Capacity degradation and internal resistance growth measure different dimensions of battery health: capacity testing evaluates how much energy a battery can still store and deliver, while resistance testing evaluates how effectively it can provide power under load.
Capacity fade is primarily an energy-retention indicator; internal resistance growth is primarily a power-delivery indicator. Laboratory battery testing systems must therefore use different test protocols, operating conditions, and failure criteria for each metric.
What Each Indicator Measures
Capacity degradation measures usable energy
Capacity is the amount of charge a cell can deliver under defined conditions, usually expressed in ampere-hours or as a percentage of its original rated capacity.
A laboratory system fully charges the cell using the recommended charging profile, then discharges it at a controlled rate to a defined low-voltage cutoff. The tester integrates the current over time to determine the practical capacity.
Capacity is commonly tracked as:
Capacity retention (%) = measured capacity / initial capacity x 100
A frequently used soft end-of-life benchmark for energy-storage and backup batteries is 80% of original usable capacity. This is a practical reference point, not a universal threshold for every chemistry or application.
Internal resistance growth measures power capability
Internal resistance represents the opposition to current flow inside the battery. As resistance increases, the battery experiences a larger internal voltage drop and loses more energy as heat during high-current operation.
The testing system can estimate resistance by applying a controlled current pulse after a rest period:
R = (V_rest - V_loaded) / I
The result depends on the pulse duration, current level, temperature, State of Charge (SoC), and measurement location. It is therefore a test-condition-specific performance indicator rather than one fixed material property.
How Laboratory Evaluation Criteria Differ
Capacity testing emphasizes repeatability and energy accounting
Capacity measurements require stable and carefully controlled conditions because the result depends on charge protocol, discharge rate, temperature, cutoff voltage, and rest periods.
A typical evaluation includes:
- A prescribed full-charge procedure
- A low-rate discharge, such as C/20
- A fixed temperature
- A defined end-of-discharge voltage
- Accurate integration of discharge current
- Repeated measurements across aging cycles or storage intervals
The principal criteria are capacity retention, energy delivered, and the rate at which capacity declines over time.
Resistance testing emphasizes dynamic response
Resistance measurements are usually faster than full capacity tests because they use a brief high-current pulse rather than a complete discharge.
A typical evaluation includes:
- A controlled rest period
- A specified SoC
- A defined current pulse, such as 1C for 10 seconds
- Measurement of voltage immediately before and during the pulse
- Calculation of the voltage response divided by applied current
- Repetition at controlled temperatures and aging intervals
The principal criteria are resistance value, resistance growth relative to baseline, and the cell's ability to maintain voltage under the intended load.
The relevant test conditions are different
Capacity tests are strongly affected by discharge rate, cutoff voltage, and temperature because these factors determine how much stored energy can be practically extracted.
Resistance tests are especially sensitive to pulse current, pulse duration, SoC, temperature, and the cell's preceding rest history. A resistance result without its test conditions is difficult to interpret or compare.
Which Indicator Matters for the Application?
Energy-storage and backup batteries
For UPS systems, telecom backup, and similar applications, capacity degradation is usually the primary health indicator.
These systems must sustain a load for a required duration. A battery with acceptable resistance but insufficient capacity may still fail the actual backup-time requirement.
Capacity testing should therefore be tied to the required discharge duration, load profile, temperature range, and end-of-discharge voltage.
Automotive starter batteries
For starter batteries, internal resistance and impedance are usually more informative.
The battery must deliver a large current for a short time to crank an engine. Resistance growth can cause a substantial voltage drop during starting even when measured capacity has declined only modestly.
Short, high-current pulse tests and low-temperature measurements are particularly relevant because starting performance is governed by instantaneous power delivery.
Traction and hybrid applications
For electric vehicles and hybrid systems, both indicators are necessary.
Capacity affects driving range and available energy, while resistance affects acceleration power, regenerative-braking acceptance, heat generation, and voltage stability. A single health indicator cannot adequately represent all of these requirements.
Why Both Indicators Are Needed for SOH Evaluation
Capacity and resistance reveal different degradation effects
Battery aging commonly produces both capacity reduction and internal resistance increase, but they do not necessarily progress at the same rate.
A cell may retain substantial capacity while its resistance has increased enough to limit peak power. Conversely, it may retain acceptable power performance while its available energy has fallen below the application's requirement.
State of Health (SOH) should therefore be defined against the battery's intended function rather than inferred from one measurement alone.
Aging history changes the interpretation
Cycle aging results from repeated charge and discharge stresses, while calendar aging continues during storage and is accelerated by high temperature and high SoC.
Laboratory systems can separate these effects by testing cells across controlled temperatures, approximately -20°C to 60°C, and SoC levels from 0% to 100%. Multiple samples also help distinguish systematic degradation from cell-to-cell variation.
The degradation trend matters
Researchers should track the slope and shape of each indicator over time, not only its latest value.
Resistance may rise rapidly near the end of life, while capacity can show a gradual decline followed by a sharper aging region. Identifying this aging knee helps establish prognostic thresholds and assess second-life suitability.
How to Configure a Laboratory Testing System
Use capacity protocols for energy-retention questions
Configure a capacity protocol when the question is:
- How long can the battery operate a defined load?
- How much energy remains after storage or cycling?
- Does a material change improve practical energy yield?
- Has usable capacity fallen below the application's service threshold?
The protocol should define the charge method, discharge rate, cutoff voltage, temperature, rest periods, and number of repetitions.
Use pulse protocols for power-delivery questions
Configure a resistance or power test when the question is:
- Can the battery deliver a required peak current?
- How much voltage sag occurs under load?
- Is heat generation increasing?
- Has the cell's conductivity or high-rate performance deteriorated?
The pulse current, duration, SoC, temperature, and rest condition must remain consistent when comparing results over the battery's lifecycle.
Combine both protocols for development decisions
Battery R&D teams evaluating electrode processing, structural changes, or active-material formulations should measure both capacity retention and resistance growth.
Multi-channel systems can run controlled cycling, periodic reference capacity tests, and resistance pulses across many cells. This exposes trade-offs between energy yield, high-rate capability, and durability.
Understanding the Trade-offs
Capacity tests are comprehensive but time-consuming
A full capacity measurement may require a complete charge-discharge sequence, rest periods, and strict thermal control.
This makes capacity testing well suited to periodic reference measurements, but inefficient as the only high-frequency diagnostic during long aging experiments.
Resistance tests are fast but condition-dependent
A resistance pulse can be completed quickly, but the result is highly dependent on SoC, temperature, current, pulse duration, and relaxation state.
Resistance values should be compared only when the test conditions are controlled or when a validated correction model is available.
Neither metric is a universal failure predictor
An 80% capacity threshold may be appropriate for some energy-storage applications but irrelevant to a starter battery whose starting power has already become inadequate.
Likewise, a resistance threshold must be linked to voltage-sag limits, peak-current requirements, thermal constraints, or another application-specific performance boundary.
Theoretical capacity is not the correct laboratory baseline
Practical capacity is lower than theoretical active-material capacity because cells contain current collectors, separators, binders, casings, and other components that do not directly store energy.
Health evaluation should compare measurements with the cell's initial measured practical capacity, using the same validated protocol, rather than with a theoretical maximum.
Making the Right Choice for Your Goal
Choose the indicator and protocol that correspond to the battery's actual duty cycle and failure consequence.
- If your primary focus is energy storage duration: Prioritize standardized capacity-retention testing under the intended load, temperature, and cutoff conditions.
- If your primary focus is instantaneous power delivery: Prioritize controlled pulse-resistance or impedance testing at the relevant SoC and temperature.
- If your primary focus is electric-vehicle or hybrid performance: Track both capacity fade and resistance growth because range, acceleration, regenerative charging, and heat generation depend on different health dimensions.
- If your primary focus is lifetime prediction: Combine periodic reference-capacity tests with repeated resistance measurements and model both degradation trends.
- If your primary focus is material or process development: Use multi-cell testing to compare energy yield, rate capability, resistance growth, and capacity retention under identical protocols.
The most reliable battery health assessment measures the indicator that matches the battery's required job and validates it under the conditions where failure would actually occur.
Summary Table:
| Indicator | Measures | Test Method | Key Criteria | Best For |
|---|---|---|---|---|
| Capacity Degradation | Energy retention | Full charge/discharge at low rate (e.g., C/20) | Capacity retention %, energy delivered | Backup, energy storage |
| Internal Resistance Growth | Power delivery | Short high-current pulse | Resistance value, growth % | Starter batteries, high-power apps |
Both are needed for complete SOH. Capacity tests are comprehensive but slow; resistance tests are fast but condition-dependent.
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