I have a Li-Ion battery-powered device (Wemos, an ESP8266 based) and I can read information about the voltage applied to its input.
Is there a relation between this voltage and its remaining charge?
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The SOC, State of Charge depends greatly with Voltage under load and no load.
The load voltage drops due to the dielectric-electrode resistance which is high initially then reduces near 3.8V or wherever the curve is flattest. The ESR rises also with temperature dropping and aging charge cycles used.
Thus pulse step load measurements are compared with open circuit or light load (or nearly 0) to get both and compare Voc, Vload with ESR to determine a better result.
I found this for 1 cell but it may differ in C rating and thus ESR value and capacitance Ah ratings from yours using a constant current load ratio of the Capacity/1h or as commonly known the C charge, discharge rating. (not to be confused with the equivalent capacitance value C which is only used by EE’s who understand the more complex equivalent circuit of a battery is many RC values.