Capacity fade in EVs

[elpwr]

Tesla is plugged in and set at 50% SOC most of the time when unused. But almost daily I charge to 70-90% early in the morning for the days use. Try to charge enough to avoid the bottom 10-15% when I arrive home.

I don't think there's any need to be obsessive about this . But I think it's clear that minimizing use of the highest and lowest SOC is a good thing, if it's reasonably possible. Same would apply for storage, too.

I followed the same battery management philosophy with the Spark EV I just sold. Sub 100 mile EV's obviously stress the battery harder as you must use a large percentage of the battery capacity. However, my Spark battery degradation was also well below the levels reported by some others in the forums who were less careful with the usage patterns.

 

[oilerlord]

I don't think there's any need to be obsessive about this . But I think it's clear that minimizing use of the highest and lowest SOC is a good thing, if it's reasonably possible. Same would apply for storage, too.

I followed the same battery management philosophy with the Spark EV I just sold. my Spark battery degradation was also well below the levels reported by some others in the forums who were less careful with the usage patterns.

And there is that too. If obsessing of my battery is going to net an extra 3-4% over 80,000 miles - is it worth the time & effort worrying about it?

What's tough is that there is so much noise out there that it's difficult to find a definitive course of action for what's right or wrong. Scientific studies are a great place to start, but when you throw TMS/BMS from Ford, GM, and Tesla into the mix - how does that affect the outcome? What about guys like me that live in cold climates? Is "cold soaking" the battery bad? If I do the low-mid SOC as studies suggest, am I fighting my car's BMS, and actually causing more harm than good? So many variables.

You're taking a moderate approach to battery management that that aligns with the "don't burn the candle at both ends" principle. From the 100's of posts from other users that do this - the resulting claims are generally positive. Things are usually good in moderation.

 

[michael]

I posted an article that says it does. They presented test results and explained the underlying electrochemical mechanism. What is your basis for making that assertion??

This is my very point. A lot of people believe mid levels of charge are best for the battery but scientific evidence shows otherwise

What is your basis?? I have explained mine

The issue with that study might be that it appears they studied capacity fade when charged to a specific level and maintained at that level. They didn't look at discharge, so the two aspects aren't mutually exclusive. It could be that storing a battery is best at zero state of charge. However, it could also be that discharging down to 0% is worse than discharging down to 50%.

We need another study that looked at various discharge cycle depths and their impact on battery capacity. Then we could try to combine the two into the most optimal battery management plan.

Here's a paper the addresses exactly that, although in batteries with different chemistry

http://www.dtic.mil/cgi-bin/GetTRDoc?AD=ADA527711


They cycled batteries at -20, 0, +20, and +40 C. For our purposes, I think we can disregard the -20 data, possibly even the 0 data (I don't know how cold the Bolt TMS allows the battery to become...in the case of my Focus, no colder than about 10 C)

They compared cells cycled between:

0 and 100%
0 and 50%
25 and 75%
50 and 100%


The report states, in part....

During analysis of the data a series of trends stood out. The first being, whenever the cell was held at an elevated temperature both the capacity and power capability of the cell decreased significantly (Figures 3-5). The one notable exception is when the cell was maintained at 0% to 50% state-of-charge (Figure 6).


Except at the very lowest temperatures (which I think do not apply to EV batteries with TMS) the lowest range provided the best battery life.

At very low temperatures, mid range was better.

People will argue things like "It's a different type of battery" "The test is old" etc. etc. That all may be true, but it's the best actual test data I have seen. I will put that ahead of opinion that is not backed up by documented test data.

And I do agree, if one avoids high temp and high SOC, the remaining benefits are comparably small. But if whenever practical one follows the best available practice, it seems a wise approach.

Personally, I keep the battery at the lowest SOC that gives me the necessary range with comfortable reserve, and I charge "just in time" to minimize the time spent at higher SOC and higher temperature.

Based on these reports, I personally will not rely on the 90% hilltop limit as sufficient. I will take the effort to kludge up a charge limit like Tesla provides with its slider.

 

[michael]

I have no intention of arguing the point with you. There is abundant evidence to suggest that the extreme top and bottom SOC's are hard on the battery. Use them only when it is absolutely necessary. And not for storage.

Teslas battery longevity is recognized as being about the best. Tesla strongly recommends keeping the car plugged in at all times. The minimum SOC when plugged in is 50%. I do this with my model S. It is 3.5 years old and has 34,000 miles on it. The battery degradation (rated range reduction) is only 1.8%. I have experienced absolutely minimal battery degradation by avoiding the extreme SOC's where possible, and keeping the car plugged in at all times - as per the recommendation.

As I have tried to point out, estimates of Tesla battery fade are almost universally based on the "rated range" display. I am not aware how this is developed, clearly it's somehow an estimate of battery capacity. It is not an actual test of the battery.

The test I cited above did actual tests and showed considerably greater fade, even disregarding calendar and temperature effects. I am skeptical about the "rated range" estimates unless someone can show how these estimates are made and demonstrates that they correlate with actual tests of the battery.

All the Tesla fans rely on this dashboard estimate. Where is there an actual test? That's not a rhetorical question...I would really like to see test results that confirm or refute the "rated range" display.

I know you don't want to argue this. I'm not trying to be contentious, I want the best solution. Where is the "abundant evidence". I've provided the evidence I've discovered to the contrary. Please cite tests to support the "mid range" theory. Not something from some nameless author at "battery university" or the like. A real test...?

 

[boltage]

http://www.dtic.mil/cgi-bin/GetTRDoc?AD=ADA527711

They cycled batteries at -20, 0, +20, and +40 C. For our purposes, I think we can disregard the -20 data, possibly even the 0 data (I don't know how cold the Bolt TMS allows the battery to become...in the case of my Focus, no colder than about 10 C)

They compared cells cycled between:

0 and 100%
0 and 50%
25 and 75%
50 and 100%

Based on figure 1, at 20C, comparing the various charge levels cycled 600 times between, from least to most degradation:

0-50% (best)
25-75% (only slightly worse)
0-100% (worse at 0-100 cycles and 500-600 cycles, but see note)
50-100% (worst by far starting at 200 cycles)

However, it is worth noting that 0-100% means that you get double the energy stored and used as the other three scenarios (in an EV, this would be like getting twice the mileage out of a charging cycle). So if you compare the degradation for the 0-100% scenario at N cycles to one of the other scenarios at 2*N cycles (meaning the same number of miles if it were an EV), it compares more favorably than just looking at the number of cycles.

If your usual daily trips are less than half of your range, you want to avoid topping off every day, except when you have a longer trip planned.

For optimal battery life, you may want to charge up only to the amount needed for your next day's trip, plus whatever buffer you want to avoid range anxiety. It would be nice if an arbitrary charge level could be set (not just 100% and hilltop reserve which seems to be 87-90%), like Tesla offers.

But it also looks like if you just want to take good care of the battery conveniently, you can charge only when the level drops below what you need the next day (i.e. no topping off if there is still enough for next day's use), and use hilltop reserve mode except when you actually need the full range. Note that this can also reduce the number of charging cycles, as noted in the comparison of 0-100% versus the other scenarios above.

Obviously, your options are more limited with a short range EV, since you are more likely to actually need the full capacity, or need to charge every day with the battery mostly in the high end of its state of charge.

 

[elpwr]

Tesla STRONGLY (their caps) recommends that the car be always plugged in when not in use. When plugged in the minimum SOC available is held at 50%. Are you telling me that you know better than Tesla about the proper storage SOC?

I'm much more inclined to follow the manufactures instructions rather than the conclusions drawn by an amateur Internet poster.

Tesla rated range is the actual battery capacity multiplied by a constant. The constant depends on the battery size and weight of the car - 290 wh/ mi in the case of my S 85. The rated range is an excellent indication of the actual battery capacity. But apparently you want to debate that, too.

I've had minimal battery degradation in two entirely different EV's that I have owned for several years. I've done this by keeping the batteries in the general mid-level state of charge went practical. This battery management philosophy is in agreement with most manufacturers and has worked extremely well for me.

Feel free to use whatever techniques you deem appropriate. Hopefully you'll have better luck with your next EV then you did with the Focus.

 

[Posatronic]

I found an interesting study

http://www.myrav4ev.com/forum/viewtopic.php?f=8&t=561&p=13780&hilit=battery+fade#p13780

published on the Rav 4 EV forum.

The author purchased Panasonic cells similar or possibly identical to those used by Tesla, which are presumably also the same or similar to those used in your Mercedes.

He tested the cells in isolation, at room temperature, and subjected them to cycling equivalent to approximately 90K miles. He said this took "many months"

In his tests, fade was about 10% at 90K miles. However, these were at moderate temperatures, and they involved primarily cycling fade, not calendar fade. Typically, a 100K mile car would need 6 to 10 years, depending on usage.

If his tests are indicative, it tends to dispute the often quoted Tesla owner report that suggests about 10% per 100K miles, even with temperature and calendar effects.

Another interesting study

http://jes.ecsdl.org/content/164/1/A6066.full#F6

From the Journal of the Electrochemical Society looks at calendar fade in NCA batteries, similar to those used by Tesla. they basically charged up cells to various states of charge and stored them, at various temperatures, for 9 months. They then measured the battery capacity when fully charged.

Look in particular at Figure 8. It shows, as expected, greater fade at higher storage temperatures and states of charge. But below about 55% SOC, fade is very low and almost independent of SOC. Above 55 %, there is a step increase in fade rate, and the fade increases with higher and higher SOC.

This study gives results which are consistent with an earlier Army study which showed that the cells lasted better when cycled between 0% and 50% compared to cells cycled between 25% and 75% or between 50% and 100%. Many people believe that "mid level" is best for the batteries, but these two studies refute that belief. Lower is better than midway.

Hey first of all, great finds. I like all that data.
Second your interpretation of figure 8 seems a little off.
You should throw out 50c. But would like to see 12.5c, more realistic temptures with battery conditioning.
So looking at just 25c(ideal temp) biggest loss at first is 0-10%. Little more 10-20% then it levels off to 55% for all 4 lines. Then from 55-65 is the step your looking at and goes down and tapers off about level the rest of the way out. At 55% for average of all 4 lines at 25c is dropping .025 capacity. That seems very small. And does not suggest way lower
Than 50% is best. It looks like mid way is just fine, not a big enough difference to leave at 0 SOC. It's such a small difference that then I'd point(55% SOC) is just fine. Then the second big step after 0-10% is 55-65~.
So to me this says having the battery at approximately 55% is just fine and then having the car charged up right before you use the car seems to be the best. So in reality then setting in he Bolt for charging to 40% right away then charge the rest of the way just before you leave seems pretty accurate. ( assuming and based on others peoples "testing" of battery that the battery is bigger then 60kwh like 66-70)
Also if you add that the Prius uses their battery from 60-80% and have one of the best track record for their battery's. ( extremely low replacement of their batteries going bad)
The mid point, to stay charged , seeems very reasonable based on reasearch and then charge right before you leave.
Even the worst 25 c line after 9.6 months is only .05.
If you find more studies I would love to read them. I like to see lots of data.

My Focus Electric had liquid battery cooling, and when charging and operating it maintained a temperature right at 98 F (37 C). The system did not cool to anything approaching 25 C and certainly not 12 C.

I don't know how the Bolt TMS will work. But the Ford system, for all practical purposes would heat up to 98F when charging or driving, and would remain very close to that for many hours, due to insulation and large thermal mass.

The point I would like to make is that many people believe the battery does best if charged and discharged above and below the midpoint. This research, together with the Army paper and the writings of Jeff Dahn suggest that the centroid should be lower. The battery has the lowest degradation at zero SOC, not at 50%.

I think the fact that a grossly over-discharged battery will be damaged suggests to many people that a battery at a very low SOC is "unhappy". As best I can see, it's very happy.

No the research does not. (Not real world also) technically but by such a small margin it's not worth noting. Mid point is ok( real world and lab). My Prius uses 60-80% and their track record is outstanding for batteries. But that also is not the same as using 100% battery.
I'll keep reading the research but meanwhile I'll drive the car. I won't let it sit at 0% and never drive the car because there is a .05% if I charge it to 55%.
Old Chinese proverb:
You know research shows an ICE engine that never is used with never wear out.

 

[LeftieBiker]

My Prius uses 60-80% and their track record is outstanding for batteries.

Not to argue for the opposition here, but your Prius, unless it's a PIP or Eco model, uses a NiMH battery pack. Very different chemistry, and NiMH packs hate to be very high or very low in charge - more so than lithium. Toyota is pretty much the only company that has had long term success with NiMH packs, and it's exactly because of that limited SOC range imposed by the BMS, probably along with higher build quality.

 

[oilerlord]

Tesla STRONGLY (their caps) recommends that the car be always plugged in when not in use. When plugged in the minimum SOC available is held at 50%. Are you telling me that you know better than Tesla about the proper storage SOC?

I'm much more inclined to follow the manufactures instructions rather than the conclusions drawn by an amateur Internet poster.

Tesla rated range is the actual battery capacity multiplied by a constant. The constant depends on the battery size and weight of the car - 290 wh/ mi in the case of my S 85. The rated range is an excellent indication of the actual battery capacity. But apparently you want to debate that, too.

We get that Tesla (strongly) recommends the always be plugged in - but we also know there is a SOC slider that allows the user to set the SOC maximum. This would suggest (at least to me) Tesla also recognizes there is benefit to charging lower than the car's default setting or else it wouldn't be there. Michael (quite politely) asked you to post research and/or data that backs up your argument, and I believe he is genuinely interested in reading it. I like reading all I can, and have read several (usually very dry) scientific studies on the subject of battery degradation. We're all free to either believe, support, or refute research we find. If you have a scientific study, and/or empirical data, that aligns with your point of view; feel free to post it. For me, I'd like to know the underlying why Tesla makes the recommendations they do, there's nothing wrong with asking questions. Knowledge is good.

 

[Nagorak]

No the research does not. (Not real world also) technically but by such a small margin it's not worth noting. Mid point is ok( real world and lab). My Prius uses 60-80% and their track record is outstanding for batteries. But that also is not the same as using 100% battery.
I'll keep reading the research but meanwhile I'll drive the car. I won't let it sit at 0% and never drive the car because there is a .05% if I charge it to 55%.
Old Chinese proverb:
You know research shows an ICE engine that never is used with never wear out.

My Prius uses 60-80% and their track record is outstanding for batteries.

Not to argue for the opposition here, but your Prius, unless it's a PIP or Eco model, uses a NiMH battery pack. Very different chemistry, and NiMH packs hate to be very high or very low in charge - more so than lithium. Toyota is pretty much the only company that has had long term success with NiMH packs, and it's exactly because of that limited SOC range imposed by the BMS, probably along with higher build quality.

Yeah, we should figure out what the Gen 1 Honda Insight did, because whatever it was, it was crap, and we should do the exact opposite! :lol:

If I had enough money I'd lease two Bolts and drive them about equally, with one set to only charge to 40% (instant recharge setting, with the charge timer set to only 1 minute so it was effectively disabled), and the other charged to Hilltop mode. Then we could see how they stacked up. Sadly that's not too practical for me to do. I could probably afford the two leases, but I wouldn't want to spend an extra $350 a month just to run an experiment.

 

[michael]

Your test would be great, but as I've suggested, one doesn't need to test the whole car, just samples of cells maintained in a similar environment.

The tests that people have already done don't seem to convince people, who give more weight to the Tesla owner's manual. And in fairness, I haven't yet found a report that exactly corresponds to the test conditions you are proposing. I really wish NREL had done capacity tests on a Tesla in the way they have done it on other cars.

I can tell you that if you go back three years on the Focus forum, you will find lots of people who believed that since Ford also said, "keep it plugged in..." this was the best procedure, Three years later, reports of battery fade are surfacing. Since I was one of the highest mileage people on that board, I was the first to report fade, and people still kept saying "Well, my car is fine..." You must have been mistreating yours. Nope. I was driving it.



Their cars aren't fine now.

 

[boltage]

Perhaps if Teslas fade less per mile than Focuses, it could be due to fewer recharge cycles. For example, if you drive 40 miles every day, then you need to recharge a 208 mile EV once every 3 or 4 days, but you need to recharge a 76 mile EV every day.

So if you drive that 40 miles 300 times per year, then you will put the 208 mile EV through 75 to 100 charging cycles, but put a 76 mile EV through 300 charging cycles. I.e. the 76 mile EV may have 3 to 4 times the battery "wear" as the 208 mile EV.

 

[michael]

Yes, that is for sure. That's one of the great advantages of a big battery. Much less cycling fade.

The rate of calendar fade, however, is the same.

Bolt will of course have the big-battery advantage over all the 80 mile class cars.

 

[devbolt]

My Prius uses 60-80% and their track record is outstanding for batteries.

Not to argue for the opposition here, but your Prius, unless it's a PIP or Eco model, uses a NiMH battery pack. Very different chemistry, and NiMH packs hate to be very high or very low in charge - more so than lithium. Toyota is pretty much the only company that has had long term success with NiMH packs, and it's exactly because of that limited SOC range imposed by the BMS, probably along with higher build quality.

The second and third generation liftback Prius models all used a NiMH pack where the SOC was kept between 40% and 80%. The PiP was the only third generation model to use a Lithium-Ion pack, which used the SOC between 23% and 85% for EV mode, and between 18% and 28% for HV mode (the two modes have some overlap depending upon the charge/discharge direction).

In the 4th generation Prius, only the low end Prius Two has a NiMH pack, all of the other ones, including the Two Eco, have a Lithium Ion pack. I do not know what the usable SOC is for the Lithium-Ion packs in the 4th generation Prius.

 

[LeftieBiker]

Thanks for that. We have a 2013 PIP, and maybe I'll tell my housemate to not worry anymore about leaving it fully charged in cooler weather. 85% should be fine.

 

[oilerlord]

Found this interesting:

"NASA reports that once Li-ion passes the 8 year mark after having delivered about 40,000 cycles in a satellite, cell deterioration caused by this phenomenon progresses quickly. Charging to 3.92V/cell appears to provide the best compromise in term of maximum longevity, but this reduces the capacity to only about 60 percent. (See BU-808: How to Prolong Lithium-based Batteries)"

http://batteryuniversity.com/learn/article/bu_808b_what_causes_li_ion_to_die

In this particular instance, the sweet spot appears to be charging to 60% SOC.

 

[SeanNelson]

"NASA reports that once Li-ion passes the 8 year mark after having delivered about 40,000 cycles in a satellite, cell deterioration caused by this phenomenon progresses quickly. Charging to 3.92V/cell appears to provide the best compromise in term of maximum longevity, but this reduces the capacity to only about 60 percent. (See BU-808: How to Prolong Lithium-based Batteries)"

In this particular instance, the sweet spot appears to be charging to 60% SOC.

That would seem to be of limited applicability to car owners. If you charged your car daily it would take over 100 years to reach 40,000 charge/discharge cycles. I suspect that it would be challenging to tease the effects of the very high charge/discharge rate from the calendar deterioration in that kind of usage. And of course there are a lot of environmental extremes in space (such as large temperature swings) that can also play a role.

 

[Nagorak]

It still does seem to suggest that in very harsh temperature conditions with an extreme duty cycle, keeping a lower charge helps. Basically it's just one more data point that suggests a lower charge results in more longevity.