OCV is the equilibrium reference, CCV is the voltage delivered under load, and “coup de fouet” is a short-lived transient—not a separate equilibrium voltage. Open Circuit Voltage (OCV) is measured with zero current and approximates the cell’s thermodynamic state and state of charge (SOC). Closed Circuit Voltage (CCV) is measured while current flows, so it is lower or otherwise different because of internal resistance, polarization, operating conditions, and battery history. The coup de fouet is a brief initial voltage dip—typically around 20 mV per cell in fully charged lead-acid cells—when discharge begins.
OCV tells you where the battery is at rest; CCV tells you how it performs under a specified load; transient behavior explains what happens during the transition between those conditions.
The Three Voltage Concepts
Open Circuit Voltage: The Equilibrium Reference
OCV is measured with the battery under zero-load conditions. No external current is intentionally drawn, allowing the terminal voltage to approach the cell’s equilibrium potential.
OCV is useful for estimating SOC, especially when the battery has rested long enough for transient effects to decay. However, it is not a perfect SOC measurement because secondary reactions, side reactions, and higher-valence electrode oxides can shift the measured value away from the theoretical equilibrium voltage.
Closed Circuit Voltage: The Operating Voltage
CCV is the terminal voltage while the battery is charging or discharging under an active load. It represents the voltage available to the application or test system, rather than the cell’s ideal equilibrium potential.
During discharge, CCV generally falls below OCV because of ohmic voltage drop and electrode polarization. The difference is commonly expressed conceptually as:
[ V_{\text{CCV}} \approx V_{\text{OCV}} - I R - \text{polarization losses} ]
The exact response depends on current, SOC, temperature, chemistry, aging, and prior charge-discharge history.
Transient Voltage Behavior: The Time-Dependent Response
A transient is a temporary voltage response caused by a change in operating conditions, such as applying a load at the start of discharge. It should not be interpreted as the battery’s stable OCV or its long-term CCV.
Some transients recover or diminish with time, while others evolve into the normal voltage behavior of the discharge. Their duration and magnitude depend on the cell chemistry, operating point, and internal condition.
Why the Voltages Differ
Current Creates Immediate Voltage Loss
When current begins to flow, the battery’s internal resistance produces an immediate voltage change. Higher discharge current produces a larger instantaneous voltage drop for the same cell condition.
This effect is part of the CCV response and is distinct from longer-lasting electrochemical polarization.
Polarization Develops During Operation
Electrode and electrolyte processes can temporarily limit the battery’s ability to sustain the applied current. The resulting polarization causes CCV to differ further from OCV and can change as the discharge proceeds.
Temperature, SOC, and aging all influence these losses. Therefore, a CCV measurement is meaningful only when its test conditions—especially current and temperature—are specified.
Battery History Matters
A battery’s previous charge and discharge history can alter its immediate voltage response. Two cells with similar OCV may therefore produce different CCV values when subjected to the same load.
This is one reason precision characterization must record not only voltage, but also rest time, current profile, temperature, SOC, and cycle history.
Understanding the “Coup de Fouet”
What the Phenomenon Looks Like
In a fully charged lead-acid cell, applying a discharge load can produce a short-lived initial voltage minimum. The characteristic dip is approximately 20 mV per cell in the reference description, although its exact appearance depends on test conditions and cell state.
The voltage may subsequently recover from this minimum before following the broader discharge trend. This makes the event a transient dip in CCV, not a sudden loss of equilibrium SOC.
Why It Occurs
The coup de fouet is associated with crystallization overvoltage at the positive electrode. It reflects a temporary electrochemical condition during the transition from the charged state into discharge.
It should not be confused with the ordinary resistive voltage drop that occurs immediately whenever current is applied. Both can contribute to the initial voltage behavior, but they represent different mechanisms.
Why It Matters in Testing
A test system that applies a strict low-voltage cutoff immediately after the load is connected may interpret the transient minimum as an end-of-discharge condition. That can terminate a valid test prematurely.
Precision protocols should therefore account for the initial voltage response through appropriate sampling, filtering or timing logic, and cutoff criteria. The goal is to distinguish a temporary dip from a sustained low-voltage condition.
Comparing OCV, CCV, and Transients
Their Measurement Conditions
| Quantity | Current condition | What it primarily indicates |
|---|---|---|
| OCV | Zero external load | Approximate equilibrium potential and rough SOC |
| CCV | Active charge or discharge load | Voltage available under defined operating conditions |
| Transient response | Immediately after a condition changes | Short-term electrochemical and electrical behavior |
OCV requires a meaningful rest period if it is intended to represent equilibrium. CCV must be reported with its load, temperature, SOC, and history because those conditions directly affect the result.
Their Diagnostic Value
OCV is useful for assessing the battery’s rested state, but it can conceal weaknesses that appear only under load. CCV exposes those load-dependent limitations, including resistance and polarization.
Transient behavior provides additional diagnostic information. An unusual initial dip, delayed recovery, or changing response can indicate that the battery’s dynamic behavior differs from its rested-voltage appearance.
Understanding the Trade-offs
OCV Is Simple but Not Fully Conclusive
OCV measurement is non-invasive and easy to perform, but it can be misleading if the battery has not rested sufficiently. Side reactions and non-reversible processes can also prevent the measured voltage from representing a purely thermodynamic state.
Use OCV as a reference rather than treating it as an exact SOC or health measurement.
CCV Is Realistic but Condition-Dependent
CCV is more representative of actual use because it measures the battery while delivering power. However, its value changes with current rate, temperature, SOC, aging, and prior cycling.
A CCV value without test conditions is incomplete. Comparing values from different protocols can lead to incorrect conclusions about performance.
Transients Can Distort Automated Decisions
Transient dips are physically meaningful, but they can interfere with automated cutoff logic. A system optimized only for steady-state voltage may falsely classify a healthy battery as discharged.
At the same time, excessive delay or filtering can conceal a genuine fault. The test design must reject brief expected transients without masking sustained undervoltage.
Making the Right Choice for Your Goal
Use the voltage measurement that matches the question you are trying to answer:
- If your primary focus is estimating rested SOC: Use OCV after an appropriate rest period, while recognizing that side reactions and electrode chemistry can cause deviations.
- If your primary focus is usable power or application behavior: Measure CCV under a defined load, current rate, temperature, SOC, and battery history.
- If your primary focus is high-rate discharge testing: Characterize the initial transient and configure cutoff logic so phenomena such as the coup de fouet do not cause premature test termination.
- If your primary focus is battery diagnostics: Compare OCV, steady CCV, and transient response together rather than relying on any single voltage measurement.
A reliable characterization separates equilibrium voltage, loaded voltage, and time-dependent transients before drawing conclusions about battery condition.
Summary Table:
| Quantity | Current condition | What it primarily indicates |
|---|---|---|
| OCV | Zero external load | Approximate equilibrium potential and rough SOC |
| CCV | Active charge or discharge load | Voltage available under defined operating conditions |
| Transient response | Immediately after a condition changes | Short-term electrochemical and electrical behavior |
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