Battery build IR/Capacity range

Not sure if this question has been asked before, currently having a bulk amount of cells and trying to figure out 2 things.

1. What is the acceptable IR and Capacity range for a single parallel group. Example building a pack and there are 4 cells in parallel, ranging from 2500mah to 2900mah, and IR testings ranging from 30moh to 100moh, what range is acceptable?

2. What is the acceptable IR and capacity range between series cells. Example Parallel pack 1 contains 4 cells in parallel at 2500mah each (10AH total) and IR is consistent between those 4 cells at 50moh. Can this be connected in series to another parallel group that has 2900mah cells and 80moh of IR?

This is for a high drain pack, im aware for a low drain pack like a power wall high IR is o.k.

adding dissimilar cells in series isnt a good thing and for sure will create a huge imbalance. putting them in parallel sounds a lot safer but still not ideal

10% is my bottom line. A cell is 10% different in IR or Capacity, the cell is rejected.

Ok so anything above 10% is no good?

I think would depend on how capable ur bms is.

How would a large capacity imbalance go with parallel cells when using a powerful bms?

I would not use the 100 milliohm cell. Thats a flashlight battery at best. When I test my cells, I use the cells that are very similar in range like 50,60 milliohm, I might use some 70's if I really need to. But I usually build 3s16p for low discharge devices I can get away with high milliohm laptop cells. When I did build a high discharge 5s6p to power my 18 volt hot glue gun, I used sony power tool cells in the 30 milliohm range. Those cells worked great.
But overall your entire pack has to be very similar, if the majority of the cells in your pack are from 50 to 70 milliohms, but in the pack there is one cell that is 100 milliohms, that one cell will cause problems, either self discharge or balance problems. All it takes is one cell.
For a high discharge pack, the 100 milliohm cell would be a weak link in the pack, its not capable of high discharge. Its worst than a laptop cell which the highest I seen are in the 70 and 80 milliohm. Most laptop cells average in the 60 range with the hard to find panasonics in the 40 range, those are premium laptop cells. I always save those for special projects, they would be a waste on a low discharge battery pack.

jonyjoe303 said:

I would not use the 100 milliohm cell. Thats a flashlight battery at best. When I test my cells, I use the cells that are very similar in range like 50,60 milliohm, I might use some 70's if I really need to. But I usually build 3s16p for low discharge devices I can get away with high milliohm laptop cells. When I did build a high discharge 5s6p to power my 18 volt hot glue gun, I used sony power tool cells in the 30 milliohm range. Those cells worked great.
But overall your entire pack has to be very similar, if the majority of the cells in your pack are from 50 to 70 milliohms, but in the pack there is one cell that is 100 milliohms, that one cell will cause problems, either self discharge or balance problems. All it takes is one cell.
For a high discharge pack, the 100 milliohm cell would be a weak link in the pack, its not capable of high discharge. Its worst than a laptop cell which the highest I seen are in the 70 and 80 milliohm. Most laptop cells average in the 60 range with the hard to find panasonics in the 40 range, those are premium laptop cells. I always save those for special projects, they would be a waste on a low discharge battery pack.

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Something interesting to note, the cells which mooch tests, the ones that are in the 50moh of resistance these are typically rated at about 10 amp discharge. Meaning 50moh indicates a low discharge cell. 25-30amp cells is what I consider high discharge.

I cull cells on 0.5% impedance and 0.25% capacity.

Depends a lot on your expectations of the use and service life.

What's it for what amp is your controller how big a motor are you going to be slugging up hills ? Flats

liveforphysics said:

Depends a lot on your expectations of the use and service life.

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Yes, and you also start with " Brand New" cells, right? Hot off the assy line? Right out the box from the manufacturer with a batch number and record? You are a accredited, professional, battery building, engineer, are you not? The only time I ever see cells with that much congruence is when they are not used. Brand new. A brand new stack of cells will have that much congruence. I have bought brand new EV modules and torn them up, and tested to this standard you put forth of a half, and/or a quarter of a percent. Yes. Brand new, might happen. However,once they are used, and you do have to test every one for you homebuilt pack.... I use 10%.

However, I come from the hobby grade consumer market. The number I present was not pulled out of thin air for me: it is the number that my datalogger ( Revolectrix DPL8, a high quality, USA designed Taiwan built charger) told me to look for. In the software, of a hobby grade datalogging charger of significant power. This number has always worked good for me, in building. .

I am always working with " Used" second life cells. I could not imagine rejecting cells based on 1/2% internal resistance and 1/4 % capacity.

That is 250mAh on a 10Ah cell. Lol. I have seen worse charge efficiency in the datalogs with the swing in daily temperature, with some used cells. Really beat up cells.

For my work, I can't afford to use anything but new cells. I still make a culling inspection cut on them, but most of the cells I work with easily hit 0.5% impedance and 0.25% capacity bounds so the rejection rate is almost nothing.

Dont feel bad, there was a point in time that due to temperature variations between tests I couldn't even get repeatability on my measurements of +-0.25%, let alone use it for screening.

I would error on scrapping an iffy cell over running it though everytime.