How to mix different internal resistances?

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rg12

100 kW
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I have a few hundred new, non branded cells with about 1.9ah capacity and checked them all for IR (and wrote on each cell) and they range from 18 to 30.
I wanted to know how should I spread them the right way since for example I have 10pcs IR 18, 20pcs IR 19, 40pcs IR 23 and so on.
I know that it wouldn't be smart making a one parallel group of IR 18, one parallel group of IR 25 because it will lose balance really fast.
So if I mix in each 10 parallel cell group IR 18-28, is there importance if the next group in series has the same added IR value in the same order?
Is it smart to calculate added IR of all the cells in the group?
 
18 to 30mohm is a pretty big variance and makes these batteries kinda suspect in my opinion, but you're going to use them anyway, right?

The other issue is the capacity. You also want to capacity match! i'd bet that these cells are all over the board in that regard, but there is probably a relation to capacity and mOhm values..

So, test all of the cells and arrange each parallel group so that it meets an average mohm AND capacity. You will need excel or some other similar tool to figure this out.

Take all the numbers of mohm readings, and average them out for all the cells. Let's say it's 24mohm. Okay, then you need to build each parallel group to be as close to 24mohm. For 2p, you can expect an average of 24ohm across 2 cells to show as 12mohm. For 10p, you can expect it to be 2.4mohm average, etc. ( mohm gets divided by the number of cells you have in parallel )
 
rg12 said:
I know that it wouldn't be smart making a one parallel group of IR 18, one parallel group of IR 25 because it will lose balance really fast.
Click to expand...

Why would it? The voltage being constrained to the same value, the string with less resistance will discharge more current under load, and then absorb some charge from the other strings when no longer under load. Parallel strings can't become imbalanced; only cells or groups in series can do that.
 
I think he means if you have assembly parallel groups of differing total IR and then put them in series. Current being the same through all parts of a series circuit mean the high IR groups will work harder, waste more charge as heat, discharge fastest and also age fastest.

I recall it being possible to use excel to automatically sort a big group of values (like 200 IR measurements) into a given number of subgroups all of the closest possible value, but how to do this escapes me...
 
The capacity and cell weight is identical in all of the cells.
So I guess I will just average them in parallel.
Is there any effect to the order of the cells? like one group being 18 18 18 28 28 28 and the next group not being the same order as the group below it, lets say 18 28 18 28 18 28

I measured 4C LG M26 cells brand new with 28mohm at 2.6ah, so if I have a 1.9Ah cell with the same IR, can I figure the since it has 27% less capacity that it will equal 27% more resistance if the 1.9ah cell was actually 2.6ah? (meaning about 35mohm)

Can you really translate mohm to C rate?

I know 4C 2.6Ah LG M26 is 28mohm and 8C 2.5Ah 25R is 11-13mohm.
If I'm not wrong, I thing the Samsung 26F is about 60mohm which is 2C (double the resistance than the LG 4C and also half the C rate) so I can figure that around 7mohm will be 16C and 3mohm will be 32C and so on...right?
 
As long as your connections between cells in the parallel group are good i..e good current sharing, the order of the cells should make no difference.

The IR is a consequence of the thickness of the conductor inside the cell. If higher capacity is desired there's less room available for conductor, hence greater IR. It should scale roughly linearly, so you can interpolate/extrapolate, just as you have done with C-rate Vs. IR.
 
Punx0r said:
As long as your connections between cells in the parallel group are good i..e good current sharing, the order of the cells should make no difference.

The IR is a consequence of the thickness of the conductor inside the cell. If higher capacity is desired there's less room available for conductor, hence greater IR. It should scale roughly linearly, so you can interpolate/extrapolate, just as you have done with C-rate Vs. IR.
Click to expand...

but capacity should be a factor also right? because the bigger the capacity the lower the IR?

EDIT: If one cell is 2.6Ah 28mohm (LG M26 which is 4C) and one cell is 1.9ah 28mohm, will the 1.9ah cell still be considered 4C because it has the same IR or will the capacity affect it?
 
Punx0r said:
I think he means if you have assembly parallel groups of differing total IR and then put them in series.
Click to expand...

Yeah-- you don't want that.

It seems to me that if you are intentionally mixing cells of different IR, you might be best served by a pack design that would otherwise be a bad idea: Build 1P strings of cells that have identical IR, each string with its own small BMS card, and link the mini-packs in parallel. If you have enough matched cells, you could make them 2P or 3P-- it wouldn't matter whether the sub-packs were all the same size as long as they're in parallel and the same voltage.
 
Chalo said:
Why would it? The voltage being constrained to the same value, the string with less resistance will discharge more current under load, and then absorb some charge from the other strings when no longer under load. Parallel strings can't become imbalanced; only cells or groups in series can do that.
Click to expand...

The simple answer is that they will wear differently over time, and the low IR groups will hit the LVC and the BMS will cut off the entire pack based on the voltage sag of the weakest group.

Same thing happens if you have a large capacity mismatch.

Voltage sag gets worse as any battery approaches 0% state of charge. So the very weak groups would prevent you from getting all the capacity out of the battery.. AND wear quicker, which means that this situation only gets worse as you put cycles on the battery.
 
rg12 said:
Is there any effect to the order of the cells? like one group being 18 18 18 28 28 28 and the next group not being the same order as the group below it, lets say 18 28 18 28 18 28
Click to expand...

No, because once you assemble the pack in parallel, they share the load, with one exception.
If you use weak nickel strips or the series interconnect is at the last or first cell, then the opposite side cell might see lower load, but this only really matters if you have a weak power bus and are really beating on the pack.
Best thing you can do is make the series interconnect in the middle of the parallel group. :)

rg12 said:
Can you really translate mohm to C rate?
Click to expand...

That's a good question... C rate is based on amp hours and resistance though, so there's a correlation, but not an exact one, i think.
 
neptronix said:
Chalo said:
Why would it? The voltage being constrained to the same value, the string with less resistance will discharge more current under load, and then absorb some charge from the other strings when no longer under load. Parallel strings can't become imbalanced; only cells or groups in series can do that.
Click to expand...

The simple answer is that they will wear differently over time, and the low IR groups will hit the LVC and the BMS will cut off the entire pack based on the voltage sag of the weakest group.
Click to expand...

I misunderstood the question. I thought he was asking whether series groups of matched cells could be placed in parallel with other series groups of matched cells. That wasn't his question, but the answer to it is yes. It complicates the use of BMS, though.
 
Ok, I have arranged the cells, put everything in excel...
It's a 24S 7P and each group of 7 cells in parallel is averaging 24mohms.
The biggest difference between cells inside a group is 21mohm and 25mohm but it's mostly 23-25ohm cells.
What do you guys think?

Would love to get an answer to this question:
If one cell is 2.6Ah 28mohm (LG M26 which is 4C) and one cell is 1.9ah 28mohm, will the 1.9ah cell still be considered 4C because it has the same IR or will the lower capacity mean higher c rate?
 
You could pair a 4C cell to a 2C cell and the result would be a 3C 2P battery.
But that's not so wise, since you have a mismatch and one cell will try to work harder, but the effective output of both is the average of both..

Do consider this about 18650's though. Some of them have chemistries that are rated for high temperatures, so they'll do a fun trick and call a 2C cell a 4C cell just because it doesn't explode while running at 3.2 volts nominal and wasting >15% of it's energy as heat. Manufacturer C ratings now no longer have to do with performance.

So the 4C cell might pair up with the 2C cell perfectly, but let mOhm be the judge as to whether that's appropriate or not :)

Another thing with different cells is that they have different voltage curves. A typical low voltage cutoff is 2.0v, 2.5v, 2.8v, and 3.0v.

Mix and match across different brands/models, or even just cells of various vintages is a bad idea unless you know *exactly* what you are doing.
 
rg12 said:
Would love to get an answer to this question:
If one cell is 2.6Ah 28mohm (LG M26 which is 4C) and one cell is 1.9ah 28mohm, will the 1.9ah cell still be considered 4C because it has the same IR or will the lower capacity mean higher c rate?
Click to expand...

No. If both cells are 28mohm then, roughly speaking, discharge capability in amps will be the same, but the c-rate will be different. This doesn't matter because c-rate is just a term of convenience - a label.
 
Punx0r said:
rg12 said:
Would love to get an answer to this question:
If one cell is 2.6Ah 28mohm (LG M26 which is 4C) and one cell is 1.9ah 28mohm, will the 1.9ah cell still be considered 4C because it has the same IR or will the lower capacity mean higher c rate?
Click to expand...

No. If both cells are 28mohm then, roughly speaking, discharge capability in amps will be the same, but the c-rate will be different. This doesn't matter because c-rate is just a term of convenience - a label.
Click to expand...

I just need to know what to expect in terms of amp draw handling.
If It's identical to a 5C or 3C then I know how to calculate the max amp draw of the pack but in mohms I have no idea.
 
999zip999 said:
Even at 3c the pack is not going to see a long long life. It will tell you on how much it sags.
Click to expand...

I still didn't say how many amps I'm going to pull out of the pack but I guess I will know that after the pack is built and I do the first load test
 
A 20mohm 1Ah cell will give you roughly the same amps at the same voltage sag as a 20mohm 5Ah cell, but obviously not for as long. Ignore c-rate.
 
999zip999 said:
You also won't tell us what it's for ??? Important.
Click to expand...

I thought it's obvious...for an ebike.
Have a QS205 motor and an 80A Greentime controller.

The pack will be around 7S (13-13.5Ah) so probably wouldn't tolerate 80A but I will do a load test and hopefully it will take it for bursts.
 
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