Li-ion Battery Voltage vs State of Charge

[JackElliott]

Am I doing this right?

I have a 14S pack of 18650 li-ion cells. 52V nominal voltage.

If a fully-charged cell is 4.20V, and if I "judgement-call" a cell at 3.00V to be fully-discharged, I get 1.20V from fully-discharged to fully-charged. In a 14S pack, that's 16.8 volts from discharged to fully-charged. Dividing this into ten equal-size steps, and labeling them as percentages, I get this for a 14S pack:

SOC  Pack voltage (resting)
100%  58.8V
90%   57.1V
80%   55.4V
70%   53.8V
60%   52.1V
50%   50.4V
40%   48.8V
30%   47.0V
20%   45.4V
10%   43.7V
0%    42.0V

Is that anywhere near right or useful? I see that the nominal voltage of 52V is closer to 60% "SOC" than it is to 50%.

Okay, this was done assuming that the resting voltage is related to state of charge, and with the assumption that it was a linear relationship. Chemistry is complicated so this is likely not very accurate.

How would you tweak my chart to more accurately reflect SOC as a function of resting voltage?

 

[spdas]

Aloha, no, that chart is no-where right or useful. Using Lithium the charge chart is no way Linear. There are steep dropoffs at the bottom and the top of the charge. Someone will explain it here.
francis

 

[JackElliott]

Thank you. Yes, my first-order approximation is going to be useless at the ends. So, how to modify it so it's somewhat more useful?

 

[A..dam]

Assuming (!) that all batteries are very similar in parameters, you can use ONE 18650 diagram as an example.
capacity vs voltage is highly current related
Google it &check images...

 

[wesnewell]

Find the cell you have here. Then look at the test data. there are many charts at different amp levels.
http://lygte-info.dk/review/batteries2012/Common18650comparator.php

 

[JackElliott]

wesnewell, thanks.