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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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  • \$\begingroup\$ The usual word is "voltage". If you are reading the cell voltage, then subject to variation under load that would give some indication of charge state. But if you are reading the voltage of a power supply input to the device, no, that has no relation to the battery remaining charge. \$\endgroup\$ – Chris Stratton Aug 30 at 16:11
  • \$\begingroup\$ I think you should definitely use the word voltage; it took me a while to realize you weren't talking about the way li-polymer cells tend to puff up (thus increasing the tension on their packaging) when they fail. \$\endgroup\$ – Hearth Aug 30 at 16:18
  • \$\begingroup\$ @jsotola - rightly or wrongly, the IoT folks told them to post here. \$\endgroup\$ – Chris Stratton Aug 30 at 17:19
  • \$\begingroup\$ The relationship is good and predictive at around full charge, and around end of charge. Observe the voltage limits there and it will stop you damaging the battery. The voltage in between is a poor estimator of the SOC, as it depends on temperature, current and charge history. The best you can say about an intermediate voltage is that the battery is not empty yet, and will take some charge. \$\endgroup\$ – Neil_UK Aug 30 at 17:27
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Yes, there is a relationship. As the capacity decreases the voltage will also decrease. However, the relationship is not linear and measuring the cell voltage is not a very accurate way of determining its capacity.

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  • \$\begingroup\$ Thank you. Is this relationship empirical (and subject to change between batteries), or tabularized (in which case I could fit my data to it). Is there a better way to measure the charge (short of tracking the cumulative energy provided by the battery) \$\endgroup\$ – WoJ Aug 30 at 16:15
  • \$\begingroup\$ @WoJ I recommend you look up the term "SoC estimation"; this is an active area of research. \$\endgroup\$ – Hearth Aug 30 at 16:19
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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.

  • Coulomb counting is another method but energy loss from Peukert’s Law is another variable.

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.

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