Measurement of Open Circuit Voltage

The measurement of Open Circuit Voltage can be time consuming as it needs to be established versus the State of Charge (SoC) of the cell. Doing this in fine steps from 0% to 100%, with the measurement being made “at rest”, means we have to charge/discharge and then let it rest before making the voltage measurement.

The “at rest” is important and requires that the cell is left to reach an equilibrium before the potential difference is measured.

open circuit voltage versus SoC for different chemistry

Depending on chemistry the OCV at a given SoC can be different depending on whether that SoC is approached by charging or discharging.

Hence, the OCV curve can take some time to establish. An experimental procedure for the measurement of OCV [1]:

StepExperiment StepCurrent RateLimits
1Thermal equilibrationt > 3h
2Constant current chargeC/10V < 4.2 V
3Constant voltage chargevariableI < 40 mA
4Voltage relaxationt > 3h
5Partial constant current dischargeC/10△Q = Qnom/50
6Voltage relaxationt > 1h
7Repeat steps 5 and 6 until V-limit reachedV >= 2.7 V
8Partial constant current chargeC/10△Q > Qnom/50
9Voltage relaxationt > 1 h
10Repeat steps 8 and 9 until V-limit reachedV <= 4.2 V

In addition there can be significant hysteresis between charge and discharge OCV and hence it is important to measure from 100% down to 0% and 0% to 100%. Hence discharging the cell in steps and making measurements and then charging the cell in steps and making a measurement.

The OCV curve below is for a Lithium Iron Phosphate chemistry and this has a significant hysteresis.

LFP OCV curve and SoC Estimation

This post has been built based on the support and sponsorship from: Eatron TechnologiesAbout:EnergyAVANT Future MobilityQuarto Technical Services and TAE Power Solutions.

References:

  1. C. R. Birkl et al, “A Parametric Open Circuit Voltage Model for Lithium Ion Batteries”, 2015 J. Electrochem. Soc. 162 A2271

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