The key measurements are capacity, resistance, self-discharge, voltage, current, impedance, and temperature. Together, these parameters show how much energy a cell can still deliver, how efficiently it delivers power, and whether degradation is accelerating. Testing must also record the conditions under which each measurement was made, including charge/discharge rate, temperature, cutoff voltage, and cycle count.
State of health (SOH) cannot be determined reliably from a single reading. The most important indicators are usable capacity relative to nominal capacity, internal resistance relative to its baseline, and self-discharge rate, supported by voltage, temperature, impedance, and cycle-life data.
What Must Be Measured During Cell Testing
Usable charge capacity
Measure the cell’s delivered amp-hour capacity during a controlled discharge and compare it with its rated or initial capacity.
[ \text{Capacity retention} = \frac{\text{Measured capacity}}{\text{Reference capacity}} \times 100% ]
Capacity retention is one of the clearest SOH indicators. A rapidly declining effective capacity indicates loss of active material, increased polarization, or other degradation mechanisms.
Cell voltage
Record cell voltage continuously during charging, discharging, rest periods, and fault events.
Voltage data reveals:
- Open-circuit voltage and approximate state of charge
- Charge and discharge voltage behavior
- Voltage sag under load
- Overvoltage or undervoltage conditions
- Cell-to-cell imbalance in a series string
The voltage profile should be interpreted together with current and temperature because identical voltage readings can represent different conditions under different loads.
Charge and discharge current
Measure and log the actual current entering and leaving the cell.
Current is required for accurate amp-hour counting and for calculating power:
[ P = V \times I ]
It also establishes the applied C-rate, which strongly affects measured capacity, voltage sag, heating, and apparent power capability.
Internal resistance and DC impedance
Measure internal resistance, preferably under defined current-pulse conditions, to determine how much the cell’s voltage falls when current is applied.
Increasing resistance generally causes:
- Greater voltage drop under load
- Reduced power capability
- Increased heat generation
- Earlier cutoff during high-current operation
The result must be reported with its test temperature, SOC, pulse duration, and current because resistance is highly condition-dependent.
Electrochemical impedance
Use electrochemical impedance spectroscopy (EIS) when deeper diagnostic information is needed.
Impedance measurements can help separate different contributions to cell behavior, such as ohmic resistance, charge-transfer limitations, and diffusion-related effects. Repeating impedance measurements over cycle life can reveal degradation trends that may not yet be obvious from capacity alone.
Self-discharge current or rate
Measure the loss of charge during a controlled rest period with the cell electrically isolated from external loads.
Elevated self-discharge can indicate internal leakage, contamination, separator damage, or other abnormal conditions. It is particularly useful for identifying cells that appear normal during a short capacity test but are unsuitable for long-term storage or series-string operation.
Temperature
Measure cell or surface temperature, and record the temperature profile throughout the test.
Temperature affects capacity, resistance, voltage, aging rate, and safety. Record both peak temperature and time spent at elevated temperatures, along with exposure to high voltage and high current.
For meaningful comparisons, test temperature should be controlled and documented rather than treated as incidental data.
Performance Metrics That Complete the Assessment
Energy delivered
Calculate delivered energy by integrating voltage over discharged capacity:
[ E = \int V , dQ ]
Energy retention can decline differently from amp-hour capacity because voltage behavior and resistance also change.
Power capability
Evaluate the cell’s ability to deliver power during defined load pulses or discharge conditions.
Power capability depends on voltage, current, internal resistance, SOC, and temperature. A cell may retain substantial capacity while losing useful high-power performance because its resistance has increased.
Coulombic efficiency
Compare charge returned to charge supplied over a complete cycle:
[ \text{Coulombic efficiency} = \frac{\text{Discharge capacity}}{\text{Charge capacity}} \times 100% ]
Small inefficiencies accumulated over many cycles can help identify ongoing parasitic reactions and predict long-term capacity loss.
Cycle count and degradation rate
Record cycle number and track capacity, resistance, energy, and temperature against cycle life.
The absolute value at one point matters, but the trend is often more informative. A rapidly declining capacity or sharply increasing resistance can signal approaching end of life even if the cell has not yet crossed a fixed limit.
Equilibrium voltage and overpotential
After allowing the cell to rest, measure its equilibrium or open-circuit voltage. Compare this with voltage under load to assess polarization and overpotential.
These measurements support SOC modeling and help distinguish thermodynamic voltage behavior from losses caused by resistance and electrochemical kinetics.
What to Record for Series-Connected Cells
Individual cell voltages
For a cell string or battery pack, measure each cell rather than only total pack voltage.
The minimum and maximum cell voltages reveal imbalance, identify the weakest cell, and show whether balancing is effective. Total voltage can appear acceptable while one cell is approaching an unsafe limit.
Cell capacity imbalance
Compare the usable capacity of cells within the string.
The weakest cell can determine the usable capacity and power capability of the entire battery, especially when the battery management system limits operation based on the first cell reaching a voltage threshold.
Cumulative resistance
Track both individual-cell resistance and the combined string resistance.
This distinguishes a weak cell from resistance added by interconnects, busbars, terminals, contactors, or other system components.
String temperature distribution
Measure temperature at multiple relevant locations when possible.
A temperature gradient may identify unequal current sharing, poor thermal design, localized resistance, or a cell undergoing abnormal degradation.
Battery-management and fault data
Where applicable, log:
- Total battery voltage and current
- Minimum and maximum cell voltages
- Temperature measurements
- Charge and discharge current limits
- Warning and fault flags
- Cell-balancing status
- Relevant CAN, SMBus, or I²C data
This information helps reconstruct behavior before a fault and connect laboratory measurements with real operating conditions.
How to Make the Measurements Reliable
Control the test conditions
Capacity and resistance are not intrinsic constants. They vary with temperature, SOC, current rate, rest time, cutoff voltage, and measurement method.
Use the same defined conditions when comparing a cell with its baseline or evaluating degradation over time.
Establish a baseline
Measure a new or known-good cell before aging tests begin.
Baseline capacity, resistance, impedance, self-discharge, voltage behavior, and temperature response provide the reference needed to calculate meaningful changes.
Verify the test system
The measurement system should be evaluated for uncertainty, repeatability, reproducibility, and linearity across its operating range.
Poor measurement fidelity can make instrumentation drift look like battery degradation. This is especially important when detecting small changes in resistance, self-discharge, or coulombic efficiency.
Log data continuously
Snapshot measurements are useful, but continuous logging provides the context needed to interpret them.
At minimum, synchronize voltage, current, temperature, cycle count, operating state, and fault information so that abnormal behavior can be traced to a specific event or operating condition.
Understanding the Trade-offs
Capacity alone is insufficient
A cell can retain much of its original capacity while its internal resistance rises substantially.
Such a cell may still store energy but fail to meet the required power demand because of voltage sag and heat generation.
Resistance is highly condition-dependent
Resistance measured at different temperatures, SOC levels, pulse durations, or current amplitudes cannot be compared directly without normalization.
A valid trend requires a consistent test protocol.
EIS provides detail but requires expertise
Impedance spectroscopy can provide valuable diagnostic separation, but the results depend on frequency range, fixture quality, excitation level, cell state, and interpretation model.
It should complement—not replace—capacity, resistance, and self-discharge testing.
Pack-level data can hide weak cells
Total voltage, total capacity, or average temperature may conceal imbalance or localized degradation.
For series-connected systems, individual cell measurements are essential for identifying the limiting cell.
Fixed SOH thresholds are application-specific
The practical end-of-life threshold depends on the application’s requirements for capacity, power, safety, and operating range.
A cell may be unsuitable for one application because of power loss while remaining usable in another with lower current demands.
Making the Right Choice for Your Goal
Use a measurement set that matches both the intended application and the level of diagnostic certainty required.
- If your primary focus is basic SOH estimation: Measure usable capacity, internal resistance, self-discharge, voltage, current, and temperature under repeatable conditions.
- If your primary focus is high-power performance: Prioritize DC resistance, voltage sag, power-pulse behavior, temperature rise, and current limits.
- If your primary focus is degradation diagnosis: Add electrochemical impedance spectroscopy, coulombic efficiency, detailed voltage profiles, and continuous cycle-life logging.
- If your primary focus is series-string or pack evaluation: Measure every cell’s voltage, capacity, resistance, temperature, balancing behavior, and the string’s cumulative resistance.
- If your primary focus is reliable production testing: Validate the test equipment for uncertainty, repeatability, reproducibility, and linearity before relying on its SOH results.
A defensible SOH evaluation combines capacity fade, resistance growth, self-discharge, operating behavior, and thermal history rather than relying on a single parameter.
Summary Table:
| Parameter | Why It Matters | SOH Indicator |
|---|---|---|
| Capacity | Measures energy storage capability; drop indicates degradation | Capacity retention % |
| Internal Resistance/DC Impedance | Affects power delivery and heat; rise indicates aging | Resistance increase % |
| Self-Discharge | Identifies internal leakage; abnormal loss indicates issues | Self-discharge rate |
| Voltage | Reveals SOC, voltage sag, and faults | Voltage profile stability |
| Current | Provides amp-hour counting and power calculation | Current capability |
| Temperature | Affects performance and aging; high temps accelerate degradation | Temperature rise and distribution |
| Electrochemical Impedance (EIS) | Separates degradation mechanisms | Impedance trends |
| Coulombic Efficiency | Detects parasitic reactions; predicts long-term capacity loss | CE % |
| Cycle Count | Tracks degradation over life | Trend of capacity/resistance |
Note: SOH must be assessed from a combination of these metrics, not a single parameter.
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