Initial SOC calibration is performed at the end of CCCV charging because the cells are closest to a known, near-equilibrium state at that point. During the CV phase, charging current gradually tapers to a very low cutoff, reducing the voltage error caused by internal resistance and polarization. The measured terminal voltage then closely approximates each cell’s open-circuit voltage (OCV), which can be mapped to an initial SOC baseline, typically near 99.4%–100%.
The end of the low-current CV stage provides the most repeatable starting point for SOC calibration: the cell is nearly fully charged, transient voltage effects are minimized, and its terminal voltage is a close practical approximation of OCV.
Why Calibration Waits Until the End of CCCV
CC Charging Produces a Voltage That Is Not OCV
In the constant-current phase, current flows continuously into the cell. The measured terminal voltage includes the cell’s equilibrium voltage plus voltage contributions from internal resistance and electrochemical polarization.
A simplified relationship is:
[ V_{\text{terminal}} \approx V_{\text{OCV}} + I R + V_{\text{polarization}} ]
Because the charging current is relatively high during CC operation, the resistance and polarization terms can be significant. Calibrating SOC from this voltage would therefore overestimate the cell’s true equilibrium voltage and potentially distort the SOC assignment.
CV Charging Allows the Cell to Approach Equilibrium
When the cell reaches the upper voltage limit, the charger switches from CC to CV mode. The charger holds the terminal voltage constant while the charging current naturally decreases.
As current tapers, the internal-resistance voltage drop becomes smaller. At the same time, lithium-ion concentration gradients and other polarization effects have more time to relax as the cell approaches electrochemical equilibrium.
The End-Current Threshold Identifies a Repeatable State
Testing systems commonly end the charge when current falls below a defined threshold, such as a small fraction of the cell’s C-rate. A threshold below approximately 1/30 C represents a low-current condition in which dynamic voltage errors are substantially reduced.
This threshold is more useful than simply detecting that the cell has reached the CV voltage limit. Reaching the voltage limit marks the start of the topping phase, not necessarily the end of charging or the point at which the cell’s OCV best represents its SOC.
Why Low Current Improves SOC Accuracy
Internal-Resistance Error Becomes Small
The resistive voltage contribution is proportional to current. As current approaches zero, the (I R) component also approaches zero.
Consequently, the terminal voltage measured near the end of CV charging is much closer to the cell’s OCV than the voltage measured during high-current CC charging.
Polarization Has Time to Decay
Polarization is a temporary voltage deviation associated with concentration gradients and electrochemical reaction dynamics. It can remain present even after the charger reaches the voltage limit.
The extended CV period allows ions to redistribute through the active material while the current decays. This reduces the difference between the measured terminal voltage and the cell’s equilibrium voltage.
Individual Cell Conditions Become More Comparable
For a series battery pack, cells may have different capacities, resistances, and polarization characteristics. Measuring them under substantial current would make those differences appear as voltage offsets that are not purely SOC differences.
Using the low-current end of CCCV charging provides a more consistent reference for comparing individual cells and establishing their initial SOC baselines.
Why Near-Full Charge Helps the Mapping
The Cell Is in a Known SOC Region
CCCV charging brings the cell to a defined upper-voltage condition and then continues until the current reaches a specified cutoff. This produces a more reproducible initial state than stopping immediately when the voltage first reaches the CV limit.
The resulting SOC is close to full charge, commonly represented in testing procedures as approximately 99.4%–100%, depending on the protocol and cell characteristics.
Voltage Can Provide Strong SOC Resolution
Near the upper end of many cell chemistries’ OCV-SOC curves, a small change in SOC produces a measurable voltage change. This voltage gradient allows the test system to distinguish small differences in the cells’ initial SOC values.
The important principle is not that voltage is always highly sensitive near full charge, but that the selected chemistry and operating region must provide a sufficiently useful OCV-SOC relationship for calibration.
The Baseline Supports Repeatable Testing
A controlled initial SOC baseline is essential for comparing capacity, energy, impedance, temperature response, and cycle-life results. If cells begin a test with different unknown SOC values, later differences may be incorrectly attributed to cell quality or degradation.
End-of-CCCV calibration reduces this source of experimental variation.
How CC and CV Work Together
CC Provides Efficient Bulk Charging
The CC phase delivers most of the charge efficiently at a controlled current. Cell voltage rises as SOC increases until the specified upper-voltage limit is reached.
However, the cell is still subject to current-dependent voltage error and may contain internal concentration gradients at the end of this phase.
CV Completes the Charge More Gradually
During CV charging, the voltage remains fixed while the current decreases. The declining current reduces voltage drop and allows the cell’s internal electrochemical state to catch up with the applied terminal voltage.
Stopping the procedure as soon as the CV voltage is reached would therefore produce a less complete and less equilibrated state.
The Cutoff Defines the Measurement Window
The end-current cutoff provides an operational definition of charge completion. It makes the calibration point reproducible across test cycles, cells, and experiments.
A well-controlled test system can capture each cell’s voltage at this point and use the result as the initial SOC reference.
Understanding the Trade-offs
OCV Is Approximated, Not Measured Instantly
Even at low current, the terminal voltage may not equal the fully relaxed OCV immediately. A separate rest period can allow additional relaxation, but it increases test duration and may not be included in every production or laboratory protocol.
Therefore, end-of-CCCV voltage should be treated as a carefully controlled practical approximation unless the procedure explicitly includes an OCV relaxation period.
The CV Stage Takes Time
The final portion of charging is slow because current tapers as the cell approaches the voltage limit. Extending the CV stage improves state definition but reduces test throughput.
The selected current cutoff must balance calibration repeatability, test duration, and the requirements of the battery chemistry.
Voltage Limits Must Be Appropriate
A CV voltage set too low can leave capacity unused and establish a baseline below the intended full-charge condition. A voltage set too high can increase voltage stress and accelerate degradation.
The upper-voltage limit and cutoff current must therefore be chosen for the specific chemistry and test protocol.
OCV-SOC Curves Are Chemistry-Dependent
The assumption that a small voltage change provides precise SOC resolution is not universal. Some chemistries have broad voltage plateaus over substantial SOC ranges, making voltage-only SOC estimation less sensitive in those regions.
Calibration should use an OCV-SOC relationship validated for the particular cell chemistry, temperature, and measurement procedure.
Pack-Level Voltage Can Hide Cell Differences
In a series pack, total pack voltage may appear correct even when individual cells have different SOC values. Initial calibration should therefore measure and evaluate individual cell voltages when the test objective is cell-level SOC matching or diagnosis.
How to Apply This to Your Project
The exact protocol should be adapted to the cell chemistry, temperature, voltage limit, and required measurement accuracy.
- If your primary focus is accurate initial SOC: Complete the CV phase until the current reaches a validated low cutoff, then use the individual cell voltages as near-OCV references.
- If your primary focus is test throughput: Use a consistent end-current threshold that is high enough to limit test time but low enough to control resistance and polarization errors.
- If your primary focus is capacity characterization: Do not terminate charging when the CV voltage is first reached; allow the tapering-current phase to complete so the cell reaches a repeatable near-full state.
- If your primary focus is long-term cell health: Avoid excessive CV voltage and unnecessarily prolonged high-voltage exposure, because the calibration benefit must be balanced against voltage-related degradation.
- If your primary focus is thermodynamic SOC accuracy: Add a defined rest period after charge and validate the resulting voltage against the chemistry-specific OCV-SOC curve.
The end of CCCV charging is used for initial SOC calibration because it combines a known near-full-charge condition with minimal current-induced voltage error, producing a practical and repeatable starting reference.
Summary Table:
| Reason | Explanation |
|---|---|
| Near-equilibrium state | Current tapers to near zero, minimizing internal resistance and polarization errors. |
| OCV approximation | Terminal voltage closely approaches open-circuit voltage (OCV). |
| Repeatable baseline | Provides a consistent near-full-charge starting point for SOC. |
| SOC resolution | At high SOC, small voltage changes map to measurable SOC differences. |
| Reduces variability | Ensures initial SOC is consistent across cells and tests. |
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