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Mixing 18650 capacities
#1
Hi All

So I have over 2000 used 18650 cells. I have capacity tested them all and now have a range of between 1200-2000mAh.

I'm looking to create the biggest 24v capacity setup I can with these cells. 7S???P.

Im still leaning but Im guessing mixing all those capacities (Using rePackr) would not be a good idea, or is it?

Assuming it is not a good idea what is the maximum capacity range I should be using? 2000mAh down to ????

Thanks in advance
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#2
If it wasn't a good idea, why would we have a link to use it????

You can mix any combination of cell capacities with any range.
The thing you need to take into consideration is what was the cells capacity at the start of its life, and what is it now. If it's more than about 25% drop, then you probably don't want to use it.

Original    3000mAh
Now         1700mAh
Difference  56%

Original    2000mAh
Now         1700mAh
Difference  85%

As you can see, the 2nd cell is much better condition than the first, even though they have same Now capacity. So it is highly recommended to compare "all" cells against their original specs
OffGridInTheCity, bogptrsn, Bobbych like this post
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#3
(02-18-2020, 01:55 PM)Korishan Wrote: If it wasn't a good idea, why would we have a link to use it????

You can mix any combination of cell capacities with any range.
The thing you need to take into consideration is what was the cells capacity at the start of its life, and what is it now. If it's more than about 25% drop, then you probably don't want to use it.

Original    3000mAh
Now         1700mAh
Difference  56%

Original    2000mAh
Now         1700mAh
Difference  85%

As you can see, the 2nd cell is much better condition than the first, even though they have same Now capacity. So it is highly recommended to compare "all" cells against their original specs
Thanks Korishan. My line saying its not a good idea was referring to mixing the cells not rePackr.
Anyway. my cells were originally 2100 and 2000 so i will exclude any below 1600mAh. (20% Loss)

Thanks Again
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#4
Mixing cells depends. If you make sure the weakest cell is the One determening max useage its fine.

Make sure to mix so you get even Number for each type in each pack. (Repackr wont do that....)

Them you get the best chance for it to last
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#5
Here is what You can do. If You have 2100 cells You can make 3 x 7 banks each 100 cells.

Prepare 21 cell holders 100P and place them on a table. (Quite a large table Wink ) Name the Banks 1 - 21.

Sort all Your cells in decending order. Start placing the highest 21 cells. Highest cell goes in Bank 1, Next highest in Bank 2, and so on until You have placed cell number 21 in bank 21. Cells 22 to 42 is placed in Bank 21 - 1, so the lowest is placed in Bank 1.
Next batch of 21 is placed 1 - 21. Next batch in 21 to 1. Continue this sequense until You run out of cells or all banks are full.

By doing it this way, each bank will be equal to the next. By building 21 banks in one go, You will end up with 21 banks equal in capacity and with the maximum capacity possible with Your batch of cells.

Best of luck Smile

ChrisD
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#6
Interesting. Would I connect sets of 3 in parallel the the backs of 7 in series?
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#7
You can connect banks 3 in parallel and then 7 in series, this gives You 7S300P, or You can connect 7 in series and then parallel the 3 resulting 7S100P.

7S100P x 3 will need 3 BMS boards.  7S300P will need only one, but significantly larger BMS.

Say You want to draw 75 Amps (3 KW at 25 Volts) each 7S100P will work with a 30A+ BMS, and they can be found at a reasonable price. 7S300P will require at least a 80A+ BMS, and You may find that this is going to cost You more.

I would go for 100P, but the choice is Yours.Smile

ChrisD
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#8
(02-20-2020, 08:43 AM)ChrisD5710 Wrote: You can connect banks 3 in parallel and then 7 in series, this gives You 7S300P, or You can connect 7 in series and then parallel the 3 resulting 7S100P.

7S100P x 3 will need 3 BMS boards.  7S300P will need only one, but significantly larger BMS.

Say You want to draw 75 Amps (3 KW at 25 Volts) each 7S100P will work with a 30A+ BMS, and they can be found at a reasonable price. 7S300P will require at least a 80A+ BMS, and You may find that this is going to cost You more.

I would go for 100P, but the choice is Yours.Smile

ChrisD

This is a great question and answer and I think would be a useful explanation, particularly in some form in the FAQ, when it comes to pack design, system setup, etc. The systems above 'sound' the same but even in this simple explanation it's clear there are considerations that, I believe, many are not inherently aware of when thinking about the design of their systems. Especially for people building 'smaller' systems, systems that are not always going to continue to grow (I think systems of that nature tend to be 'easier' to plan because you're always adding stuff).

To highlight some of the more important elements, from the aspect of the size of the system (3kw system in the above example from a 24V battery) and understanding the ramifications and how they, directly, impact system setup. Things like BMS requirements as well as wiring/busbars (see below).

Also, correct me if I am wrong, but going the 3 x 7s100p system would also allow smaller gauge wire/busbars for the 100p's versus going the 7s300p route? I am ONLY discussing the wire/system for the busbars for the 100p or 300p packs ... I understand that when doing the 3 x 7s100p system, you will still ultimately end up with the same requirements for wire gauge to draw the 75amps, for example going to a 3000w inverter.
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#9
(02-18-2020, 05:07 PM)daromer Wrote: Mixing cells depends. If you make sure the weakest cell is the One determening max useage its fine.

Make sure to mix so you get even Number for each type in each pack. (Repackr wont do that....)

Them you get the best chance for it to last
 
Yes the SLS rePackr won't do that but the Nemos Repackr does.
You do have to have M$ Excel but if you have your cells numbered it will create the packs for you with the best ballance and the most Ah.

After the Repacker is done it will assign a pack number to each cell then you can sort by cell identifier (number) and you can easily find the cell and place it into the corresponding pack.



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