How to Conduct Effective Cycle Testing for Custom Battery Packs

What Is Battery Cycle Testing?

Battery cycle testing is a controlled process of repeatedly charging and discharging a battery pack while recording changes in electrical performance over time.

A complete cycle generally includes:

• Charging the battery according to specified charging protocols
• Allowing the battery to rest
• Discharging under controlled current
• Recording voltage, current, temperature, capacity, and resistance
• Repeating the process hundreds or thousands of times

The primary objective is to determine cycle life, which is commonly defined as the number of charge-discharge cycles completed before the battery capacity decreases to approximately 80% of its original rated capacity.

Depending on the application, testing may continue beyond this threshold to better understand long-term degradation characteristics.

Unlike a simple capacity test, cycle testing evaluates how the battery ages over time under realistic operating conditions.

Why Cycle Testing Matters for Custom Battery Packs

Every custom battery pack is unique.

Different products require different:

• Cell chemistries
• Charge rates
• Discharge currents
• Operating temperatures
• Mechanical designs
• Protection circuits
• Duty cycles

Because of these differences, manufacturers cannot rely solely on cell supplier specifications.

Instead, the complete battery pack—including cells, BMS, wiring, connectors, enclosure, and firmware—must be validated as an integrated system.

Proper cycle testing provides valuable insights into:

• Actual service life
• Capacity degradation trends
• Internal resistance growth
• Thermal stability
• Charging efficiency
• Voltage consistency
• Cell balancing effectiveness
• BMS protection performance
• Product reliability
• Warranty risk prediction

For OEMs, this information reduces uncertainty before launching products into the market.

Key Performance Indicators Measured During Cycle Testing

A professional cycle test measures far more than battery capacity. Multiple performance indicators are tracked throughout the battery's life to provide a complete picture of reliability and aging behavior.

These parameters allow engineers to identify subtle degradation long before complete battery failure occurs.

Typical Cycle Testing Conditions

To generate meaningful and repeatable results, cycle testing must be performed under controlled laboratory conditions. Variations in temperature, charging methods, or discharge rates can significantly affect measured battery life.

Below is an example of common testing conditions used for lithium-ion and lithium polymer battery packs.

While these conditions are widely used for baseline evaluation, additional testing at high and low temperatures is often necessary for batteries intended for outdoor, industrial, or medical environments.

High-quality testing equipment with precise current control and accurate data acquisition is essential for obtaining reliable, repeatable results.

How to Conduct Effective Cycle Testing for Custom Battery Packs

Effective cycle testing is much more than repeatedly charging and discharging a battery. A well-designed test plan should replicate the battery's real operating conditions while maintaining strict control over variables that could influence the results.

Based on our experience developing custom lithium battery packs for medical devices, industrial equipment, GPS trackers, and smart electronics, we recommend the following eight-step testing process.

Step 1: Define the Testing Objective

Before connecting the battery to a cycler, determine what you want to validate.

Typical objectives include:

• Estimating battery lifespan
• Comparing different cell brands
• Evaluating a new battery design
• Verifying BMS functionality
• Validating customer specifications
• Accelerating product qualification
• Predicting warranty performance

Different objectives often require different testing profiles. For example, a wearable device battery may use shallow discharge cycles, while an industrial battery pack may undergo deep discharge testing at higher currents.

Clearly defining the objective prevents unnecessary testing and produces more meaningful results.

Step 2: Select Representative Battery Samples

Testing only one battery pack rarely provides statistically reliable data.

Instead, manufacturers typically evaluate multiple samples from the same production batch to assess manufacturing consistency.

Sample selection should consider:

• Cell production lot
• Battery pack assembly batch
• Connector configuration
• Protection circuit version
• Battery management firmware
• Manufacturing date

Testing multiple samples helps identify variations caused by production processes rather than battery design itself.

Step 3: Stabilize Environmental Conditions

Temperature is one of the most influential factors affecting lithium battery performance.

For baseline cycle life evaluation, batteries are commonly tested in a temperature-controlled chamber maintained at 25°C ±2°C.

Additional environmental testing may include:

• Low-temperature operation (-20°C)
• High-temperature storage (60°C)
• Thermal cycling
• High-humidity exposure
• Combined temperature and vibration testing

Maintaining stable environmental conditions ensures that performance changes are caused by battery aging rather than external variables.

Step 4: Apply the Correct Charge and Discharge Profile

The charging and discharging process should closely match the battery's intended application.

Typical parameters include:

Charging

• Constant Current (CC)
• Constant Voltage (CV)
• Specified cutoff current
• Rest period

Discharging

• Constant current discharge
• Defined cutoff voltage
• Controlled discharge rate (commonly 0.5C–1C)
• Cooling interval before the next cycle

Testing outside the recommended voltage or current limits may artificially shorten battery life and produce misleading results.

For specialized applications such as drones, power tools, or medical equipment, customized duty cycles often provide more realistic performance data than standard laboratory profiles.

Step 5: Continuously Monitor Key Performance Data

A professional battery cycler records hundreds of data points throughout every charge and discharge cycle.

Important parameters include:

• Capacity (Ah or mAh)
• Voltage curve
• Charge efficiency
• Discharge efficiency
• Internal resistance
• Surface temperature
• Cell voltage balance
• Cycle number
• Charging duration
• Discharging duration

Rather than focusing solely on the final cycle life, engineers analyze long-term performance trends to identify early signs of degradation.

For example, a gradual increase in internal resistance often appears long before significant capacity loss occurs.

Step 6: Evaluate Capacity Retention

Capacity retention is one of the most widely accepted indicators of battery health.

It compares the battery's remaining capacity with its original rated capacity throughout the testing process.

The calculation is straightforward:

Capacity Retention (%) = Remaining Capacity ÷ Initial Capacity × 100

Many lithium-ion batteries are considered to have reached the end of their useful service life when capacity falls to approximately 80% of the initial value.

However, some medical, aerospace, or industrial applications may require stricter acceptance criteria depending on reliability requirements.

Step 7: Analyze Battery Degradation Trends

Cycle testing should identify not only when performance declines but also why it declines.

Typical degradation mechanisms include:

• Growth of the solid electrolyte interphase (SEI)
• Loss of active lithium
• Electrode material fatigue
• Electrolyte decomposition
• Lithium plating caused by improper charging
• Increased internal resistance
• Mechanical stress from repeated expansion and contraction

Understanding these degradation mechanisms allows engineers to optimize:

• Cell selection
• Charging algorithms
• Thermal management
• Mechanical structure
• Protection circuit design

This analysis often leads to longer battery life without changing the overall battery size.

Step 8: Compare Results Against Design Requirements

The final step is determining whether the battery meets the original project specifications.

Typical evaluation criteria include:

A battery pack that successfully meets these criteria is generally ready for further product validation or mass production.

Best Practices for Reliable Battery Cycle Testing

While testing equipment continues to improve, the accuracy of cycle life evaluation still depends heavily on proper testing methodology.

Based on industry experience, we recommend the following best practices:

• Test the complete battery pack rather than individual cells whenever possible.
• Use calibrated battery cyclers and regularly verify measurement accuracy.
• Allow adequate rest periods between charging and discharging to stabilize cell voltage.
• Record all environmental conditions, including temperature and humidity.
• Test multiple samples to account for manufacturing variation.
• Avoid interrupting long-term tests unless required by the test protocol.
• Analyze voltage curves and internal resistance trends in addition to capacity loss.
• Store raw test data for traceability and future product improvement.
• Repeat critical tests when significant design changes are introduced.
• Follow internationally recognized standards to ensure consistent and comparable results.

By combining disciplined testing procedures with comprehensive data analysis, manufacturers can identify potential issues early, reduce development costs, and deliver battery packs with more predictable long-term performance.

Common Mistakes That Reduce the Accuracy of Cycle Testing

Even with advanced battery cyclers, poor testing practices can produce misleading results. We've seen cases where a battery appeared to have a short lifespan simply because the test conditions did not reflect its intended application.

Below are some of the most common mistakes and their potential impact.

Avoiding these issues helps ensure that cycle life data is both repeatable and meaningful, allowing engineers to make confident design decisions.

International Standards Related to Battery Cycle Testing

Although every battery application has unique requirements, internationally recognized standards provide a common framework for evaluating performance and safety.

The most frequently referenced standards include:

IEC 61960

IEC 61960 specifies performance testing methods for secondary lithium cells and batteries. It defines procedures for measuring capacity, energy efficiency, cycle life, and electrical characteristics under standardized laboratory conditions.

It is widely referenced when comparing the performance of lithium-ion cells used in portable electronic devices.

Cycle life testing is commonly performed according to internationally recognized standards such as IEC 61960, which defines standardized methods for evaluating the performance of rechargeable lithium batteries. For detailed technical information, refer to the IEC 61960 Battery Performance Standard (https://www.iec.ch).

IEC 62133

IEC 62133 focuses on the safety requirements for rechargeable lithium batteries used in portable equipment.

In addition to electrical safety, it includes mechanical, thermal, and environmental tests that help verify the reliability of battery packs under normal and abnormal operating conditions.

UL 2054

UL 2054 is a North American safety standard for household and commercial battery packs. It evaluates electrical protection, enclosure integrity, mechanical robustness, and resistance to abuse conditions.

Many OEM products entering the U.S. market require battery packs designed to comply with UL safety requirements.

Battery packs sold in North America are frequently evaluated against UL 2054 safety requirements. More information can be found on the official UL Solutions website (https://www.ul.com).

UN38.3

UN38.3 is mandatory for the transportation of lithium batteries by air, sea, rail, and road.

Although it is not a cycle life standard, passing UN38.3 confirms that a battery pack can safely withstand transportation-related stresses such as vibration, shock, altitude simulation, thermal cycling, and external short circuits before commercial shipment.

Lithium batteries intended for international shipment should comply with UN38.3 transportation requirements. The official UN Manual of Tests and Criteria is available from the United Nations Economic Commission for Europe (https://unece.org/transport/dangerous-goods).

Manufacturer-Specific Validation Protocols

In addition to international standards, many OEMs develop their own validation procedures based on actual product usage.

Examples include:

• High-rate pulse discharge testing for power tools
• Continuous low-current cycling for IoT devices
• Frequent partial-state-of-charge cycling for wearable electronics
• High-temperature endurance testing for automotive and industrial equipment
• Deep discharge validation for portable medical devices

These customized protocols often provide a more realistic assessment of field performance than standardized laboratory testing alone.

How OEMs Can Use Cycle Testing Results to Improve Product Development

Cycle testing is not simply a quality-control exercise. It is a valuable engineering tool that supports better product design and more informed business decisions.

Well-documented test data can help OEMs:

• Select the most suitable lithium cell chemistry for the application.
• Compare suppliers using objective performance metrics rather than nominal specifications.
• Optimize battery pack structure to improve thermal management and extend service life.
• Fine-tune charging and discharging strategies through Battery Management System (BMS) programming.
• Estimate product lifespan and warranty periods with greater confidence.
• Reduce the likelihood of costly field failures or product recalls.
• Demonstrate compliance during customer audits and regulatory reviews.

By incorporating cycle testing early in the product development process, manufacturers can identify potential issues before mass production, reducing both development time and long-term support costs.

Why Work with A&S Power for Custom Battery Pack Validation?

At A&S Power, we believe that a reliable battery begins with rigorous testing—not assumptions.

With more than 15 years of experience in custom lithium-ion battery and lithium polymer battery manufacturing, we support OEM customers through every stage of battery development, from prototype evaluation to mass production.

Our engineering approach includes:

• Comprehensive charge-discharge cycle testing
• Capacity and internal resistance verification
• Cell matching and consistency analysis
• Battery Management System (BMS) functional validation
• High- and low-temperature performance evaluation
• Mechanical reliability assessments
• Transportation and safety testing support
• Production traceability and quality documentation

Whether your application is a portable medical device, industrial handheld terminal, GPS tracker, smart wearable, or consumer electronic product, our validation process is designed to help reduce technical risk and improve long-term reliability.

Rather than relying solely on theoretical specifications, we provide data-driven insights that enable our customers to make confident engineering decisions.