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Behaviour of cells in parallel take 2
#31
I actually picked those two at random from the cell database, but I was looking for 2 fairly common cells with similar capacity and different nominal voltages. I have no idea if they have similar IR ratings from the factory, so hopefully they do. I'm looking forward to your test results!
Check out my long-term capacity test of 18650s: https://secondlifestorage.com/showthread.php?tid=6868
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#32
Don't forget to consider this :

"With all batteries, SoC affects the internal resistance. Li-ion has higher resistance at full charge and at end of discharge with a big flat low resistance area in the middle."

https://batteryuniversity.com/learn/arti...resistance

With second life cells these variations in IR vs SoC may create other issues that single reading IR values miss...

Like the data and testing as it is this sort of work, experimentation and research that uncovers the reality... ++1
If you can't quantify how much they cost, it's a deal, I'll buy 5 of them for 3 lumps of rocking horse ......
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#33
(06-20-2020, 04:00 AM)Generic Wrote: I actually picked those two at random from the cell database, but I was looking for 2 fairly common cells with similar capacity and different nominal voltages. I have no idea if they have similar IR ratings from the factory, so hopefully they do. I'm looking forward to your test results!

Well here are the results of these cells tested.
Quite normal results of 2 reasonably decent batteries. I do not believe the Nom V had much influence on anything.
Wolf

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#34
Thank you Wolf!! During the discharge, right around 3.6V, cell 1 and 3 (3.6V cells) started to contribute more current than cells 2 and 4 (3.7V cells). However, the increase in current was not as dramatic as I expected. The 3.7V cells were around 0.9 Amps and the 3.6V cells were around 1.15 Amps. Likewise, on the recharge, the 3.6V cells took about 1.25 Amps each vs. the 3.7V cells 0.75 Amps each initially, before slowly reversing as more charge was absorbed.

So, nominal voltage makes a small difference, but not enough to cause concern when assembling packs, as long as there is about a 50-50 distribution within the packs.
Check out my long-term capacity test of 18650s: https://secondlifestorage.com/showthread.php?tid=6868
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#35
Worth emphasis: even if you match the IR and capacity well, unlike cells in packs can still end up quite imbalanced by the end of a discharge due to chemistry mismatches - which can lead to very different voltage profiles under load, e.g. see the graph below. This means that some cells will be under much more stress than others near end of discharge, so they will degrade much faster than others (and this is a positive feedback loop, which ends up knocking down the weakest cells in a domino effect, till the capacity of the entire pack is brought down).

E.g. below note that at 3.6V the LG has about 44% left but the Sanyo only has about 14%. so even if their voltages are matched their SOCs may be way off (but precisely how they balance under load will depend on many factors. e.g. IR, temp, etc)
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#36
(07-19-2020, 11:41 PM)gauss163 Wrote: Worth emphasis: even if you match the IR and capacity well, unlike cells in packs can still end up quite imbalanced by the end of a discharge due to chemistry mismatches - which can lead to very different voltage profiles under load..............
@gauss163

That is very true hence we try to encourage powerwall users to charge to ~ 4.0V and discharge to a maximum minimum of ~ 3.4 as the cells tend to wander at that point even if they are the same manufacturer, same part number, same production date, same capacity, very close IR etc.
I also encourage IR differences no greater than 15mΩ and capacity differences of no more than 400mA and if possible even less than that in a pack build.
No sure if you have looked at my Harvested cell analysis sheet with over 6000 cells recorded. Out of those 6000 used and tested cells I was able to get
1200 cells that passed my stringent tests of 40mΩ to 65mΩ and 2250mAh to 2650mAh with a minimum 85% SOH. I also placed same manufacturer and part number cells next to each other in the pack so that they would react similarly in a group.
If you have looked at all my traces it shows how cells behave with different neighbors having a small difference of capacity and IR and large differences of IR and capacity and a combination of both. I have not seen another study like that anywhere but I may be mistaken. Discharge curves are going to be different for each cell especially different chemistry such as ICR chemistry is going to discharge different than INR NCA IMR. That we do know. Again I do not personally advocate mixing high drain and low drain cell in a pack.

The 2 batteries shown in your chart are more than likely 2 different chemistries I can find the LG18650HE4 and it is a high drain low IR cell. Designed for power tool packs.
http://www.batteryspace.com/prod-specs/10679.pdf

I have to guess at the Sanyo though as it is just (Red) no identifiers of the manufacturer part number and yes I did look at that chart and  this website is already in my favorites anyway so  an educated guess is that it is a ICR chemistry made for laptops, with a much higher IR. My guess it is a UR18650FM  as the discharge curve is strikingly close to the one displayed above.

So kind of comparing apples and oranges and I have done those test also coming to the conclusion that it really isn't the best practice to mix chemistries.


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#37
(07-20-2020, 03:04 AM)Wolf Wrote: [...] So kind of comparing apples and oranges

But that's exactly my point - when you mix very different chemistries in a pack they are less likely to remain comparably balanced during discharge due to their very different voltage profiles.

Re: your remark on energy vs power cells: this doesn't really matter here since the same voltage profile differences can still occur when both cells are energy cells (I simply picked the first examples I found from the list that show the voltage profile difference). It really has more to do with major chemistry differences (e.g. LiCo vs. NMC) that radically alter voltage profiles.
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#38
All I can say very true I completely agree.
Your point is well taken.

My research shows that after ~3.5V all cells no matter what Li-Ion chemistry it is will drop of causing the other stronger cells to take over if there is a demand, creating stress on them. It is quite obvious what to do. Do not discharge below 3.5V
As far as the discharge curve is concerned if IR and capacity are relatively close the cells will not be stressed till they get to the magic voltage which keeps popping up in my tests ~3.5V
Unfortunately the discharge curve of 1 cell compared to the other cannot give you the true relationship of that discharge curve when both of these cells are in parallel.
It rudimentarily changes that discharge curve as each cell will contribute according to its ability and will hybridise that discharge curve as the voltage of the cells will be very close.
I do find  LiCoO2 and LiNiCoAlO2 chemistry somewhat accommodating to each other. For some reason the seem to play well. Almost kind of like LiFePO4 with Pb.
I don't know why but they do.
Not so much with  LiNiMnCoO2 and  LiMn2O4. Must be the manganese that's all I can think of.

So much more research needs to be done as far as used cells is concerned.  What can, cannot and or should not be done.

Now mind you many powerwall are running quite well, some without much of a regard to many of these factors.
Mine is running quite well. At ~3.99 V all my packs start to congeal and come very close together to generally within 0.02 volts. At full charge the packs are at 4.04 to 4.05. None of my packs ever get to the point of the batrium BMS to step in and need to balance, YET,  my packs have not been running that long only 10 weeks we will see what happens in a year.
The maximum delta I have seen so far is 0.06 V at ~3.5 V at discharge.
You see most of the research that is done is on new cells, not used cells, of which we have no idea how many cycles they have been through.
All we can do with the equipment that is readily available and reasonably priced is come up with a method to at least get an idea that this cell is worthy.
After the testing we have some idea of the SOH of that cell.
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#39
(07-20-2020, 01:17 PM)Wolf Wrote: [...] Unfortunately the discharge curve of 1 cell compared to the other cannot give you the true relationship of that discharge curve when both of these cells are in parallel [...]

Yes, such discharge curves are only a starting point, but they already suffice to show how big chemistry mismatches can greatly disturb balancing. To get more precise results you'd need to do some dynamic modelling (as do fuel gauges), necessarily incorporating all the pertinent parameters (IR, temp, etc), which is quite hairy. Instead, it is usually much easier to run some tests and attempt to glean some intuition from the results - as you have done. Even then it is often difficult to make sense of the results - even for a few cells, let alone 10 times as many. But don't give up, it seems you've already gained some insight.
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#40
(07-20-2020, 01:17 PM)Wolf Wrote: Now mind you many powerwall are running quite well, some without much of a regard to many of these factors.
Mine is running quite well. At ~3.99 V all my packs start to congeal and come very close together to generally within 0.02 volts. At full charge the packs are at 4.04 to 4.05. None of my packs ever get to the point of the batrium BMS to step in and need to balance, YET,  my packs have not been running that long only 10 weeks we will see what happens in a year.
The maximum delta I have seen so far is 0.06 V at ~3.5 V at discharge.
Yes - I'm seeing similar results with my 2 oldest packs at 720 daily cycles w/average 40% DOD in 4.0v -> 3.5v range - they've never needed an explicit balance other than to bring new batteries online.  These 2 oldest packs (14s120p - e.g. 260ah packs) are a random mix (both parallel and serial) of 'new' 36v 20cell ebike packs made up of LG-MF1-18650 and ICR18650-22P  before I was aware of any of these topics.     

Since then, I've regularly added batteries of green NCR18650A, grey LGABB41865, and blue LGDAS31865.   BUT, even these are high quality (e.g. new'ish) compared to dead laptop batteries.   1 battery at 400 cycles, 1 at 300 cycles, and the latest couple at 100 cycles -  I find that all 84 packs play nicely
with a 3.54v bottom and a max high of 4.0v (same as you).   I've let it go up to 4.15 a few times (40? times out of 720 cycles) but not much / had no noticeable affect.  

I've only had to turn on balance when I add a new battery.  Have gone as long as 9 months with no balance so far - but during that time, one pack did drift from 0.06v max diff to a 0.09v max diff.   I added 3 new cells to it and it's been find since...  so maybe a bit under powered?   It was an early pack before I actually tried to balance ah / pack.    One of several background questions I live with. 

(07-20-2020, 01:17 PM)Wolf Wrote: The maximum delta I have seen so far is 0.06 V at ~3.5 V at discharge.
You see most of the research that is done is on new cells, not used cells, of which we have no idea how many cycles the have been through.
I've never tried to balance beyond 0.03v diff when I add a new battery because things will vary during the cycle from a 0.03v hi/low pack to 0.06v high/low pack from day to day no matter what I've tried - and of course anything <100mv is perfectly fine for my operation.     I've tried 'individual pack charging' (tweak individual pack voltages) and 'add / remove cells here and there' (for suspect ah compatibility) but there seems to be some amount of fuzziness in the system - cells or longmons or reported measurements - and  a 30mv -> 60mv gap up/down each day in no way takes away from the use I need from the battery bank or seem to have any long term affect. 

*I'm amazed at the stability of the battery bank (without balance) - even though individual packs vary a bit over time and the daily max difference between packs goes up/down a bit each day.  In spite of this,  the battery bank remain stable!

I did avoid using those MOLI cells that you tested - showing quite a different discharge curve from the NCR18650As.    

I did have to discard 5 packs with Green Sony G5/G7 as they would not balance with other packs - the hi/low voltage would swing 150-200mv wilder than the others (don't remember exactly) but I put that down primarily to their 200-250 IR?  rather than their discharge curve but I don't know.  One of the ongoing mysteries I carry from this journey.

(07-21-2020, 05:33 PM)gauss163 Wrote: Yes, such discharge curves are only a starting point, but they already suffice to show how big chemistry mismatches can greatly disturb balancing. To get more precise results you'd need to do some dynamic modelling (as do fuel gauges), necessarily incorporating all the pertinent parameters (IR, temp, etc), which is quite hairy. Instead, it is usually much easier to run some tests and attempt to glean some intuition from the results - as you have done. Even then it is often difficult to make sense of the results - even for a few cells, let alone 10 times as many. But don't give up, it seems you've already gained some insight.
Yes sir.   I'm amazed that I've had such a good experience so far (700 cycles) and no problems with the newest packs.   But of course when you add time (1,000s of more cycles) I'm unsure what to expect to maintain the battery bank.

As packs degrade.....   will they degrade more or less 'similarly'?  There are several maintenance scenarios I can imagine
- a slow process of tweak/repairing packs one by one as they start failing to keep balance 
- replacing failed packs with new / less-AH packs to match the remaining packs so the battery bank continues... BUT I expand the DOD % (or the size of the battery bank) to keep supplying the daily power needed.
- all or nothing
- other issues.

The continued operation looms as each cycle accumulates and has become more front/center in my thinking.  Time will tell Smile
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