Fabian Steger thank you so much for such a very detailed instructions. I would like to use C/20 to find out the remaining capacity. do you think it is suffice? or should I go to even lower current to find out the capacity?

Fabian is correct - a complex topic.

But you can streamline and simplify your test strategy Raam to establish a very accurate SOC/Self Discharge polynomial for any particular cell type, and lock it in.

But for what this will be worth going forward is dubious beyond an academic exercise because self-discharge implies no external constant load current (except for periodic short duty cycle high impedance parameters measurement polling) effectively the cells under test are open circuit cells discharging on a shelf and the resultant data will not be useful in calculations relating to real world dynamic variable loads; and charging and discharging scenarios.

Select three cells from the same batch. All three are SOC/Self discharge test cells and charged together to 100% at a nominal voltage for x period.

Because the most blatant error of the voltage-based SOC test process occurs when disturbing a battery with a charge or discharge is not in play in this test scenario, only one key parameter V of each cell needs to be precision polled, measured, and logged simultaneously to an SD Card by a Datataker at a nominated frequency (say every 1 minute).

Set the test to work, walk away and come back in 30 days to extract the SD Card flat file for processing.

The resultant cartesian coordinate plane (V/T) for the three cells, will accurately resolve to what you set out to establish.

Moving a bit off topic – a two measurement polling strategy (V and IR) is employed in some DC Coupled Solar Energy Generation Off Grid plus Battery Storage Systems, dynamically on the fly, at the Battery String Level rather than the Battery Cell Level, when evaluating Series Battery String Health; Aging and Performance against each other to gauge overall battery bank health, by identifying individual series strings that seem to have strayed beyond a nominated set point away from the average of a percentage of the best performing strings (not cells – String.Tot.V and String.Tot.IR combination).

In this real world usage scenario, this is a very useful remote monitoring and control option for system administrators to have in their toolbox through IoT App.

Lawrence Coomber

Thank you all for such detailed explanation

Raam Kasinathan:

The complex syndrome of the self-discharge is a very slow and weak (multi-)function[1] of the SOC for a medium to big size (parameter, commercial) new LFP battery. Apart size effect[1] (and the type of the LFP battery), you must fix many external parameters, using an environmental test chamber for Temperature, Humidity, vibrations, etc.

Anyway, the most sensitive tool for this task is G-EIS. Try to measure[2] one spectrum/day, for a month or more.

1. Deterministic models of Li-ion battery aging: It is a matter of scale https://www.sciencedirect.com/science/article/pii/S2352152X18303098?via%3Dihub

2. iDC=0uA, iAC=quite Low (say near an, iAC value corresponding to iDC~C/100, or less), in a very wide Frequency spectrum: 1mHz

thank you Prof.Samaras.