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Documentation Index

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Open-circuit voltage (OCV) is the voltage measured across a battery’s terminals when no current is flowing. It reflects the electrochemical potential difference between the electrodes and is fundamental to understanding battery operation—from predicting state of charge to modeling discharge behavior.

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Terminology: OCV vs OCP

Let’s distinguish between two related but different concepts:

Open-Circuit Voltage (OCV)

Defined for the entire battery as the electrical potential difference between the two electrodes when no current flows between them. This concept is widely used in electrochemical systems and electronic devices.

Open-Circuit Potential (OCP)

Defined for each electrode independently. It corresponds to the OCV measured between that electrode and a reference electrode. The OCV of the battery equals the difference between the OCPs of the positive and negative electrodes.

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How OCPs Change with Lithiation

For most electrode materials, the OCPs of each electrode (and thus the OCV of the battery) are not constant—they change as each electrode lithiates and delithiates:
Lithiation StateOCP
Higher lithiationLower OCP
Lower lithiationHigher OCP
This is why the OCV of a battery is often used as a proxy for its state of charge (SOC), which quantifies how much energy remains in the battery. As the battery discharges, its OCV decreases.
While most electrode materials exhibit an OCP that changes with lithiation state, some materials—like metallic lithium—have an OCP that remains nearly constant regardless of lithiation. This unique property makes lithium metal useful as a reference electrode in laboratory measurements and contributes to its high energy density in battery applications.

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Role in Intercalation Reactions

The OCPs play a critical role in the intercalation reactions at the surface of the active material:
  • When the electrode is at its OCP: Forward and backward intercalation reactions are in equilibrium—no net current at the interface
  • When electrode potential rises above OCP: Lithium deintercalates from the particles
  • When electrode potential falls below OCP: Lithium intercalates into the particles

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Connection to Charge/Discharge

This ties back to the definition of positive and negative electrodes. When a battery is discharging:
  1. The voltage drops below the battery’s OCV
  2. The potential of the positive electrode is lower than its OCP
  3. The potential of the negative electrode is higher than its OCP
  4. Result: Lithium deintercalates from the negative electrode and intercalates into the positive electrode
There are many nuances glossed over here, such as the contributions of the electrolyte or the connection between OCPs and electrochemical potentials.