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surfingslovak
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Re: Optimal Portion of Battery to Use for Longest Battery Li

Sun Aug 28, 2011 10:24 am

ericsf wrote:When it comes to chemistry, properties of elements are very difficult to "guess". Look at carbon. Could you guess that diamonds and coal are made of the same thing?

Eric, I appreciate your response and your thoughtful post. What I see are fundamentally two problems, there is no research data available for Leaf's AESC cells and the information Nissan and NEC provide is extremely sparse. The second problem is the lack of battery capacity warranty. If Nissan wishes to provide very simple battery care guidance, that's fine, so long they back up their words with a warranty and reward the owner for adhering to their recommendations.

The posters on this forum come from a wide variety of backgrounds and what you say is perhaps applicable to someone who does not have strong engineering roots or was never fond of chemistry classes. Although I'm not the most experienced poster when it comes to battery care on this forum, please rest assured that I'm taking this approach as scientifically as I can and I'm certainly not "guessing". Unfortunately, unless someone buys a few AESC modules and conducts rigorous testing, everything we say is a matter of opinion. I'm more inclined to believe experienced posters with EV and battery research background than anyone else and the consensus there is not to worry about the battery. While that's the most likely scenario, I'm finding it really sad that Nissan does not provide more adequate battery warranty. That alone can put an end to the wide-spread angst among the less experienced folks and new adopters of this technology.

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Re: Optimal Portion of Battery to Use for Longest Battery Li

Sun Aug 28, 2011 11:08 am

Good information from all posters, but does anyone know of a Lithium ion battery chemistry that does WORSE by shallow cycling the battery and trying to keep the average SOC around 50%? If there aren't any current examples, then it seems unlikely this would be problematic for this particular battery chemistry.

Also, what information is available for other Lithium ion chemistries re: depth of discharge (DOD) for useful cycle life? I know the useful cycle life goes down dramatically if you overcharge the battery, but at what point do you not get any increase in cycle life? If 80% to 30% (what I do on a typical commute day) is the point at which cycle life can't be increased, then I am already doing the best I can. However, if a 30% DOD gives a longer cycle life, then it would pay for me not to charge to 80% on my non-commute days.
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surfingslovak
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Re: Optimal Portion of Battery to Use for Longest Battery Li

Sun Aug 28, 2011 11:37 am

Stoaty wrote:Good information from all posters, but does anyone know of a Lithium ion battery chemistry that does WORSE by shallow cycling the battery and trying to keep the average SOC around 50%? If there aren't any current examples, then it seems unlikely this would be problematic for this particular battery chemistry.

The closest thing I found was a JPL report for the Mars Rover mission. They found the capacity fade from cycling to be about six times higher at 60% DOD when compared to 30% DOD. They used SAFT LiNiO2 cells with graphite anode and cylindrical stainless steel hardware. The cells were tested in 30% DOD regime (5000 cycles) with average energy fade rate at 4.0V at 0.000704% per cycle and 60% DOD regime (500 cycles) with average energy fade rate at 4.0V at 0.00430% per cycle.

There are other reports as well, some of them for lithium manganese cells even, but I could only find fairly coarse graphs. The JPL report was the only one that spelled out the exact amount of capacity fade per cycle with different DOD regimes. I don't have a lot of data to quantify this statement, but based on other observations, the SAFT cells used for the Mars Rover mission were durable and very well built. Even though we have the benefit of a decade of research and development, I would be surprised if Leaf's cell were an order of magnitude better in terms of cycling losses.

That being said, the other data I looked at and the statements from other posters, lead me to believe that calendar life will be more relevant for Leaf owners than capacity fade from cycling.

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Re: Optimal Portion of Battery to Use for Longest Battery Li

Sun Aug 28, 2011 11:54 am

What I see are fundamentally two problems, there is no research data available for Leaf's AESC cells and the information Nissan and NEC provide is extremely sparse. The second problem is the lack of battery capacity warranty.


I totally agree with you about the second problem. The battery warranty section of the manual does talk about "normal" decrease but doesn't say a word about that is "normal". It's very frustrating and it's probably why all the threads on this topic get so long and passionate.

About the first problem, I don't think it's reasonnable to expect Nissan or NEC to disclose information about their design. This is a competive industry and they are in to beat their competition and make money. I can only see drawbacks for them to publish more information.

Appologies if I sounded patronizing or lecturing.

My opinion is that it's virtually impossible for us to find out the answer to the question asked in this thread (what is the best use patern to increase battery longevity) without the full knowledge of what's going on inside the LEAF's battery pack. We seem to disagree on this but I find the information you provided very interesting and I hope you're enjoing this discussion as much as I do :-)
SL w/ QC (3/5/2011) - 11bars on 1/23/2014 @ 54k miles - 10 on 12/23/2014 @ 73k - 9 on 10/5/2015 @ 85k - 8 on 7/3/2016 @ 96k

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surfingslovak
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Re: Optimal Portion of Battery to Use for Longest Battery Li

Sun Aug 28, 2011 12:06 pm

ericsf wrote:My opinion is that it's virtually impossible for us to find out the answer to the question asked in this thread (what is the best use patern to increase battery longevity) without the full knowledge of what's going on inside the LEAF's battery pack. We seem to disagree on this but I find the information you provided very interesting and I hope you're enjoing this discussion as much as I do :-)

Absolutely, thank you for your response :-) You were right and those points needed to be raised. I came to regard Leaf ownership primarily as a learning experience, and an introduction to the world of EVs. I'm indebted to the people I have met on this forum and at our monthly gatherings. I wish this website had better search capabilities, since some of the hotly contested topics have been discussed over the course of 12 months and it's quite educational to review them with the benefit of hindsight.

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planet4ever
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Re: Optimal Portion of Battery to Use for Longest Battery Li

Sun Aug 28, 2011 3:53 pm

surfingslovak wrote:I wish this website had better search capabilities, since some of the hotly contested topics have been discussed over the course of 12 months and it's quite educational to review them with the benefit of hindsight.

Better than Google? And you would propose?

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ericsf
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Re: Optimal Portion of Battery to Use for Longest Battery Li

Sun Aug 28, 2011 6:46 pm

The closest thing I found was a JPL report for the Mars Rover mission. They found the capacity fade from cycling to be about six times higher at 60% DOD when compared to 30% DOD. They used SAFT LiNiO2 cells with graphite anode and cylindrical stainless steel hardware. The cells were tested in 30% DOD regime (5000 cycles) with average energy fade rate at 4.0V at 0.000704% per cycle and 60% DOD regime (500 cycles) with average energy fade rate at 4.0V at 0.00430% per cycle.

The battery chemistry NASA seem to have used look to me like one is a Li-phosphate and the other is an NMC type. See:http://batteryuniversity.com/learn/article/lithium_based_batteries. Apparently those are both rated with twice the life cycle of Li-manganese. So let's assume that the LEAF's battery would perform twice worse than this and see what we get:

Let's see what driving 100,000 miles (the LEAF's battery warranty) in each case would do. At 30% DOD it would take 4567 cycles to drive that far. In that case the battery capacity would decrease by 6.4% (used the NASA decrease rate of 0.000704% multipled by 2 to account for the chemistry difference). At 60% DOD it takes 2283 cyles to drive 100,000 miles. In that case the capacity would have dropped by 19.6%. That's *only* 3 times more than the 30% DOD scenario.

I find several things interesting :

1) What you found (the NASA study) clearly shows that shallow discharge cycles (less than 30% DOD) is much better than deeper discharges (more than 60%) when it comes to battery longevity. When considering the number of miles driven instead of the # of cycles, the difference between 30% DOD and 60% DOD less dramatic but still very significant.

2) By barely massaging the NASA numbers to account for a factor 2 performance difference in longevity between their batteries and LEAF's batteries the results are pretty darn close to Nissan's claim that after 8 years most batteries would still have 80% of their original capacity : 2283 cycles (what it takes to drive 100,000 miles with 60%DOD) would take 6.25 years at 1 cycle per day... or 8.75 years at 5 cycles per week.

3) Finally, this other source (http://www.gpina.com/pdf/highpower-cat.pdf) which specifically rates Li-Manganate batteries gives about 85% capacity remaining after 1000 cycles @ 70% DOD. With the *massaged* NASA numbers the same number of cycles would get 91.4% capacity @ 60 % DOD... That's consistent if we agree that 10% DOD difference can explain the 6.4% increase in capacity loss.

I really hope that in a few years Nissan will publish the data they will get from all the LEAF's battery and how they were used so that we can finally know if we are even close :geek:
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Re: Optimal Portion of Battery to Use for Longest Battery Li

Sun Aug 28, 2011 7:29 pm

ericsf wrote:1) What you found (the NASA study) clearly shows that shallow discharge cycles (less than 30% DOD) is much better than deeper discharges (more than 60%) when it comes to battery longevity. When considering the number of miles driven instead of the # of cycles, the difference between 30% DOD and 60% DOD less dramatic but still very significant.
Does shallow discharges of 30% DOD mean in the Leaf scenario going from 80% SOC down to 50% SOC, then recharging back to 80% afterward?

As opposed to deep discharges of 60% DOD means in the Leaf scenario going from 80% SOC down to 50% SOC, then instead of charging back up to 80% SOC, you leave it alone and discharge it another 30% so you have 20% SOC left before recharging to 80% SOC again?

So in effect this relates to our current discussion of recharging to 80% SOC asap as opposed to not recharging to 80% SOC asap, but keeping it in 40-50% SOC for as long as possible?

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Re: Optimal Portion of Battery to Use for Longest Battery Li

Sun Aug 28, 2011 7:44 pm

I do not believe that leaving the car idle for a while (even days) before driving it again counts as 2 shallow discharges. My understanding of the DOD is that it's the SOC difference between 2 recharge cycles. So your second example (80% SOC to 50% - stop then drive to 20% SOC) would be considered as one 60% DOD.

For me it's clear that it's way better for the battery to recharge from 50% SOC back to 80% if you can instead of keeping it at 50% SOC and driving further down to 20% SOC.
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Re: Optimal Portion of Battery to Use for Longest Battery Li

Sun Aug 28, 2011 8:05 pm

ericsf wrote:I do not believe that leaving the car idle for a while (even days) before driving it again counts as 2 shallow discharges. My understanding of the DOD is that it's the SOC difference between 2 recharge cycles. So your second example (80% SOC to 50% - stop then drive to 20% SOC) would be considered as one 60% DOD.

For me it's clear that it's way better for the battery to recharge from 50% SOC back to 80% if you can instead of keeping it at 50% SOC and driving further down to 20% SOC.


Hope you guys are right. At first I'd charge to 100% on a weekend and recharge just once at the end of the day. Last few weeks I've gone to 80% charging regardless of the day of week, and topping off whenever there is a 2 bar deficit or more from 80% at the weekends. Unfortunately I don't have the opportunity to charge at work, so I user 50% each weekday, sometimes more.
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