Open Circuit Voltage - Battery Design

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 potential difference between the anode and cathode changes based on the ion concentration and hence versus State of Charge (SOC).

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* **2016**, 9, 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.

LFP

Lithium Iron Phosphate (LFP) has a very flat OCV vs SOC characteristic compared to other chemistries. This is described as a ‘two-phase’ transition process where the surface concentration of lithium ions is almost the same during both intercalation and de-intercalation, resulting in a flat potential.

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.

This relationship between OCV and SoC underpins the terminal voltage method of SoC estimation, where a measured cell voltage is compared against a stored OCV curve to infer state of charge.

Parameterisation of OCV and Diffusion Coefficient

ICI extrapolation can be used as a fast parameterisation technique to determine the OCV of a material.
The diffusion coefficient calculated using ICI was highly comparable to GITT but recorded much faster. However, it was at a slightly higher magnitude.
Subsequently, this technique will be applied to other materials.