Optimal charging at 20% SOC, 6 temp bars, cool ambient temp

The reason for this post is that I demand a lot of my LEAF, I own the car outright, and am willing to go out of my way to prolong the life of its battery pack.

My question is, at a non-optimal, low SOC of 20% (or sometimes even a tad under LBW), would be better for battery pack longevity to let it cool upon arrival at home, rather than charging immediately? Some evenings I must charge upon arrival because of additional driving that we do after I get home from work. However, there are many evenings I could let the car cool first.

These days I've been arriving home from work (after a ~5000' mountain climb) with roughly 20% SOC (one bar of charge for those without an SOC meter), six temperature bars, and an ambient temperature around 70°F. Normally I plug in and start charging upon arrival, manually stop the charge at roughly 40%, and use a timer to complete the charge early the next morning shortly before leaving for work. My guesstimate is that the battery pack temperature is at minimum 85°F when I arrive home. Thanks to my relatively cool mountain location, the pack always drops down to five temperature bars overnight, and does not typically reach 6 temp bars until later in the day thanks to my access to shaded parking at work in the hot Valley.

Based on recent data from Arizona and Texas owners, it now seems that high temperatures are much more significant in terms of pack degradation than SOC. On the other hand, the negative effects of letting the car sit at a non-optimal low or high SOC could be amplified by higher temperatures.

my understanding is that 30% is the dividing line for longish term battery storage. you could charge up to 30-50%, stop manually and then let the end timer finish the job. the amount of battery warming from L2 charging in the middle of the pack is miniscule.

I've been wanting a whole new charging system lately. I, like you have decided to be extra nice to the battery. I'm doing this in part because I want to make up for the fact that I plan to quick charge many times a day on rare occasion to make it to far away places (so far my record is 7 partial QC's in a day with nominal warming). I wish there was a simple way to delay initial charging a bit. we need multiple charging timers that will all work together and can easily be tweaked a bit. a charging station that keeps the times on the main screen with an up down arrow right there for you to adjust as you plug in would be good. manually starting and stopping charging is a bore and it's easy to forget to stop the initial charge and end up at 80 or 100% (did that last night). I do wonder if we are being too pedantic about this? Is the benefit so fractional compared to ambient temps that it's not worth the hassle? if only we knew where the limit is with temp/charging and battery longevity!

GaslessInSeattle said:

my understanding is that 30% is the dividing line for longish term battery storage. you could charge up to 30-50%, stop manually and then let the end timer finish the job. the amount of battery warming from L2 charging in the middle of the pack is miniscule.

I've been wanting a whole new charging system lately. I, like you have decided to be extra nice to the battery. I'm doing this in part because I want to make up for the fact that I plan to quick charge many times a day on rare occasion to make it to far away places (so far my record is 7 partial QC's in a day with nominal warming). I wish there was a simple way to delay initial charging a bit. we need multiple charging timers that will all work together and can easily be tweaked a bit. a charging station that keeps the times on the main screen with an up down arrow right there for you to adjust as you plug in would be good. manually starting and stopping charging is a bore and it's easy to forget to stop the initial charge and end up at 80 or 100% (did that last night). I do wonder if we are being too pedantic about this? Is the benefit so fractional compared to ambient temps that it's not worth the hassle? if only we knew where the limit is with temp/charging and battery longevity!

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+1. And sounds like a chris1howell project to me ... hint ... hint

Six temperature Bars is roughly 74 to 98 degrees F.

If you charge at 75 ºF, probably not an issue,
but at 98 ºF you might want to wait a bit.

Experiment:
1. via the back-seat Emergency Sht-Off hatch, take temperature readings,
and try to calibrate them with the 5-to-6 Bar transition (74.5 ºF).

2. When charging at home (I presume L2, 240v 16A),
take temp. readings before and after charging.

3. Report your findings.

4. If the temp. gain during L2 charging keeps you under ... guess 85 ºF ...
then you (most likely) have little to worry about, and
charging convenience (or TOD costs) might be more important factors.

I bought a small, digital-readout, IR temp meter from Harbor Freight.
I should try using it.

abasile said:

My question is, at a non-optimal, low SOC of 20% (or sometimes even a tad under LBW), would be better for battery pack longevity to let it cool upon arrival at home, rather than charging immediately?

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surfingslovak posted this chart elsewhere that shows the effect of SOC on battery degradation on LiMN batteries.



Anyway - you can see that the difference between 20-40% is nominal. I am curious as to why the dissolve rate goes down above 80% SOC, though! I wonder how different the chart might look with more data-points.

I would think that the difference between sitting at 20% until your normal charge start time and charging an hour to 40% is probably negligible. It might make a bigger difference if the battery is below VLBW - at which point I might consider charging an 60-90 minutes, but IMO unless that happens regularly, I wouldn't sweat it.

Yeah I'm a bit alarmed that degradation peaks at 80%. I would have thought it would peak at 100%. Is this why it's recommended to set ONLY an end time for timer charges - to minimize the amount of time the battery remains at 80% SOC?

dandrewk said:

Yeah I'm a bit alarmed that degradation peaks at 80%. I would have thought it would peak at 100%. Is this why it's recommended to set ONLY an end time for timer charges - to minimize the amount of time the battery remains at 80% SOC?

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The amount of dissolved Mn as plotted by the chart isn't the only factor in degradation since it's well proven that in general the higher the SOC above ~40% the faster the battery degrades.

The point of the end-timer is to minimize the amount of time spent at high SOC - whether that's 80% or 100% doesn't matter - more time spent at lower SOC should = more battery life (though temperature appears to be the overriding factor).

Given that Leaf has 24kWh (I was told in this forum), then wouldn't an 100% usable charge of 21 kWh is actually ony 87%, and an 80% charge (which is 16.8kWh) in reality only 70% ?

i would take Gary's suggestion "partially" in that if concerned, remove the back seat, open the hatch. this allows hot air to vent out and should cool the pack "slightly" quicker.

other than that, you are charging at 240 and based on your elevation, guessing you never charge beyond 80%, i think you are over thinking the process. just plug it in, time it to be done just before you leave the next morning.

one thing i might do is charge right away in case you have an errand you want to run locally.

i personally think the risk of sitting at a low SOC for what would essentially be no more than 6-8 hours is very minimal along with sitting at 80% for the same

The pack is under the car and sealed so I don't believe it would make any practical difference.

DaveinOlyWA said:

i would take Gary's suggestion "partially" in that if concerned, remove the back seat, open the hatch. this allows hot air to vent out and should cool the pack "slightly" quicker.

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When I arrive home, I am closer to 10%. I have been considering installing a hot water timer inline with my AV charger to cut the power for 30 minutes about 2 hours after I typically arrive home. Then, when I get home I just have to hit the override to charge until the water timer cuts it off which should get me above the 30% line. Then the car will finish in the morning since I set the END timer only. *I think* this will work - I know that if I come out and unplug and replug it in it will stop the manual override and revert to the timer settings. Seems reasonable that a power outage will have the same effect.

mkjayakumar said:

Given that Leaf has 24kWh (I was told in this forum), then wouldn't an 100% usable charge of 21 kWh is actually ony 87%, and an 80% charge (which is 16.8kWh) in reality only 70% ?

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I believe that Phil managed to get exact SOC readings out of the battery ECU and determined that full charge corresponded to 94 or 95%. 80% charge is what it says, and not less, as many of us have previously assumed.

In addition to what's been already said, please consider that the charge sustaining mode on the Volt kicks in around 22 or 23% SOC. Presumably, you can drive the vehicle in this mode for months on end, and GM engineers thought that this SOC offered a good trade-off between range and battery longevity. I would not be concerned to let the Leaf sit at 20%, but if it was parked for an extended period of time (days and weeks), I would bring the SOC above 35% if possible.

The charts we are looking at are usually for generic LnMnO4 cells and they don't apply 100% to the Leaf. I would use them to get an idea of the characteristics and behavior of these cells, not an exact prediction of how the Leaf will behave. In addition to manganese dissolution in electrolyte Dave posted above, there are other processes that degrade the battery. I believe that at low SOC, Jahn-Teller effect will start impacting the spinel structure of the cell.

The owners manual (page EV-23) and these studies give us an idea of the speed of these processes as well. Dissolution into electrolyte, which seems to be dominant at high SOC, is typically measured in a matter of days. Spinel distortion due to Jahn-Teller effect is measured in a matter of weeks.

As a practical recommendation for a daily routine, I would try to keep the Leaf above the low battery warning (17% SOC). Dipping down to the very low battery warning (8% SOC) should be no problem, but you might not want to do it as part of your daily routine if possible (Luke is probably exempt from this). Turtle mode appears to be set fairly conservatively from a cell voltage perspective, but the SOC is so low that I would be concerned, and would not recommend doing that on a regular basis.

If you look at the annual battery report that Phil posted earlier, Nissan keeps track of the number of turtle events and the number of hours spent at high SOC. It's pretty clear that they are doing this, because they think that it will impact battery performance. Note that they are lumping low SOC (0-20%) into one band and high SOC (>85%) into another (PKTIMEWHENHIGHSOCAFTERCHARGE, PKTIMEWHENHIGHSOCAFTERDRIVE, SOCATCG and TURTLELAMP).

LEAFer said:

+1. And sounds like a chris1howell project to me ... hint ... hint

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I have been thinking about changing the charge rate over time... Kinda backwards from this topic. For Example on a 80A breaker you can use up to 80A non-continuous. If the load is continuous (over 3 hours) than you can use only 80% of the breaker value. So I was thinking of adding a mode to charging at 100% of the breaker rating for the first 2 hours and 59 minutes and then drop to 80%.

With OpenEVSE it would be pretty easy to come up with a charge rate curve based on time or just simply charge for 2 hours then go to ready to wait for the charge timer. There is already a charge timer running during charge on OpenEVSE.

chris1howell said:

I have been thinking about changing the charge rate over time... Kinda backwards from this topic. For Example on a 80A breaker you can use up to 80A non-continuous. If the load is continuous (over 3 hours) than you can use only 80% of the breaker value. So I was thinking of adding a mode to charging at 100% of the breaker rating for the first 2 hours and 59 minutes and then drop to 80%.

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That sounds interesting, particularly for a L1 charger. Chris, what is involved in the EVSE changing the charging current while charging? Does it require the EVSE to terminate the charge and then submit a new pilot, or is there some way to renegotiate the current level while the charging is ongoing? (Sorry, I'm just not familiar with the protocols.)

TIA!

RegGuheert said:

what is involved in the EVSE changing the charging current while charging? Does it require the EVSE to terminate the charge and then submit a new pilot, or is there some way to renegotiate the current level while the charging is ongoing?

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The EVSE changes the pilot duty cycle and the EV has 5 seconds to comply.

chris1howell said:

The EVSE changes the pilot duty cycle and the EV has 5 seconds to comply.

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O.K. Sounds straightforward. Thanks!

Thank you for the responses, particularly surfingslovak's detailed and informative reply! Trying to gauge the actual battery temperature, as suggested by garygid, is not a bad idea.

In general, it seems sensible for daily driving to try to use the LEAF's battery within parameters similar to those imposed by the more conservative Volt. In terms of SOC, my commute is pretty close to that, as my daily SOC range is roughly 20% - 80%. Beyond that, one can "safely" charge to 100% daily if charging is completed shortly before driving. Based on Tony Williams' experience turtling his 2011 LEAF on many occasions and subsequent capacity loss, however, it indeed seems like a very bad idea to frequently use the very low end of the battery.

Back to my original question, I think I'll continue charging to ~40% upon arrival from work, even if only to have some reserve in case we decide to go somewhere that evening. But if I have reason to believe the pack is particularly hot (closer to 98 F), I'll probably hold off on charging.

surfingslovak said:

mkjayakumar said:

Given that Leaf has 24kWh (I was told in this forum), then wouldn't an 100% usable charge of 21 kWh is actually ony 87%, and an 80% charge (which is 16.8kWh) in reality only 70% ?

Click to expand...

I believe that Phil managed to get exact SOC readings out of the battery ECU and determined that full charge corresponded to 94 or 95%. 80% charge is what it says, and not less, as many of us have previously assumed.

Click to expand...

The other piece of this that mkjayakumar may have overlooked is that zero usable charge (dead turtle) is not 0 kWh in the battery. So if you have 21 kWh usable it might be from 2 kWh minimum to 23 kWh maximum. (I'm not claiming those are the real numbers - just pointing out that the origin is not really zero, and that messes up assumptions you may have on % calculations.)

Ray

surfingslovak said:

If you look at the annual battery report that Phil posted earlier, Nissan keeps track of the number of turtle events and the number of hours spent at high SOC. It's pretty clear that they are doing this, because they think that it will impact battery performance.

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Battery Life.. but otherwise what you said

Keep the battery as cold as possible and in the 20-80% range for longevity, and never fast charge when the battery is near human body temperature.

Per Phil a GID is the stored power in the battery, full charge at 281 GID (95%) and contactor opens at 5 GID (2%).. so a total of 276 GID is stored when the car is new, 22kWh. If the computer allowed us to charge up to 300 GID then the total stored energy would be 24kWh, battery life would be very short.

Thus 22kWh is the usable energy when the car is new... but note that not all of that will end up on the road due to motor and inverter losses, but those are dependent on how you drive thus different for everyone.