separator starts melting.

 

[dgpcolorado]

It is possible that since the battery heater wasn't designed for use at warmer temperatures that it could cause localized damage to cells if used that way. Those cells next to the heater elements might get quite a bit hotter than the rest of the battery (or the battery temperature sensors). At -4ºF to 14ºF that likely wouldn't be a problem. But at 50ºF it might be an issue.

Just guessing.