CC–CV cycling ages the cell under a controlled, repeatable load, while EIS periodically measures how its internal electrochemical resistance changes. A typical test charges a lithium-ion cell at constant current until it reaches an upper voltage limit, then holds that voltage until the current falls to a defined cutoff. The cell is subsequently discharged at constant current to a lower voltage limit, with EIS sweeps performed at scheduled states of charge, temperatures, or cycle intervals. High-precision testing systems are essential because small errors in current, voltage, timing, temperature, or impedance measurement can become large errors in State of Health (SOH) and Remaining Useful Life (RUL) predictions.
The value of degradation testing depends on measurement fidelity. CC–CV cycling provides the controlled aging history, EIS provides complementary information about evolving internal resistance and interfaces, and precision instrumentation ensures that the resulting capacity, voltage, efficiency, and impedance trends reflect the cell—not the test system.
How CC–CV Cycling Is Configured
Constant-current charging
The cycle begins with constant-current (CC) charging. The tester applies a fixed current—for example, 1.5 A—until the cell voltage reaches its programmed upper limit, such as 4.2 V.
This phase provides a repeatable charge input and makes it possible to compare capacity, voltage response, and cycle time across aging intervals.
Constant-voltage charging
Once the upper voltage limit is reached, the system switches to constant-voltage (CV) charging. The tester holds the cell at that voltage while the charging current gradually decreases.
CV charging ends when the current falls below a specified cutoff, such as 20 mA. This ensures that each cycle reaches a consistent terminal charging condition without holding the cell at high voltage indefinitely.
Constant-current discharge
After charging, the cell is discharged at a controlled current—for example, 2 A—until it reaches the lower voltage cutoff.
The tester integrates discharge current over time to calculate available capacity:
[ Q = \int I(t),dt ]
Because capacity depends on current, temperature, voltage limits, and prior operating history, these conditions must remain consistent when comparing degradation from one cycle to the next.
Rest periods and test conditions
Protocols commonly include controlled rest periods between charge, discharge, and diagnostic measurements. Resting allows transient voltage relaxation to decrease and improves the repeatability of subsequent capacity or impedance measurements.
Temperature, depth of discharge, C-rate, upper and lower voltage limits, and state-of-charge history should be controlled and logged because each can influence apparent degradation.
How EIS Is Integrated Into Degradation Testing
Applying a small AC perturbation
Electrochemical Impedance Spectroscopy (EIS) applies a small alternating-current or alternating-voltage perturbation around a defined DC operating point. The system measures the resulting voltage and current response as a function of frequency.
The response is represented as complex impedance:
[ Z(f) = Z'(f) + jZ''(f) ]
where the real and imaginary components reveal different aspects of the cell’s electrochemical behavior.
Selecting the measurement point
EIS is normally performed at a controlled state of charge, temperature, and rest condition. Measurements may be taken after charging, after discharging, or at selected SOC points during the aging protocol.
The exact choice depends on the diagnostic objective. A fixed SOC and temperature improve comparability, while measurements at multiple SOC values can reveal how degradation interacts with operating condition.
Choosing the frequency range
A frequency sweep may cover approximately 0.1 Hz to 5 kHz, as in the reference protocol. Other systems use broader ranges, such as roughly 10 kHz down to 10 mHz, when the instrumentation and cell response justify it.
The frequency range is not universal. High frequencies can reveal ohmic and contact-related effects, mid-frequency features can reflect charge-transfer and interfacial processes, and low frequencies can capture diffusion and mass-transport behavior.
Tracking impedance evolution
As a cell ages, EIS can reveal growth in:
- Ohmic resistance, associated with current collectors, electrolyte, contacts, and other resistive paths.
- Charge-transfer resistance, associated with electrochemical reaction kinetics.
- Interfacial features, including changes in surface films such as the solid-electrolyte interphase.
- Diffusion-related response, which becomes more visible at lower frequencies.
The most useful prognostic signal is often the change in impedance features over time, rather than one isolated spectrum.
Scheduling EIS during cycling
EIS may be performed periodically—for example, after a defined number of cycles or at regular capacity-retention intervals. The cycling test continues until a defined End of Life (EOL) threshold is reached, such as 30% loss of rated capacity.
Periodic EIS creates a time series that can be correlated with capacity fade, voltage-profile changes, coulombic efficiency, and cycle time.
Why Measurement Precision Determines Prognostic Quality
Small errors accumulate across cycles
Capacity is calculated by integrating current over time, so even a small current offset can accumulate into a meaningful capacity error over long tests.
Likewise, small voltage inaccuracies can change the point at which CC charging transitions to CV charging or the point at which discharge terminates. This changes the effective stress applied to the cell and can distort comparisons between cycles.
Accurate coulombic efficiency reveals early degradation
Coulombic efficiency compares the charge removed during discharge with the charge required during the preceding charge. It is close to 100% in healthy lithium-ion cells, so detecting small deviations requires highly accurate current and time measurement.
Repeated losses associated with parasitic reactions can consume active lithium and accelerate capacity fade. High-precision systems can identify these small changes early, before large capacity losses become obvious.
Stable control prevents instrumentation-induced aging
A battery tester is part of the experimental system, not merely a data recorder. Poor regulation, switching transients, channel drift, or inconsistent cutoff behavior can create artificial differences in aging rate.
Stable current and voltage control ensures that observed degradation is caused primarily by the cell’s chemistry and operating conditions.
High-fidelity data improves SOH and RUL models
Data-driven SOH and RUL algorithms depend on consistent relationships between measured features and actual cell condition. Useful inputs can include:
- Capacity retention.
- Voltage curves.
- Incremental-capacity features such as changes in (dQ/dV) peaks.
- Mid-voltage behavior.
- Coulombic efficiency.
- Cycle time.
- EIS resistance and frequency-domain features.
If these signals contain drift, noise, or systematic bias, the model may learn the tester’s artifacts rather than the cell’s degradation mechanisms.
Combining EIS With Other Degradation Indicators
Voltage-profile features can provide earlier warning
EIS is valuable, but it should not be treated as the only diagnostic measurement. Incremental capacity, mid-voltage, and cycle-time features can sometimes reveal degradation earlier than impedance changes during full cycling.
This is why robust test plans combine capacity, voltage, efficiency, timing, and impedance data instead of relying on a single prognostic feature.
Capacity measures the practical outcome
Full discharge from 100% SOC to the lower cutoff provides a direct capacity measurement when the test conditions are controlled. When a full discharge is not available, capacity can also be estimated from charge transferred between sufficiently separated SOC points.
Capacity retention remains a central EOL metric because it directly represents the cell’s ability to deliver usable energy or charge.
Impedance explains internal change
Capacity indicates how much performance has been lost, while EIS can help identify how the cell’s internal behavior is changing.
The two measurements are complementary: a cell can exhibit rising resistance before substantial capacity loss, or it can lose active material without an immediately proportional impedance increase.
Understanding the Trade-offs
EIS is sensitive to test conditions
Impedance depends strongly on temperature, SOC, rest time, excitation amplitude, wiring, contact resistance, and fixture quality. A spectrum collected under different conditions may not be directly comparable with an earlier spectrum.
Every EIS result should therefore be accompanied by its operating point and measurement configuration.
Wider frequency coverage increases complexity
Extending a sweep toward very low frequencies improves access to diffusion-related behavior but increases measurement time and susceptibility to drift. High-frequency measurements, meanwhile, are sensitive to cables, fixtures, contact resistance, and instrument bandwidth.
The optimal range is the one that captures relevant mechanisms with adequate signal quality and practical test duration.
More diagnostics can reduce throughput
Frequent EIS and diagnostic cycling produce richer data, but they also lengthen each test and reduce the number of cells that can be evaluated in parallel.
The schedule should match the purpose of the experiment: mechanism study may justify frequent diagnostics, while production screening may require a faster protocol.
Precision does not replace protocol discipline
A highly accurate tester cannot correct for inconsistent cell assembly, poor electrical contacts, uncontrolled temperature, or variable rest periods.
For solid-state and laboratory cells especially, uniform electrode density, compaction, pellet thickness, interfacial contact, and surface planarity are necessary to prevent measurement artifacts from being mistaken for electrochemical behavior.
Making the Right Choice for Your Goal
A suitable configuration should be selected according to the degradation question and the required prediction horizon.
- If your primary focus is capacity-fade measurement: Use repeatable CC–CV charge and constant-current discharge limits, controlled temperature, and accurate current integration.
- If your primary focus is resistance and interface evolution: Add periodic EIS at fixed SOC, temperature, rest time, and measurement amplitude.
- If your primary focus is early degradation detection: Combine EIS with incremental-capacity, mid-voltage, cycle-time, and coulombic-efficiency features.
- If your primary focus is reliable SOH/RUL modeling: Use a high-precision, multi-channel system with synchronized data logging, stable regulation, calibration, and traceable test conditions.
- If your primary focus is accelerated aging: Increase electrical or thermal stress only within a clearly defined protocol, while preserving enough measurement accuracy to separate real degradation from test-induced artifacts.
A well-controlled CC–CV/EIS workflow turns battery aging from a simple cycle-counting exercise into a quantitative prognostic experiment.
Summary Table:
| Configuration Aspect | CC-CV Cycling | EIS Integration |
|---|---|---|
| Purpose | Controlled aging under repeatable load | Periodic measurement of internal resistance and impedance changes |
| Procedure | CC charge to upper voltage, CV hold until current cutoff, CC discharge to lower limit | Apply small AC perturbation at fixed SOC/temperature, sweep frequency (e.g., 0.1 Hz–5 kHz) |
| Key Parameters | Current, voltage limits, cutoff current, rest periods, temperature | Frequency range, AC amplitude, measurement point (SOC/temperature), rest state |
| Data Obtained | Capacity (via current integration), coulombic efficiency, cycle time | Ohmic, charge-transfer, interfacial, and diffusion resistances |
| Role in Prognostics | Direct capacity fade, SOH, and cycle-life tracking | Early detection of resistance growth, complementary to capacity data |
| Precision Requirement | Current/voltage accuracy to avoid cumulative capacity errors | Impedance measurement fidelity to avoid artifacts from contact/wiring; stable control to prevent induced aging |
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