The Open Circuit Voltage (OCV) is a fundamental parameter of the cell. The OCV of a battery cell is the potential difference between the positive and negative terminals when no current flows and the cell is at rest.
OCV needs to be established versus the State of Charge (SoC) of the cell from 0% to 100%. Meaning that the cell will need to be discharged in known steps and from a known SoC. The “at rest” is important and requires that the cell is left to reach an equilibrium before the potential difference is measured. Also, 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 [Ref 1]:
| Step | Experiment Step | Current Rate | Limits |
| 1 | Thermal equilibration | – | t > 3h |
| 2 | Constant current charge | C/10 | V < 4.2 V |
| 3 | Constant voltage charge | variable | I < 40 mA |
| 4 | Voltage relaxation | – | t > 3h |
| 5 | Partial constant current discharge | C/10 | △Q = Qnom/50 |
| 6 | Voltage relaxation | – | t > 1h |
| 7 | Repeat steps 5 and 6 until V-limit reached | – | V >= 2.7 V |
| 8 | Partial constant current charge | C/10 | △Q > Qnom/50 |
| 9 | Voltage relaxation | – | t > 1 h |
| 10 | Repeat steps 8 and 9 until V-limit reached | – | V <= 4.2 V |
The typical lithium battery OCV curves versus SoC then looks like:
Some points to consider:
- Hysteresis in the discharge versus charge curve can result in errors in estimation of the state of charge if the BMS uses cell voltage to estimate SoC.
- OCV curves are dependent on the chemistry of the cell.
References:
- C. R. Birkl et al, “A Parametric Open Circuit Voltage Model for Lithium Ion Batteries”, 2015 J. Electrochem. Soc. 162 A2271
- C. Zhang et al, “A Generalized SOC-OCV Model for Lithium-Ion Batteries and the SOC Estimation for LNMCO Battery”, Energies 2016, 9, 900. https://doi.org/10.3390/en9110900