DIY matching cell parameters?

[MattiFin]

Most seem to be concerned about matching capacity and internal resistance for the cells in battery pack.

IMO that is wrong approach if your concern is long-term pack balance.
What you would need is to match is self-discharge and coulombic efficiency as these are the actual players driving your pack to imbalance.

Both tricky and time-consuming to measure so how about:
A) elevated temperature self-discharge test. A week in 45Cel storage is only something like 1.5% discharge, possibly too tricky to reliably measure?
B) Measuring coulombic efficiency directly is really hard and would need super precision equipment.
But there might be a work-around and it is simply to cycle for multiple times (20? 100?) the series connected pack between 80% and 20% SOC.
After finishing the cycling discharge to 0% to find out which cells drifted farthest from the rest.

 

[MattiFin]

What? Nobody to even argue? ?

 

[Gazoo]

Assuming the cells are properly parallel top balanced, and then connected in series, you will know the delta of the cells at the top and the bottom by fully discharging and then fully charging the pack. It does require having a smart BMS.

I think many of us realize the cells we have received are not capacity matched to begin with. So I don't understand your point unless you are referring to multiple packs that make up a system. In that case each cell can be capacity tested and matched up to individual packs accordingly.