Open Circuit Voltage

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.

The typical lithium battery OCV curves versus SoC then looks like:

typical OCV vs SoC
C. Zhang et al, “A Generalized SOC-OCV Model for Lithium-Ion Batteries and the SOC Estimation for LNMCO Battery”, Energies 20169, 900. https://doi.org/10.3390/en9110900

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.
open circuit voltage versus SoC for different chemistry

The open circuit voltage versus SoC curves (average of charge/discharge values) shows quite different forms for the different chemistries.

open circuit voltage table of values

Battery Calculations Workbook

The Battery Calculations Workbook is a Microsoft Excel based download that has a number of sheets of calculations around the theme of batteries.

Including: OCV curves – exactly what it says on the sheet, average charge/discharge open circuit voltage curves for NMC, LFP, LTO and NaB.

Measurement of OCV

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.

References:

  1. C. R. Birkl et al, “A Parametric Open Circuit Voltage Model for Lithium Ion Batteries”, 2015 J. Electrochem. Soc. 162 A2271
  2. C. Zhang et al, “A Generalized SOC-OCV Model for Lithium-Ion Batteries and the SOC Estimation for LNMCO Battery”, Energies 20169, 900. https://doi.org/10.3390/en9110900