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Space batteries are not manufactured as a single component. They are built progressively from individual electrochemical cells into larger assemblies capable of delivering the voltage, current and stored energy required by a spacecraft.
ECSS-E-HB-20-02A describes a hierarchy in which individual cells are electrically connected to create strings. Strings can be incorporated into cell building-blocks or bricks. Building-blocks can be assembled into modules, and modules are connected to form the complete battery.
This hierarchy supports scalability, configuration control and progressive verification. Engineers can characterise electrical performance, environmental robustness, life capability and safety behaviour before committing higher-value hardware to the next level of assembly.
Under the handbook, a test item can be a single cell, a series or parallel string, a building-block, a module or a complete battery. However, this does not mean every test must be repeated at every level. Test applicability depends on the programme phase, test objective, product architecture, qualification status, mission environment and agreed verification approach.
Supporting documentation normally includes a test plan or test specification, an approved test procedure and a test report. Test reports should identify the test item, configuration, test conditions, results, data analysis and associated nonconformances.
Understanding the battery hierarchy is important for spacecraft designers, battery engineers, Assembly, Integration and Test teams, product assurance engineers and verification managers. It determines how requirements are allocated, how representative test items are selected and how evidence is carried forward into qualification and flight acceptance.
Important: ECSS-E-HB-20-02A is a guidance handbook. Compliance is established against the applicable project requirements and normative ECSS standards, not against the handbook alone
ECSS-E-HB-20-02A defines a battery as:
“One or more cells, or modules, electrically connected to provide the required operating voltage, current and energy storage levels.”
A battery is therefore the final electrical and mechanical assembly created from lower-level electrochemical and electrical building blocks.
A structured battery architecture can:
Increase voltage through series connections.
Increase capacity and current capability through parallel connections.
Support modular product design.
Allow representative testing at lower assembly levels.
Improve configuration control and traceability.
Reduce the cost and programme impact of late-stage failures.
Provide defined interfaces for monitoring and protection functions.
| Programme stage | Battery construction |
|---|---|
| Design | Select the cell chemistry, cell format, capacity, voltage, electrical topology, mechanical arrangement and thermal approach. |
| Manufacturing | Receive, screen, identify and assemble cells into strings, building-blocks, modules and batteries. |
| AIT | Integrate the battery with spacecraft electrical, thermal, mechanical and monitoring interfaces. |
| Verification | Demonstrate electrical performance, environmental suitability, life capability and safety behaviour. |
| Flight readiness | Confirm final configuration, acceptance status, battery health and readiness for spacecraft integration and operation. |
Step 1: Cell
A cell is the smallest electrochemical energy-storage unit within the battery architecture.Common physical formats include:
The cell is the fundamental energy-storage element. Its chemistry, capacity, resistance, voltage limits, temperature limits, ageing characteristics and safety behaviour influence the design of the complete battery.
Step 2: String
A string is an electrically connected group of cells. The handbook recognises both series and parallel cell arrangements as possible test items.
Series arrangement: ns1p
Cells are connected positive-to-negative.
Primary effect:
Example:
Three nominal 3.6 V cells connected in series produce a nominal 10.8 V string.
The notation 3s1p means three cells in series and one parallel path.
Parallel arrangement: 1smp
Cells are connected positive-to-positive and negative-to-negative.
Primary effect:
Ampere-hour capacity increases.
Current capability can increase.
Nominal voltage remains equal to the voltage of one cell.
The notation 1s3p means one series position containing three parallel cells.
Parallel connection requires careful consideration of cell matching, current sharing, interconnection resistance and protection. It should not be treated as a purely arithmetic increase in capacity.
Step 3: Cell Building-Block or Brick
ECSS-E-HB-20-02A defines a cell building-block or brick as:
“A sub-assembly unit which consists of identical electrically connected cells.”
Building-blocks or bricks are connected to form a module or battery. They can provide a practical intermediate level for manufacturing, inspection, testing and configuration control.
Step 4: Module
The handbook defines a module as:
“A set of any number of identical cells, electrically connected.”
A module is a mechanically distinct, deliverable assembly. This distinction is important. A cell brick is primarily an electrical sub-assembly, while a module has a defined mechanical identity and can normally be handled, transported, tested and integrated as a separate item.
A module can include:
Cells and cell restraints.
Busbars and electrical interconnections.
Temperature sensors.
Voltage-monitoring connections.
Protective devices.
Mechanical interfaces.
Thermal interfaces.
Internal wiring and insulation.
Step 5: Battery
The final battery is created by integrating the required cells, building-blocks or modules into a complete assembly.
Depending on the design, the battery can contain:
Cells.
Series and parallel connections.
Cell building-blocks or bricks.
Modules.
Busbars and power connections.
Voltage and temperature sensing.
Protective devices.
Balancing provisions.
Mechanical containment.
Thermal-control interfaces.
Spacecraft electrical and data interfaces.
The completed battery becomes the flight hardware installed within the spacecraft.
Risk Reduction
Progressive testing can identify unsuitable cells, electrical mismatch, weak interconnections, mechanical problems or thermal-control limitations before they become embedded in a high-value flight battery.
This does not eliminate risk automatically. Lower-level testing only provides useful evidence when the test item, interfaces, loading, mounting and environmental response are representative of the intended application.
Compliance
A defined hierarchy supports requirement allocation and the selection of appropriate verification methods. It allows the programme to identify which requirements are verified by cell data, which require module-level evidence and which can only be closed using the complete battery.
Verification Confidence
Evidence can be collected progressively across development, lot acceptance, qualification and acceptance activities.
The strength of the evidence depends on:
Test-item representativeness.
Configuration control.
Manufacturing traceability.
Test-level justification.
Equipment accuracy and calibration.
Defined pass and fail criteria.
Proper treatment of anomalies and nonconformances.
Mission Assurance
A controlled hierarchy improves confidence that the final battery can survive handling, launch and the intended mission environment while maintaining the required electrical and safety performance.
However, assembly-level success cannot compensate for an incomplete mission profile, weak life model or poorly justified acceleration method.
Cell
↓
String or cell group
↓
Cell building-block or brick
↓
Module
↓
Battery
Planning
Define:
Battery architecture and interfaces.
Applicable requirements.
Test-item levels.
Model philosophy.
Development, qualification and acceptance approach.
Lot acceptance strategy.
Mission and environmental profiles.
Verification methods.
Pass and fail criteria.
Data acquisition requirements.
Safety controls.
Nonconformance arrangements.
Preparation
Prepare and control:
Cell, string, building-block, module or battery configuration.
Cell identity and lot traceability.
Test fixtures.
Mechanical interfaces.
Electrical connections.
Thermal-control arrangements.
Instrumentation.
Calibrated test equipment.
Test specification.
Test procedure.
Hazard assessment.
Test Readiness Review evidence.
Execution
Potential activities include:
Initial electrical characterisation.
Capacity and energy measurement.
Internal resistance measurement.
AC impedance measurement.
Self-discharge testing.
Charge-retention testing.
Rate-capability testing.
Vibration and shock.
Thermal vacuum testing.
Leak and hermeticity testing.
Radiation testing.
Calendar ageing.
Cycling and mission-profile testing.
Safety and protective-device testing.
Not every test applies to every test item or programme phase. The applicability must be justified.
Review
Review:
Test configuration.
Raw and processed data.
Pass and fail criteria.
Measurement uncertainty.
Deviations from procedure.
Anomalies.
Nonconformances.
Test-item condition.
Requirement coverage.
Closure
Issue controlled reports containing:
Test-item identification.
Configuration.
Test conditions.
Equipment and instrumentation.
Results.
Data analysis.
Deviations.
Nonconformances and dispositions.
Conclusions.
Verification references.
Close all actions before relying on the evidence for qualification or flight acceptance.
Relevant ECSS Documents
The principal documents referenced by ECSS-E-HB-20-02A include:
ECSS-E-HB-20-02A: Lithium-ion battery testing handbook.
ECSS-E-ST-20: Electrical and electronic engineering.
ECSS-E-ST-10-03: Testing.
ECSS-E-ST-10-04: Space environment.
ECSS-Q-ST-20-08: Storage, handling and transportation of space hardware.
ECSS-Q-ST-70-02: Thermal vacuum outgassing testing for space materials.
ECSS-S-ST-00-01: ECSS system glossary of terms.
The applicable issue and revision of each document must be confirmed within the project documentation.
Handbook Versus Standard
ECSS-E-HB-20-02A provides practical guidance, test descriptions, documentation guidance and an applicability matrix. It does not replace the applicable normative project requirements.
A project should not claim that a battery is “ECSS certified” solely because selected handbook tests were performed.
A defensible compliance position requires:
Identification of applicable requirements.
Approved tailoring.
Defined verification methods.
Controlled test specifications and procedures.
Traceable evidence.
Formal closure through the project verification process.
Tailoring Considerations
Battery architecture and verification are mission-specific. Tailoring can be influenced by:
Spacecraft power demand.
Peak and continuous current.
Mission duration.
Orbit and eclipse profile.
Depth of discharge.
Charge and discharge profile.
Thermal environment.
Radiation environment.
Launch loads.
Spacecraft configuration.
Redundancy philosophy.
Hazard controls.
Cell heritage and qualification status.
Crewed or uncrewed operation.
Tailoring should be agreed and recorded. It should not be used simply to remove difficult or expensive tests without an evidence-based justification.
Customer Requirements
Customers, prime contractors, launch providers and safety authorities can impose requirements beyond the ECSS baseline. These requirements can include additional screening, lot controls, safety tests, transport evidence, mission-profile testing or independent review.
The verification baseline should reconcile ECSS requirements, customer requirements, product specifications, supplier limitations and mission-specific constraints.
Battery Management Subsystem Limitation
ECSS-E-HB-20-02A states that Battery Management Subsystem testing is not included in the handbook issue.
The handbook defines the Battery Management Subsystem, but it should not be used as the sole basis for validating BMS hardware, software, balancing logic, fault detection or protection algorithms. Those activities require a separate and properly defined verification basis.
Electrical Characterisation
Typical electrical activities include:
Capacity measurement.
Energy measurement.
Internal resistance measurement.
AC impedance measurement.
Open-circuit voltage measurement.
Cell electromotive-force characterisation.
Self-discharge testing.
Charge-retention testing.
Rate-capability testing.
Measurements must be performed under controlled and reported conditions. Capacity and resistance results are not meaningful without the associated temperature, state of charge, current profile, voltage limits, rest periods and measurement method.
Environmental Testing
Potential environmental tests include:
Low-level sine vibration.
High-level sine vibration.
Random vibration.
Shock.
Thermal vacuum.
Leak testing.
Hermeticity testing.
Radiation testing.
Corona testing for relevant high-voltage batteries.
Where cells or building-blocks are tested separately, their fixture should be mechanically representative of the intended installation. Input levels can require adjustment to account for amplification within the module or battery structure.
Life Testing
Life verification can include:
Calendar ageing.
Cycle ageing.
Mission-profile cycling.
GEO eclipse profiles.
LEO cycling profiles.
Real-time testing.
Accelerated testing.
Wear-out testing.
Capacity-retention trending.
Internal-resistance trending.
Accelerated testing requires particular care. Increased temperature, higher current or shortened rest periods can alter the dominant degradation mechanism rather than merely accelerate it.
Safety Testing
Potential safety tests include:
Overcharge.
Overdischarge.
External short circuit.
Internal short-circuit simulation.
Vent-pressure testing.
Burst-pressure testing.
Protective-device characterisation.
Balancing-system testing where applicable.
Safety testing requires a specific hazard assessment, controlled exclusion arrangements and suitable containment. It should not be treated as routine environmental testing.
Technical Risk
Cells can differ in capacity, resistance, self-discharge and ageing behaviour. Poor matching can cause current imbalance, voltage divergence and uneven thermal loading.
Interconnection resistance, sensing accuracy and thermal gradients can become more significant as the assembly grows.
Schedule Risk
Calendar and cycle-life programmes can take months or years. Late definition of the mission profile can invalidate an existing test programme or require additional testing.
Cost Risk
Failure of a complete battery can be significantly more expensive than failure at cell or building-block level. However, excessive lower-level testing can also consume schedule, samples and budget without closing meaningful requirements.
Verification Risk
The wrong test item can produce misleading evidence. A cell-level test might not represent module restraint, thermal paths, electrical interconnections or protective functions.
Documentation Risk
Weak traceability between incoming cells, manufacturing records, test samples and flight hardware can prevent valid qualification read-across.
Acceleration Risk
An accelerated test can create degradation mechanisms that are not representative of flight. Acceleration factors must be justified through data and modelling.
Configuration Risk
Changes to cell chemistry, manufacturing process, production lot, firmware, protection devices, interconnections or thermal design can undermine previously generated evidence.
| Activity | Cell | String or brick | Module | Complete Battery |
|---|---|---|---|---|
| Electrical characterisation | Common | Possible | Common | Common |
| Capacity and energy measurement | Yes | Possible | Yes | Yes |
| Internal resistance | Yes | Possible | Yes | Yes |
| Vibration and Shock | Yes, with representative fixtures | Yes | Yes | Yes |
| Thermal Vacuum | Yes | Possible | Yes | Yes |
| Calendar ageing | Primary cell level | Uncommon | Project-specific | Project-specific |
| Mission-profile cycling | Common | Possible | Common | YCommon |
| Safety testing | Common | Possible | Common | Common |
| Protective-device testing | Where fitted | Where fitted | Where fitted | Where fitted |
| Term | Meaning |
|---|---|
| Battery | One or more cells or modules electrically connected to provide the required operating voltage, current and energy-storage levels. |
| Cell | An individual electrochemical energy-storage unit. |
| Cell can | The cell packaging or container. |
| String | An electrically connected group of cells. The handbook discusses series and parallel strings but does not provide a separate formal glossary definition for “string”. |
| Cell building-block or brick | A sub-assembly consisting of identical electrically connected cells. |
| Module | A mechanically distinct, deliverable set of identical electrically connected cells. |
| Battery Management Subsystem | Electronic circuitry that prevents operation outside specified voltage, current and temperature ranges and manages cell-to-cell imbalance. |
| Accelerated test | A test intended to shorten cycle-life testing and estimate average life under normal operating conditions. |
| Activation | Introduction of electrolyte into an assembled cell during manufacture. Formation is included within activation. |
| Ageing | Permanent change in characteristics or performance caused by repeated use or passage of time. |
| Calendar loss | Available charge, normally expressed in ampere-hours. |
| Cycle life | Number of cycles completed under specified conditions before defined performance criteria are no longer met. |
| Cycle loss | Gradual and irreversible performance degradation caused by electrical cycling. |
| OCV | Open-circuit voltage measured when external current is zero. |
| EMF | Electromotive force under open-circuit steady-state conditions. |
| DoD | Depth of discharge, expressed as removed ampere-hours relative to nameplate capacity. |
| DoDE | Depth of discharged energy, expressed as removed watt-hours relative to nameplate energy. |
| EOCV | End-of-charge voltage. |
| EODV | End-of-discharge voltage. |
| SoC | State of charge, expressed as a percentage of available capacity. |
| Protective device | A device intended to interrupt or reduce current to prevent hazardous failure. |
| Test item | A cell, series or parallel string, module, building-block or battery selected for testing. |
Maintain Configuration Control
Assign a unique identity to every controlled cell, building-block, module and battery. Record lot, manufacturing history, storage history, test exposure and disposition.
Ensure Requirement Traceability
Link each requirement to:
The selected verification method.
The applicable test item.
The test specification.
The result.
The supporting report.
Any associated nonconformance or waiver.
Control State of Charge
Define and record the state of charge before, during and after testing. State of charge affects electrical response, safety behaviour and environmental-test risk.
Use Qualified Test Equipment
Use calibrated equipment with suitable range, accuracy, sampling rate and environmental compatibility.
Control Thermal Conditions
Record cell, module and ambient temperatures. Battery performance and ageing are highly temperature-dependent.
Use Representative Fixtures
Fixtures for cell, brick or module testing should represent the intended mechanical boundary conditions where environmental evidence is expected to support the flight configuration.
Conduct Independent Reviews
Use independent technical review at key points, including:
Architecture review.
Test-strategy review.
Procedure review.
Test Readiness Review.
Anomaly review.
Qualification closure.
Preserve Raw Data
Retain raw time-history data together with processed results. Summary plots alone are insufficient for investigating anomalies or reproducing calculations.
Close Actions Before Progression
Do not move to the next assembly level while unresolved anomalies could affect design validity, safety or qualification status.
Requirement Compliance
The battery satisfies the allocated electrical, environmental, life and safety requirements.
Verification Closure
Applicable verification activities are completed, reviewed and formally closed.
Hardware Integrity
The test item survives the required test sequence without unacceptable damage, degradation or hazardous behaviour.
Performance Retention
Capacity, energy, resistance and other key parameters remain within the defined limits.
Traceability
The programme maintains traceability from cell type and production lot through the tested assemblies to the flight battery.
Configuration Validity
The tested configuration is representative of the qualified or accepted flight configuration.
Flight Readiness
The battery has an approved configuration, acceptable health status, closed verification evidence and defined handling, storage and integration controls.
Planning Support
Resonate Testing can support the development of a proportionate verification strategy covering cell, building-block, module and complete-battery test levels.
Technical Guidance
Independent technical review can challenge assumptions concerning test-item representativeness, environmental levels, acceleration methods, instrumentation and pass criteria.
Test Campaign Development
Support can include the development and coordination of qualification and acceptance campaigns covering relevant electrical, vibration, shock and thermal-vacuum activities.
Documentation Support
Resonate Testing can review test specifications, procedures, readiness evidence, configuration records, data packages and test reports.
Independent Review Support
An independent review can identify gaps between the proposed test campaign, the mission profile and the evidence required for verification closure.
Qualification and Flight-Readiness Assistance
Support can continue from early planning through test execution, anomaly resolution, evidence review and qualification closure.
Whether you’re looking to contact us for the first time or have another testing requirement, we’d love to hear from you.
