Is it necessary to test capacity of each cell?

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ozz93666

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Feb 22, 2017
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I'm fairly new to this , so brainstorming an idea which may be wrong ....

It seems a lengthy part of the procedure is to test the capacity of each cell and sort them accordingly ... if using these cells for a powerwallit seems sensible to use all , even those with 1/2 AHr .... but it mustbeessential to remove any that rapidly self discharge as this will drainthe whole pack...

First charge each cellup to4V... leave for a day and checkvoltage .... those that are still high can be wired in parallel making large blocks... then before puttingthem in series they should be balanced , so each block has the same capacity , one block may have a 100 cells , another 98 , another 97 , .. but they all need roughly the same AHr capacity for best performance of the battery .
 

daromer

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Ok im not going to say if you are wrong or right here but im going to show what my thoughts are about packs and cells. Note that my thoughts are mixed with personal experience including several years of work with batteries including colored personal thoughts :D

** Whats worth taking in account that i would say does not make 0.5Ah cells subject to be used in any pack: **
* Cells that have 0.5Ah most likely are in the end of their life... Why? Because most cells you get hold of is 2Ah or more new. Cells that have 1/4rd left would have used 90-95% of its number of cycles. (If you check capacity vs cycles vs life charts)
* Your time is worth money working on the packs. Adding 0.5Ah cells when you can add 2Ah cells... 4 times more time so question is if you could have bought new cells (3Ah as example) instead of using 0.5Ah and just gone to your normal work earning money?
* Older cells that are close to its end of life have some factors. 1. Their IR going up causing the other cells to take larger load ie you need to calculate avg current on each cell. 2. Their self discharge can/will grow. May not be big when tested but what if its only 20 more cycles untill they actually starts to go bad?
*If you dont test all cells -> How would you know if a pack is even? Unless you test the full pack of course. What if you have 100 cells in that pack? How do you know what cells to swap out for better ones if the capacity is 10% lower than the other packs?

** Is there any advantages of using above? **
Yes! But they are few and if you ask me use those in separate packs and only if you have spare time from your wife/kids/girlfriend/family :)

* You will get some more Wh out of the total pack
* You dont have to throw cells away directly
* You dont have any other cells available

So basically it all ends up to: Are your time worth using small capacity cells? Are your time worth the hazzle potentially having cells that die alot earlier than others?

Personally:
-- I only use >2Ah in my main packs! And they need to pass certain amount of tests.
-- Cells between 1500-1999 goes into secondary smaller packs that i can hook in and out easily:
-- Cells 1000-1499 is left for kids toys and stuff where it doesn't matter that much...
-- Cells below i toss in the bin to use as fireworks or exchange for new ones later on.

https://snag.gy/BSTmVl.jpg
 

ozz93666

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Feb 22, 2017
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341
daromer said:
Ok im not going to say if you are wrong or right here but im going to show what my thoughts are about packs and cells. Note that my thoughts are mixed with personal experience including several years of work with batteries including colored personal thoughts :D

** Whats worth taking in account that i would say does not make 0.5Ah cells subject to be used in any pack: **
* Cells that have 0.5Ah most likely are in the end of their life... Why? Because most cells you get hold of is 2Ah or more new. Cells that have 1/4rd left would have used 90-95% of its number of cycles. (If you check capacity vs cycles vs life charts)...


https://snag.gy/BSTmVl.jpg

Thanks , that's some good reasons ... but it seems this whole matter is clouded in uncertainty .. Does 0.5 Ahr capacity mean the cell is near the end of it's life? , if so surely the other 5 cells from the samelaptop pack should also have the same capacity, but I don't think that's the case ...

Someone told me a while ago that laptop batteries don't die a natural death , they're terminated after a specific time (I forget weather he said this was by the computer , or by chips in the battery pack) this makes sense ... my personal laptopbattery had virtually no use , but went dead after 18months , when dismantled the cells were fine.... cellscoming of a production line should be the same ... all have the same life , be very predictable.


Great video , daromer ....Fascinating to see how everyone has their own style and different way to build ....

It seems to me what we should be doing is fine tuning our techniques so we can mass produce these , demand will only rise for vehicular and solar use , and there's an unlimited supply of laptop batteries.. this is a BIG industry of the future ....

I saw in the video you were soldering in fuses for each cell , have you tested the blowing current the way they are soldered into the pack ? it won't be the same as a free wire .... which wire do you use , I'd like to calculate the power consumed by these fuses in normal use .
 

daromer

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Thing is all cells react a bit different. It may have been that the cell with 0.5Ah had more heat on itself during the life or whatever? It can also be that that one died some cycles earlier than the other? Who knows? If you want to test feel free to do so :)
I agree that if 1 cell is close to 0.5 the others may be too. But in general i see that the ones in parallel always come close to each other. At the same time i have seen cells fail in packs just weeks old... With that said there is many factors.

As i said i have not seen anything that clearly states xxx follows yyy. In the beginning they may be.. But not in the end. Same goes to many things out there. Because we are using packs that are in their end of life we need to expect that some are worse than others. I have had batteries people said "those are unusable and dead" that i have been using on other appliances for years.. And that was people working daily with them.

For instance i was on one place the other day... This is horrible because they made me angry...
Women hand in her phone and asked why her battery time is so low.
Tech guy answers: Look here at this chart. As you can see you have xxx time left on a charge. this equals to 490 charge cycles. Your battery only last 500.
Women:But i only have owned this for 6 months! Can i do something not to cause this or prevent this
Tech guy: Yes you can. Always do full cycles.. Because every time you set the phone in your charger it counts as 1 cycle.. No matter if you charge 1minute or 2 hours.
Women: But but we always charge in our car and so.
Tech Guy: Stop with that. Charge once a day at most!

This is bullshit! We all know that Lithium batteries has most cycles in middle of its SOC... To bad such people can continue to work on such places.
That guy totally blindly looked at his chart and had no clue on how it works in real life. They only wanted to sell the service to her to change the battery and then instructed her with false.

Yes some chargers do more harm than they should but its not about what he told that women. I actually told here how it actually works after when we went out of this shop.. unfortunately i could not yell at him because its a place i tried to get batts from :)


Naturall death.. Yes and no. Many batteries dies due to that the electronic dies... Thats also a factor and then you might find good cells. But at the same time it differs if the pack had been stored somewhere for another year and then you might end up with a pack with all cells at around 1V.....

With that said if the cells never have been abused ie they are in the normal area they most likely will act the same. But if BMS or 1 cell fail we do not have that normal usage any more.. And they will start differ.

Yes i have tested them in place too. And i have also tested them free with same busbar approach. Since they blow due to the heat generated the soldered busbar takes away alot.
Im using 35AWG thinned fuse wire rated to 5A. In real life thats more like 6-12A depending on how its mounted.

Power consumed is something i also have calculated in another thread here. Dont find it now but we are talking about 0.1% wasted energy or around. I did an error in that calculation based on wrong length. I calculated with 5mm but i actually have ~10mm in avg.
 

FireFrog

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Jan 22, 2017
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Would be nice if phones or other electronics had cut off voltages to help preserve the cells inside but unfortunately that wouldn't make more sales.
 

daromer

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Phones have that. Most devices have cut-off. But what you want is different setpoints?
 

daromer

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Dont know.
 

Janis

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Jan 22, 2017
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At least for a few phones I saw the the low cut-off is about 3.4 till 3.5 V, so it's reasonably high, leaving time to find the charger. Upper cut off in the range 4.1 - 4.2 V, which is also ok. No serious manufacturer wants trouble, so it's pretty conservative.
 
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