Battery capacity retention measures the percentage of a battery’s reference capacity that remains after a defined storage period or ageing process. It helps battery manufacturers, OEMs, quality assurance teams, and procurement professionals evaluate capacity degradation, verify product specifications, and compare battery performance under controlled test conditions.
What Is Battery Capacity Retention?
Battery capacity retention is the percentage of a battery’s reference capacity that it can still deliver after storage or ageing. It is calculated by dividing the measured capacity by the reference capacity and multiplying by 100.
For example, if a battery has an initial reference capacity of 100 Ah and delivers 90 Ah during a subsequent test, its capacity retention is 90%.
Battery capacity retention formula:
Capacity Retention (%) = (Discharge Capacity at Cycle N (Ah) / Initial or Reference Discharge Capacity (Ah)) x 100
A higher retention percentage indicates that more of the reference capacity remains. However, the result must be interpreted against the test conditions, battery application, and applicable acceptance criteria.
How Is Battery Capacity Retention Tested?
Battery capacity retention testing measures the capacity a battery delivers after a specified storage period or aging process. The procedure depends on whether the objective is to assess storage performance or capacity degradation after repeated charge-discharge cycles.
Battery Capacity Retention Testing Procedure
- Establish the reference capacity: Measure the battery’s initial capacity using a defined charging and discharging procedure.
- Apply the test conditions: Store the battery for a specified duration or subject it to a prescribed number of charge-discharge cycles.
- Control operating parameters: Follow the specified temperature, state of charge (SoC), current levels, and voltage limits.
- Measure the remaining capacity: Discharge the battery under the prescribed conditions and record the delivered capacity.
- Calculate capacity retention: Divide the measured capacity by the reference capacity and multiply by 100.
- Evaluate the result: Compare the calculated percentage against the relevant standard, product specification, or agreed acceptance criteria.
Storage-retention testing and cycle-life testing are different. Storage testing evaluates capacity after a defined period of storage, whereas cycle-life testing examines performance after repeated charge-discharge cycles.
Key Parameters in Battery Capacity Retention Testing
The following parameters help ensure that test results are interpretable and comparable:
- Reference and measured capacity: Establish the baseline and remaining capacity in ampere-hours (Ah) or another specified unit.
- Temperature: Record storage and test temperatures because temperature influences battery performance and degradation.
- Charge and discharge current: Apply the prescribed current levels during capacity measurements.
- State of charge: Control the battery’s charge level during storage and testing.
- Storage duration or cycle count: Document the elapsed storage period or number of completed cycles.
Which Standards Apply to Battery Capacity Retention Testing?
The applicable battery testing standard depends on the battery chemistry, design, and intended application.
IEC 62660-1:2018 specifies performance and life testing for secondary lithium-ion cells used in electric road vehicles. Its scope includes testing related to capacity, storage life, and cycle life.
This standard is relevant to lithium-ion cells intended for electric vehicle propulsion; it should not be treated as a universal standard for every battery type. Manufacturers and buyers should identify the applicable standard and test protocol for their specific application.
Why Is Battery Capacity Retention Important?
Battery capacity retention helps businesses evaluate how much capacity remains after storage or aging and whether a battery meets defined performance requirements.
Importance for Different Battery Industry Professionals
- Battery manufacturers: Assess capacity degradation, product consistency, and compliance with specifications.
- OEMs and system integrators: Determine whether cells or battery packs meet application-specific capacity requirements.
- Procurement teams: Compare supplier test reports and verify declared performance.
- Quality assurance teams: Identify deviations from acceptance criteria and investigate potential degradation.
- Battery operators: Assess remaining capacity to support operational planning and replacement decisions.
Capacity retention results can support product qualification, warranty assessment, and lifecycle planning. However, capacity retention alone does not establish battery safety, power capability, energy efficiency, or total service life. These require additional assessments appropriate to the application.
What Factors Affect Battery Capacity Retention?
Several factors influence battery capacity retention and how test results should be interpreted.
1. Temperature
High temperatures can accelerate certain degradation mechanisms, while low temperatures can reduce the capacity available during a measurement.
2. Charge and Discharge Rates
Current levels affect measured performance and can influence degradation over repeated use.
3. Depth of Discharge
The extent to which a battery is discharged during operation can affect cycle aging.
4. Storage Conditions
Storage duration, temperature, and state of charge influence retained capacity.
5. Battery Chemistry and Design
Different chemistries, electrode materials, and cell designs exhibit different degradation characteristics.
6. Operating History
Previous cycling and demanding operating conditions can affect remaining capacity.
For reliable comparisons, batteries should be tested against a clearly defined reference using consistent, application-appropriate procedures.
Battery Capacity Retention vs. Capacity Fade vs. State of Health
Battery capacity retention, capacity fade, and state of health (SoH) describe related but distinct aspects of battery performance.
| Parameter | Meaning |
| Battery capacity retention | Percentage of reference capacity remaining after a defined storage period or aging process. |
| Capacity fade | Reduction in capacity relative to the reference capacity. |
| State of health (SoH) | Assessment of battery condition relative to a defined baseline, potentially incorporating capacity, resistance, and other indicators. |
For example, a battery retaining 90% of its reference capacity has experienced 10% capacity fade, provided both values use the same reference and measurement basis. Capacity retention is one useful indicator of battery condition, but it is not a complete measure of SoH.
How Should Businesses Interpret Battery Capacity Retention Results?
A capacity retention percentage is meaningful only when the reference measurement, test method, and acceptance criteria are clearly defined.
When evaluating battery test reports or comparing suppliers, businesses should verify:
- ✓Initial reference capacity and measured capacity.
- ✓Applicable testing standard and procedure.
- ✓Temperature, charge-discharge rates, and voltage limits.
- ✓Whether the result follows storage or repeated cycling.
- ✓Acceptance criteria specified in the product specification or procurement agreement.
- ✓Measurement uncertainty and repeatability, where reported.
For example, consider two suppliers that both report 90% capacity retention. A procurement team should verify that both results use comparable reference capacities, storage durations, temperatures, and measurement procedures before treating the results as equivalent. The same percentage does not necessarily indicate the same performance under different test conditions.
Conclusion
Battery capacity retention is a key metric for assessing remaining battery capacity after storage or aging. Consistent testing, clearly defined reference measurements, and appropriate acceptance criteria help battery manufacturers, OEMs, and procurement teams evaluate performance, compare suppliers, and make informed product qualification decisions.
Frequently Asked Questions (FAQs)
1. What does 90% battery capacity retention mean?
It means the battery delivers 90% of its defined reference capacity under the specified test conditions. For a reference capacity of 100 Ah, this corresponds to a measured capacity of 90 Ah.
2. How is battery capacity retention calculated?
Divide the measured capacity by the reference capacity and multiply by 100. Both measurements should use consistent units and a clearly defined measurement basis.
3. Is battery capacity retention the same as battery efficiency?
No. Capacity retention measures the proportion of reference capacity remaining, while battery efficiency measures how effectively charge or energy is transferred, depending on the efficiency metric used.
4. Does high battery capacity retention guarantee a longer battery life?
No. High capacity retention indicates that more reference capacity remains under the specified conditions. Total battery life also depends on operating conditions, power capability, degradation mechanisms, and application requirements.
5. Which standards are used for battery capacity retention testing?
The applicable standard depends on battery type and application. IEC 62660-1 covers performance and life testing of lithium-ion cells for electric road vehicles, including capacity, storage-life, and cycle-life testing.