> ## Documentation Index
> Fetch the complete documentation index at: https://docs.ionworks.com/llms.txt
> Use this file to discover all available pages before exploring further.

# Open-Circuit Voltage and Potential

> What open-circuit voltage (OCV) and open-circuit potential (OCP) mean, how they differ, and how they change with lithiation.

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](/guide/batteries-101/state-of-charge) to modeling discharge behavior.

## Terminology: OCV vs OCP

Let's distinguish between two related but different concepts:

<CardGroup cols={2}>
  <Card title="Open-Circuit Voltage (OCV)" icon="battery-full">
    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.
  </Card>

  <Card title="Open-Circuit Potential (OCP)" icon="circle-half-stroke">
    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.
  </Card>
</CardGroup>

## 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 State  | OCP        |
| ----------------- | ---------- |
| Higher lithiation | Lower OCP  |
| Lower lithiation  | Higher 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.

<Note>
  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.
</Note>

## 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

## 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

<Note>
  There are many nuances glossed over here, such as the contributions of the
  electrolyte or the connection between OCPs and electrochemical potentials.
</Note>

## Related Topics

* [Electrode Essentials](/guide/batteries-101/electrode-essentials)—electrode structure and terminology
* [State of Charge](/guide/batteries-101/state-of-charge)—how OCV relates to remaining charge
* [Battery Capacity](/guide/batteries-101/battery-capacity)—measuring how much charge a battery can store
* [Reaction Kinetics](/guide/batteries-101/reaction-kinetics)—how OCP drives intercalation reactions