cell matching, internal resistance?

hello
is there a majic formular or calculator, when making packs regarding internal resistance,
or is it just a case of matching capacities?

I do Capacity Only But Others here do Both Capacity and IR.

I 'sort of' focus on IR. I'm OK with 90-120mOhms whereas <50mOhms is sometimes recommended as long as the overall battery has similar cells.

I had an experience where 5 (of 14) packs in a series were made of Sony G5/G7s with 200mOhms whereas the other packs were a different brand and more in the normal range of <100mOhms. These 5 packs had pretty wild voltage swings (I could not easily balance with other 9) and I believe it was largely due to IR difference. After replacing those 5 packs with similar IR and discharge curve - my balance issues vanished.

I only do capacity bad then i random the rest.

I don't test resistance. None of the cheap charger/testers most of us are using can accurately measure internal resistance. Try testing the same cell a few times and you'll see it's never consistent. A capacity test typically weeds out most (if not all) that have high internal resistance.

Also testing the resting voltage and then again in another month of rest for Self dischargers Does the trick for me

With ir you can get away with cheaper testers, but you will need to find the fabric specifications on the cellsir.
If there optimum ir is for an example 25, and you measure 50mohm, that cell wont do you any good anymore.
Or if the cells optimum ir is 60 and you have 80mohms or 40mohms then there is something wrong with the cell.
If you stick to the fabrics optimum ir value and you take a max deviation of 10% you will be good.
As mentioned earlier, with the capacity test you will weed out most, not all.

Now there are two ways that i like: or make packs as random as possible.
Or match only hi/low ir cells together.
There are people here that dont like mixing hidrain cells(low ir low capacity) or low drain cells(high ir and high capacity)
Some mix(i do) some dont..
But with both methods make sure your packs are in the end of the same capacity.

For the general use in our powerwall i belief this is not an issue.
Unless you are only going to operate heavy machinery on your wall, that will drain more than 20a per cell per hour.
Than go with hi drian(low ir) cells
Most of us drain just 50 to 500 mah per cell, i wonder if daromers cells would ever see above 50mah drain....
Hi or low ir is in my opinionno issue, only what they can store and how many cycles they have left.

Save yourself a lot of work and buy a yr1030 ir tester incl the cell holder and probes.
It is fast and accurate testing for volt and ir.
General rule of thumb, only keep the ones below 75mohm.

The biggest bonus with testing ir in my humble opinion: if you test first on ir before charging or capacity test, then you will have to test less cells, you take out the already bad ones without testing.
How long does it takes to test 100 cells properly(the actual testing) and how long does a ir plus v check take?
~4 hour charging 4 hours discharging handling ect vs a look on a display that is telling you the v and the ir.

When i open some "fresh" packs, i sort them per manufacturer, get the proper data for that cell andtest them first with my ir meter, odd ir is out, below 2v is out.
There are ways to bring back0 to 2v cells, but that is not my territory or practice, sorry

hope this was helpful, best