CVP differs from conventional CCCV charging by controlling the cell’s polarization voltage rather than only its external terminal voltage. CCCV holds current constant until the terminal voltage reaches an upper limit, then holds voltage constant while current tapers; this can produce a rapid rise in polarization during the late charging stage. CVP continuously varies current, using lower current in the early and late stages and higher current in the middle, so polarization remains approximately constant at a lower level and the operating voltage more closely follows the cell’s equilibrium OCV curve.
The central distinction is what the charger regulates: CCCV regulates terminal voltage, while CVP regulates the voltage associated with electrochemical polarization. By keeping polarization low and stable, CVP separates the cell’s true SOC-dependent OCV behavior from charging-induced voltage rise, reducing degradation while enabling faster charging.
What CCCV Controls
Terminal Voltage Is the Primary Feedback Signal
In conventional CCCV charging, the cell receives a fixed current during the initial phase. When the terminal voltage reaches a predefined upper limit, such as 4.2 V for many lithium-ion cells, the charger switches to constant-voltage operation.
The charger therefore responds to the external terminal voltage, which includes more than the cell’s equilibrium voltage. It reflects the combined effects of ohmic resistance, electrochemical polarization, and open-circuit voltage.
Late-Stage Polarization Can Distort the Voltage
As SOC increases, the cell generally accepts current less readily. Under a fixed-current phase, this causes polarization voltage to rise, often sharply during the late charging stage.
The terminal voltage can consequently reach its upper limit before the cell’s internal state has been cleanly represented by its OCV. The charger then enters the CV phase, but the high polarization has already created additional electrochemical and thermal stress.
The Upper Voltage Limit Can Be Reached Too Quickly
A terminal voltage near 4.2 V does not necessarily mean that the cell’s OCV alone has reached 4.2 V. Part of that voltage may be the result of ohmic drop and polarization caused by the charging current.
This makes terminal-voltage control an imperfect proxy for equilibrium SOC, particularly at higher current and near full charge.
How CVP Tracks the OCV Curve
CVP Regulates Polarization Directly
Constant Polarization Charging varies the charging current throughout the profile to maintain polarization voltage, (U_P), within a relatively low and stable range.
The current is typically lower where the cell is less able to accept charge, especially toward the beginning and end of charging, and higher during the middle region where the cell can accept more current without excessive polarization.
The Voltage Relationship Explains the Difference
The relevant relationship is:
[ U_{\text{terminal}} - U_{\text{ohmic}} = U_P + U_{\text{OCV}} ]
Here, terminal voltage contains both the cell’s equilibrium contribution, (U_{\text{OCV}}), and the dynamic polarization contribution, (U_P), after accounting for ohmic voltage.
When (U_P) is kept low and relatively constant, changes in the controlled operating voltage more closely reflect changes in (U_{\text{OCV}}). The charging trajectory can therefore follow the cell’s inherent SOC-OCV curve more closely than a fixed-current profile with rapidly increasing polarization.
Current Becomes an Adaptive Control Variable
CCCV specifies current first and reacts when terminal voltage reaches a limit. CVP instead adapts current according to the cell’s polarization behavior at each point in the charging process.
This allows CVP to supply more current when polarization remains manageable and reduce current when the same current would cause excessive voltage displacement from the OCV curve.
How Lower Polarization Reduces Degradation
Less Overvoltage Stress
A large polarization voltage pushes the operating voltage above the cell’s equilibrium voltage. Maintaining a lower (U_P) reduces this overvoltage stress and helps prevent the charger from driving the cell aggressively against its electrochemical limits.
Fewer Side Reactions
High polarization and elevated terminal voltage increase the conditions that promote unwanted internal reactions. These side reactions consume active materials and can contribute to capacity fade.
By limiting polarization across the full charging profile, CVP reduces the driving force for these reactions compared with a profile that permits strong late-stage polarization.
Lower Internal Energy Loss
Polarization represents energy dissipated within the cell rather than stored reversibly. Keeping it low reduces internal losses and associated thermal stress during charging.
The result is a charging process that can balance charging speed with reduced aging pressure more effectively than a fixed-current profile.
Performance and Life Implications
Charging Speed Can Improve
Because CVP does not require the current to remain conservatively low throughout the entire charge, it can use a higher current during the middle portion of the profile.
The supplementary test results describe a reduction in charging time to roughly 53% of the CCCV time in one example, from approximately 3 hours to 1.6 hours, while also reporting a small improvement in capacity efficiency.
Capacity Retention Can Remain Strong
A faster charge is not automatically better for cell life. Its value depends on whether the increased current creates damaging polarization and thermal stress.
The cited 100-cycle testing indicates that CVP maintained a low capacity-degradation rate comparable to a low-rate, 1/3 C CCCV profile, suggesting that polarization control can improve speed without imposing the same aging penalty as uncontrolled high-current charging.
OCV Tracking Improves Interpretation
For battery testing, closer tracking of the SOC-OCV curve makes the measured voltage more representative of the cell’s equilibrium behavior rather than its temporary dynamic response.
This is useful when comparing cells, evaluating capacity, analyzing rate capability, or studying degradation mechanisms.
Understanding the Trade-offs
CVP Requires More Measurement and Control
CCCV is widely used because its two-stage logic is simple, standardized, and easy to implement. CVP requires reliable estimation or measurement of polarization-related voltage and a control system capable of adjusting current continuously or through sufficiently fine steps.
The quality of CVP results therefore depends on sensor accuracy, control bandwidth, cell characterization, and the suitability of the polarization model or calculation method.
OCV Is Not Measured Instantaneously Under Load
The OCV curve represents equilibrium behavior, while a charging cell is in a dynamic, nonequilibrium state. CVP can make the operating voltage track the OCV relationship more closely, but it does not turn a loaded terminal-voltage measurement into a direct instantaneous OCV measurement.
Accurate OCV characterization still requires appropriate rest periods, relaxation analysis, or other validated measurement methods.
Chemistry and Cell Condition Matter
The appropriate polarization target and current profile depend on cell chemistry, design, temperature, SOC, aging state, and operating limits. A CVP profile developed for one cell type should not be transferred unchanged to another.
Protection limits for voltage, current, and temperature remain necessary even when polarization is controlled.
Standardization May Favor CCCV
For production charging and cross-study comparisons, CCCV remains important because it is familiar and commonly specified in test standards and datasheets. CVP is most valuable when the objective includes optimizing charging behavior, reducing stress, or understanding the cell’s electrochemical response.
How to Apply This to Your Project
CVP is best viewed as a control strategy for separating useful charging current from avoidable polarization stress.
- If your primary focus is charging speed: Use CVP to permit higher current in the middle SOC range while reducing current during regions where polarization rises rapidly.
- If your primary focus is cycle life: Keep the polarization target low and stable, particularly during late-stage charging, to reduce overvoltage, side reactions, and thermal stress.
- If your primary focus is OCV or SOC characterization: Use the CVP voltage relationship as a closer approximation of the SOC-OCV trajectory, while validating true OCV with rest or relaxation measurements.
- If your primary focus is standardized benchmarking: Retain CCCV as a reference profile and compare CVP against it using charge time, efficiency, temperature, polarization, and capacity retention.
The practical choice is to use CCCV for simple standardized control and CVP when precise polarization management, OCV tracking, and long-term cell preservation are central objectives.
Summary Table:
| Feature | CCCV (Constant Current Constant Voltage) | CVP (Constant Polarization Voltage) |
|---|---|---|
| Control Parameter | Terminal voltage | Polarization voltage (U_P) |
| Current Profile | Fixed current then tapering | Adaptive current (lower early and late, higher middle) |
| Polarization Level | Can rise sharply late in charge | Kept low and stable |
| OCV Tracking | Indirect, distorted by polarization | Closer tracking of SOC-OCV curve |
| Degradation Stress | Higher due to overvoltage and side reactions | Reduced by limiting polarization |
| Charging Speed | Typically slower in late stage | Faster (e.g., ~53% of CCCV time) |
| Cycle Life | Can degrade faster with high current | Comparable to low-rate CCCV |
| Complexity | Simple, standardized | Requires advanced control and measurement |
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