Another cell grouping question

Ken140tdi

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May 16, 2017
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sorry if this is old ground and been covered before but I cant quite find the answer Imlooking for.

Ive now got around 1000 cells that I have recovered from old leptop battery packs. I now have them all tested and re charged and have also got them all set aside in boxes grouped from 1500 ma to
2850 ma. Ofc I dont have many at or close to the 2850 range.
I boxed them like this 1500 to 1650 then 1650 to 1750. 1750 to 1850 and so on right up to 2750 to 2850 and higher.

My question is on grouping the cells to make my 14s battery. I have seen that some of you dont use cells that are more than 200 ma between the highest and lowestcapacity. Unfortunately I dont have enough cells that are with in this range to build a battery large enough to be worth while so Im asking what you guys think would be the down side of using cells with a larger difference would be or is it simply a very bad idea. ????
Of course I would build each pack so that all 14 packs had the same total capacity as each other and the same number of cells in each pack as well.

If for example I had the highest cell in a pack at say 2800 ma and the lowest cell at 2200 ma capacity would it work like this during charging and discharging. As apack started to discharge the lower capacity cell would start to loose voltage faster than the highest one so the highest one would give out more to compensate so that when the pack was flat each cell would still be at the same voltage as all the others in the pack. And during charging the opposite would apply the larger capacity cells would absorbe more charge than the low capacity ones.
If Im corect then it shouldnt matter if a pack has different capacity cells providing the total capacity of each pack with in the 14s battery have the same total capacity as each other
 
It's always the same:

14 packs should have the same capacity, otherwise they will get out ouf balance very easily and you can't use their full capacity.
Within the packs you want the cells to be as close as possible. What that means depends on what you have to work with and what your load is. The higher the load the closer the cells have to be. There are no definitive rules.

And please get the units right. "ma" is milliyears. What you are looking for is mAh.
 
Ken140tdi said:
Of course I would build each pack so that all 14 packs had the same total capacity as each other and the same number of cells in each pack as well.

That's really the primary goal. Each pack in the series just needs to be super close in capacity. The further the deviation, the harder the BMS has to work to keep it balanced.


but, if you can, keep pack variances low as well. This will help keep the stress level down on those cells. But you could go as much as 500mAh variance and still be fine. You just wouldn't want to put a 2400mAH with 1200mAh cells.
 
Ok that really helps a lot. Its a shame to waste my best cells but perhaps I should base this next battery Im going to build around the capacity cells I have the most of which is 1850 to 2250 Mah. I could add the few 2800 to 2850 mah cells I have to my first battery I built from new 2850 cells.

I presume it would not hurt to build smaller capacity batteries which I could add cells to to later to bring the battery size up to the same as my first 5 kWh one
 
I do it like this:
 
daromer said:
I do it like this:
Yes I have watched this video several times. So I see in your video you used 2000mah as the lowest capacity cell all the way up to 2700mah in the same pack. So are you effectively saying it does not mater what capacity cells you use providing that the total capacity of each pack is very close to each other.
 
If your current per cell basis is low then no it doesnt matter. Just beware of that you done really mix 20 year old cells with new ones. Thats just perhaps not the best idea since we dont know how long old cell will last.

But yes as long as total capacity and number of cells are the same in each pack Im fine with that. My system is still running and the first packs is up to 1.5 year now or something
 
Hello All!

So, a cell with 80x2300 would not be able to be used with a cell of 108x1700 am I missing something here?
 
No, you can do this, although it isn't ideal. You shouldn't mix the 1700mAh cells with the 2300mAh cells if possible, that's what this here is about. Actually, don't do either if you can avoid it.
 
This is separate cell packs. with equal capacity. Am I wrong? They are not mixed cells in the same pack (battery)
I understand this is to be avoided, but, what is the draw back? capacity is the same, voltage is the same.
 
Chiptosser said:
Hello All!

So, a cell with 80x2300 would not be able to be used with a cell of 108x1700 am I missing something here?

Yes I think you err missing something, but only the terminology. A cell is a single 18650. A pack is how ever many cells in one pack and a battery is how ever many packs linked together in parallel to get the voltage you require.

My understanding is that you can put different capacity batteries side by side as they will self ballence
 
Chiptosser said:
This is separate cell packs. with equal capacity. Am I wrong? They are not mixed cells in the same pack (battery)
I understand this is to be avoided, but, what is the draw back? capacity is the same, voltage is the same.

The pack with the lower capacity cells will not behave like the one with the bigger capacity cells.
Also, please check the definitions: http://secondlifestorage.com/t-Frequently-Asked-Questions-FAQ#pid20003
 
I would mix them. Same amount of lower cells as higher cells. That would in theory make them behave equally.
 
daromer said:
I would mix them. Same amount of lower cells as higher cells. That would in theory make them behave equally.

That makes sense to me as we know that cells with in a pack self ballence and also that batteries (one battery being 14 packs in series) will self ballence between other batteries in parallel.


What is the maximum difference capacity you would risk usingin a pack between the highest and lowest capacity cells.

I have good cells ranging from just over 1500 mah up to 2850 mah
 
Only in theory. And it has nothing to do with balancing. Depending on how used they are they will have a bigger resistance in relation to the other packs. Again, this is potentially a problem under bigger loads, just like different capacities within a pack.

This isn't about risks as such. It's about whether the battery will work properly and delivers consistent results or not.
In a pack you will have uneven current flow and can't calculate, can't even estimate, the load per cell.
Between packs you might have individual packs underperforming and they will have an impact on the battery performance. Some packs might hit the low voltage threshold earlier than others, although their capacity should be the same, and you can't use the remaining theoretical capacity of this pack and the actual capacity of others.

The smaller the load the smaller the impact of those effects are.

You want numbers but no one can tell you the right numbers. It doesn't work like that. Have some confidence, do some calculations (or estimations if you have to), run some tests and then you will find out. At the moment I'm testing seven 40p packs I've built. The cell capacities range from 1960mAh to 2400mAh and they are distributed randomly into seven packs. I will find out if that worked out or not.

What I'm saying is that I would never advise someone to do it or to leave it. Instead you should make your own decisions based on knowledge your are getting. It isn't wise to build something and put it into production. You want to test it first anyway and you want to know about it.
You don't want to rely on something said by us dummies, we aren't the ones that have to deal with what you will build.
 
Ok I get it. I have 8 kWh of battey storage so far and Im hoping that this next battery will take it up by another 5kwh. All my cells have been opus tested at 1 amp drain and the max my inverter can draw is 3.7 kWh. By my calculations if Im right none of my cells should ever see even half an amp drain. So hopefully cell risistance wont really come into play.
 
DarkRaven, Are you saying that: internal resistance is a factor in a battery module performance?

As in the example I was inquiring about( 80x2300 and 108x1700) two separate modules?


By the way your reference to the FAQ does not awnser the guestion.
 
Yes he is and the higher the charge rates and load rates the more that resistance will afect battery performance.

Have you not tested your cells one by one and found some cells that get quite hot to touch even under only half an amp charge rate ?? Those cells get hot because of their internal resistance
 
Yes, sure, the internal (and external for that matter) resistance is always a factor in battery performance.

The FAQ entry was just about the definitions of pack and battery, you have been mixing them up a bit.
 
Ken- Yes, I have thousands of cells here that I have tested. Not near as many as others.

Yes, I am aware of the different resistance values of the different capacity cells.

I am just throwing out questions here, I am being the devils advocate .

Sorry! I said cell,, I did give a number count and cell capacity for two different packs,modules.

1200 of my cells are new, unused 1700 cells, one year from manufacture date 12/6/16
An other 700+ are from Terry from Texas, they range from 2100-2800 undetermined age.
Plus a mix of 600 other new cells that I have to resort.

From the responses, of the original question it seems as thou people are thinking that I want to mix cells in the same pack.
One pack of 80@2300 and one pack of 108@ 1700

The capacity of the two packs are within 400mA total capacity of each other. Yes, You are normally going to have resistance variation thru-out every pack (module) that you assemble.

So, assuming the IR is as close to the same in each pack, with the values that I have given for an example, do you think these two packs be considered very close to equal?
 
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