That may be a language barrier.
Take a cell, and subject it to full charge (take note of the temperature & misc. Env conditions). Then let the cells rest until the terminal voltage stabilizes. Cell manufacturers usually standardize this time for new cells as 30 minutes. Plot terminal voltage vs. time. When it settles, that is the FCV.
Full charge being as per datasheet to 3,65V and CV or CP. Then cell rests and lowers voltage to FCV.
I think everyone is in agreement with the above. This is the method for the SOC OCV plot supplied by the manufacturer. This test is done at a variety of temperature intervals to form the SOC OCV curves that characterise the cells.
I can assure you the Eve datasheet is correct and they hover at the higher values at those temperatures (after hours). Are you sure your cells aren’t B grade?
So to confirm:
• Does your Method calculate the FCV or do you use the SOC OCV curve from manufacturer?
• Does your method compensate for changes in temperature?
• Does your method calculate the FCV based on capacity alone?
• Do you still think everyone is doing it wrong?
This is where I have to call you wrong. Cell FCV (essentially a voltage) is a function of chemistry and temperature, but not Internal Resistance.
You can attach an external resistance in series with a cell and check for yourself that it still won't change its FCV.
How can it be a function of Chemistry and not have an effect on internal resistance. The FCV will decrease as the cell looses capacity. The increase in internal resistance will affect the FCV.
Are you implying that a resistor placed In series does not incur a voltage drop? The measured voltage will be lower. I suggest you use a resistor with a proportional resistance to that of a cell internal resistance increase.
A cell in a battery pack may see an increase in IR later down the road. I as an Engineer like to go beyond "should not have" and "might face this down the road during design life". A cell may have high IR but be nominal capacity.
It is our job to build systems that can tolerate simple imperfections like this. Convergent balancing is exactly that.
The literature is clear on this. Capacity and internal resistance are inversely proportional. A decrease in capacity will see a rise in internal resistance ( and vice Versa).
The majority of balancing algorithms will easily deal with this. The goal of an impedance matched pack is to have all cells age similarly under standard balance conditions. I appreciate your altruism as an engineer but I feel it is misplaced. I too am an engineer not that it’s important. We all want to design batteries to last.
I absolutely disagree that there is a general voltage for all LFP. The 3,37V is not standard across the board. I feel this is misleading. compare this to GFB SOC OCV that hovers at 3,4…It does seem we’ve reach agreement on the importance of temperature.
Your sample set doesn’t sound large enough to be drawing such strong conclusions. Especially if your cells are B grade, aged or not directly from Eve. I think EVE knows exactly what the are doing. I have seen the higher FCV on the regular but it will vary with age and cycle life.