As a cell’s state of health (SOH) declines, its polarization voltage generally increases at the same charging current and test condition. Aging raises internal losses, including ohmic resistance and electrochemical overvoltage, so the measured cell voltage deviates further from its equilibrium voltage. This makes SOH a critical variable in battery testing: if it is ignored, the test system can underestimate voltage response, heat generation, and the severity of internal degradation.
Core takeaway: A higher polarization voltage at the same current, SOC, temperature, and rest condition is a strong indicator of aging-related impedance growth. Including SOH-dependent behavior in the polarization model improves resistance tracking, thermal prediction, lifecycle analysis, and end-of-life assessment.
What Polarization Voltage Represents
The difference from equilibrium voltage
When current flows, the operating voltage differs from the theoretical equilibrium voltage because of total polarization. During charging, the required terminal voltage rises above the equilibrium value; during discharge, the available terminal voltage falls below it.
Total polarization includes both ohmic voltage drop and electrochemical overvoltage. The latter includes charge-transfer limitations and concentration effects caused by ion transport, diffusion, and migration.
Why current creates polarization
A higher current produces larger internal voltage losses. These losses occur because electrons and ions encounter resistance, while electrochemical reactions and mass transport cannot respond instantaneously to the imposed current.
Consequently, polarization is not a fixed cell property. It depends on current rate, SOC, initial resting state, temperature, and SOH.
How SOH Changes Polarization Voltage
Aging increases voltage loss at the same current
For a given charging current and comparable test conditions, an aged cell typically develops a larger polarization voltage than a new cell. Experimental comparisons across cycling—for example, early-life versus substantially cycled conditions—show a significant increase in the established polarization amplitude as SOH declines.
This means that two cells with the same nominal voltage and charging current can require different charging voltages because their internal losses are different.
The physical sources of the increase
SOH degradation can increase ohmic resistance through active-material loss and deterioration of electrical contacts within the electrode structure. It can also increase polarization resistance by worsening reaction kinetics and mass-transport behavior.
The relative change in these components is not necessarily identical. During calendar aging, for example, ohmic resistance may increase by a greater percentage than polarization resistance, while the electrical time constant remains relatively stable.
The response can become strongly nonlinear
The SOH effect does not operate independently of the test condition. Polarization is also strongly affected by current rate and SOC, and it can rise sharply near extreme SOC levels or near the end of charging.
Therefore, an observed voltage increase cannot automatically be attributed to aging. The test must control or account for current, SOC, rest history, temperature, and charge protocol.
Why SOH Matters in Battery Testing
It prevents underestimation of voltage response
A model calibrated only on a fresh cell will generally underestimate the voltage required to charge an aged cell at the same current. It may also underestimate discharge voltage loss.
That error becomes important in fast-charge studies, voltage-limit testing, and lifecycle simulations, where small modeling errors can alter the apparent operating margin.
It improves thermal-generation estimates
Polarization represents energy dissipated inside the cell. As polarization increases, more electrical energy is converted into irreversible heat during operation.
If SOH-related polarization growth is omitted, thermal models and test algorithms may predict less heat generation than the aged cell actually produces. This can lead to inaccurate temperature forecasts and misleading safety conclusions.
It improves diagnostic interpretation
A change in total resistance does not identify its source. Separating ohmic resistance from polarization resistance provides more useful information about whether degradation is associated primarily with electrical contacts, active-material loss, electrolyte transport, or reaction kinetics.
Methods such as current interruption and electrochemical impedance spectroscopy can help distinguish these contributions.
Incorporating SOH into the Polarization Model
The role of the aging distortion factor
A linear RC polarization model can be extended with an SOH-dependent term:
[ U_P = U_P(0) + I\sum_k A_k + B_{\mathrm{SOH}} ]
Here, (U_P(0)) represents the initial polarization state, the (A_k) terms describe the RC polarization contributions, and (B_{\mathrm{SOH}}) represents the voltage distortion associated with cell aging.
The precise implementation of (B_{\mathrm{SOH}}) should be identified from controlled aging data rather than assumed to be universal across cells, chemistries, or operating conditions.
Why the correction is useful
Including an SOH term allows the model to reproduce polarization behavior across the cell’s lifecycle instead of treating every tested cell as if it were new.
This improves:
- Polarization-impedance tracking
- Voltage prediction during charge and discharge
- Thermal-generation estimation
- Fast-charging protocol evaluation
- Remaining-life and end-of-life analysis
SOH must be combined with other state variables
An SOH correction alone is insufficient because polarization also depends on SOC, current rate, and initial polarization state. A practical model may therefore require separate terms or parameter maps for SOC, initial rest condition, current, and SOH.
This is particularly important for chemistries such as LiFePO₄, where polarization can increase substantially at very low or very high SOC and at higher charge or discharge rates.
Using Polarization to Analyze Cell Aging
Track the same test condition over time
The most informative comparison is made at the same current, SOC range, temperature, and rest protocol. The change in polarization voltage can then be attributed more confidently to aging rather than to a changed operating condition.
A useful workflow records voltage curves over repeated aging intervals and extracts the polarization response at defined operating points.
Examine the shape, not only the amplitude
A larger polarization amplitude indicates increased internal losses, but the curve shape also contains information. Changes near the end of charge, for example, may reveal increasingly constrained reaction or transport behavior.
At very advanced aging, voltage behavior may become more complex. During low-rate recharge, overcharge voltage can initially rise with aging, but near the end of life a sudden decline may occur as charge efficiency and effective SOC behavior deteriorate.
This late-life behavior should not be interpreted as evidence that degradation has reversed. It may indicate that the cell has entered a different degradation regime.
Combine polarization with resistance decomposition
Tracking only total resistance can conceal the underlying degradation mechanism. Separating ohmic and polarization components provides a stronger basis for diagnosing structural, contact, electrolyte, and kinetic changes.
The combined trend is more valuable than any single voltage metric because different aging mechanisms can produce similar terminal-voltage changes.
Understanding the Trade-offs
Higher polarization does not equal SOH by itself
Polarization is influenced by multiple variables, so it is not a standalone SOH measurement. A hot cell, a high-rate test, an extreme SOC, or an insufficient rest period can all increase the measured voltage deviation.
SOH conclusions require controlled conditions or a model that explicitly accounts for these variables.
A simple linear model has limits
The RC formulation is useful for engineering prediction, but real battery polarization is nonlinear. The response may vary with SOC, current direction, current rate, temperature, and proximity to the end of charge.
A single constant (B_{\mathrm{SOH}}) may therefore be adequate for a limited operating window but inadequate across the full lifecycle and operating envelope.
Increased polarization can create feedback
Higher polarization raises internal heat generation. Temperature changes can then alter resistance and reaction behavior, affecting the measured voltage again.
For accurate testing, voltage, current, temperature, SOC, and aging history should be recorded together rather than analyzed as isolated signals.
Parallel cells add another complication
In parallel-connected cells, unequal polarization causes current imbalance. The more polarized cell may carry a different share of the dynamic current, accelerating capacity loss and reducing pack efficiency.
Cell-level polarization measurements are therefore important for understanding pack-level aging, not merely for characterizing individual cells.
Applying This to Battery Aging Tests
The most reliable analysis treats SOH as a state-dependent model input rather than a label applied after testing.
- If your primary focus is voltage prediction: Calibrate polarization parameters at multiple SOH levels while holding current, SOC, temperature, and rest conditions consistent.
- If your primary focus is degradation diagnosis: Decompose total resistance into ohmic and polarization components instead of relying only on terminal-voltage shift or total resistance.
- If your primary focus is fast charging: Include SOH, SOC, and current-rate dependence because aged cells can develop substantially higher polarization and heat generation at elevated rates.
- If your primary focus is end-of-life detection: Track both the growth of polarization and late-life changes in charge-voltage-curve shape, using charge efficiency and capacity trends as supporting evidence.
- If your primary focus is battery-pack reliability: Measure cell-to-cell polarization variation because non-uniform losses can drive current imbalance and accelerate individual-cell aging.
Treating SOH-dependent polarization as a measurable aging signature leads to more accurate models, safer test limits, and better decisions about cell lifetime.
Summary Table:
| Factor | Effect |
|---|---|
| SOH decline | Increases polarization voltage at same current |
| Ohmic resistance | Increases due to active material loss and contact deterioration |
| Polarization resistance | Increases due to worsened kinetics and transport |
| Thermal generation | Increases as polarization grows, affecting temperature forecasts |
| Voltage prediction | Underestimated if SOH ignored, impacting fast-charge and lifecycle studies |
| Diagnostic capability | Improved by separating ohmic and polarization components |
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