Module Testing

Module testing is the bridge between cell-level characterisation and full pack validation. A module is rarely just “a box of cells” — it’s a defined group of cells connected in parallel (to build capacity) and then in a limited number of series steps (to build voltage), interconnected by busbars and instrumented by the BMS. This is the first level at which cell-to-cell interaction becomes the thing under test, rather than the cell itself.

That distinction matters. A cell test tells you how one electrochemical unit behaves in isolation. A module test tells you how a population of those units behaves once they’re electrically and thermally coupled — whether current splits evenly across a parallel group, whether a busbar joint runs hot under load, whether the BMS can actually see and balance every cell it’s wired to, and — critically — whether a fault in one cell stays contained or propagates to its neighbours. The tests naturally fall into eight categories:

• Performance Data: module-level capacity, energy, internal resistance, and power capability, including current sharing within parallel groups
• Electrical / Interconnect Testing: busbar resistance, joint and weld quality, insulation resistance, and HVIL continuity
• BMS Sensing & Data Validation: voltage sensing accuracy, temperature sensor placement, balancing function, and communications
• Thermal Tests: thermal imaging, cooling system validation, gradient mapping, and cell-to-cell thermal coupling
• Mechanical Tests: vibration, shock, crush, and structural integrity of the assembled stack
• Ageing & Parallel Group Divergence: module-level cycle and calendar ageing, and how cells within a group drift apart over life
• Abuse & Thermal Runaway Propagation: single-cell trigger methods and how a fault propagates through a populated module
• Legislative / Certification Testing: standards that test directly at module level, or that are commonly scaled down to it during development

Performance Data

Module performance testing repeats much of the cell-level performance characterisation — capacity, resistance, power — but the result is no longer a simple multiple of the constituent cell data. Busbar resistance, weld quality, and connection topology all introduce losses and asymmetries that only show up once cells are assembled.

Module Capacity & Energy

• Module Capacity Test
  • Procedure: CC-CV charge to module Vmax (sum of series-step Vmax); rest; CC discharge to module Vmin at a reference C-rate
  • Outputs: Module capacity (Ah) and energy (Wh); comparison against the theoretical sum of constituent cell capacities
  • Relevance: The gap between measured and theoretical capacity is a direct indicator of interconnect losses and parallel-group imbalance
• Module Power Capability (HPPC at module level)
  • Procedure: Hybrid Pulse Power Characterisation applied at module terminals across SoC and temperature
  • Outputs: Module DCIR map; pulse power capability (W); comparison against the sum of cell-level DCIR plus calculated interconnect resistance
  • Relevance: Confirms the busbar and joint design isn’t eroding the power capability the cell selection promised

Internal Resistance & Current Sharing

• Parallel Group Current Sharing
  • Procedure: Individual current probes (Hall-effect or shunt) clamped on each parallel leg within a group; module cycled under representative load profiles
  • Outputs: Current imbalance between legs (%); circulating current at rest (driven by OCV mismatch between parallel cells)
  • Relevance: Even small weld or busbar resistance variance between legs causes asymmetric current sharing — the lowest-resistance leg ages fastest, eventually making the imbalance worse
• Module OCV & Cell-to-Cell Voltage Spread
  • Procedure: Full rest period after charge; individual cell or cell-group voltages logged via the BMS sensing taps
  • Outputs: Voltage spread across the series string (mV); initial balance state
  • Relevance: Establishes the baseline spread the BMS balancing strategy has to manage from day one

Electrical / Interconnect Testing

This category exists almost entirely because of the module: it doesn’t apply at cell level, and by pack level the individual joints are largely inaccessible. Module testing is the practical window to verify busbar and joint design before it’s locked into the full assembly.

Busbar & Joint Resistance

• Four-Wire (Kelvin) Resistance Mapping
  • Procedure: Kelvin probes across each busbar joint and weld; micro-ohm measurement at low current to avoid self-heating
  • Outputs: Joint resistance map (µΩ) across the module; identification of outlier joints
  • Relevance: A single high-resistance joint becomes a localised hotspot under load and skews current sharing in its parallel group
• IR Thermography Under Load
  • Procedure: Module cycled at rated/peak current while monitored with a thermal camera
  • Outputs: Joint and busbar temperature rise (°C) vs. current; hotspot identification
  • Relevance: Correlates directly with the resistance map above; the practical pass/fail check for busbar design
• Weld / Joint Quality (Peel, Pull, Micro-section)
  • Procedure: Destructive sampling of production-representative joints: peel or pull-to-failure force; cross-section and micrograph of weld nugget
  • Outputs: Joint strength (N); weld penetration depth; porosity and inclusion assessment
  • Relevance: Mechanical integrity of the joint under vibration is as important as its electrical resistance

Insulation & Dielectric

• Insulation Resistance (IR) Test
  • Procedure: DC voltage applied between HV terminals and module chassis/baseplate; resistance measured
  • Standard: Typically referenced against ISO 6469-1 / IEC 60664 series principles
  • Outputs: Insulation resistance (MΩ); compared against minimum Ω/V requirement
• Dielectric Withstand (Hipot)
  • Procedure: AC or DC high voltage applied between HV circuit and chassis for a defined dwell time; leakage current monitored
  • Outputs: Pass/fail on leakage current threshold; confirms no breakdown path exists
• HVIL Continuity & Fault Injection
  • Procedure: Continuity check through the HVIL loop with the module connector mated and unmated; deliberate loop-break fault injected
  • Outputs: Confirms the interlock correctly signals a fault when the loop is broken — first practical opportunity to verify this above cell level
• Creepage & Clearance Verification
  • Procedure: Physical measurement of air gap and surface tracking distance between adjacent busbars and between busbars and chassis
  • Outputs: Compared against voltage-class requirements; confirms no margin has been eroded by manufacturing tolerance stack-up

BMS Sensing & Data Validation

A module is the first unit at which the full BMS sensing architecture — voltage taps, temperature sensors, balancing circuits, and communications — is exercised together. Cell-level BMS characterisation data tells you what the cell does; module testing tells you whether the BMS can actually see and act on it once wired into a real harness.

Voltage Sensing

• Cell Tap Harness Validation
  • Procedure: Reference voltmeter measurement at each cell tap compared against the value reported by the BMS over its communication bus
  • Outputs: Sensing accuracy (mV) per channel; identification of any cross-wired or open taps
  • Relevance: A wiring error here silently corrupts every downstream SoC, balancing, and protection decision
• Common-Mode Voltage Range Check
  • Procedure: Verify each cell-voltage measurement channel performs correctly across its actual operating common-mode voltage within the series string, not just at the cell’s own terminal range

Temperature Sensing

• Sensor Count & Placement Validation
  • Procedure: Compare measured temperature distribution (via a denser reference thermocouple grid) against the readings from the production sensor count and locations
  • Outputs: Confirms whether the specified sensor count actually captures the hottest cell and the worst-case gradient
  • Relevance: This is exactly the question addressed in How Many Temperature Sensors are Required? — module-level testing is where that sensor-count decision gets validated against real data rather than assumption
• Virtual / Estimated Temperature Validation
  • Procedure: Where a virtual temperature sensor model is used to infer cell temperatures the physical sensors don’t directly cover, module test data is used to validate the estimator against ground-truth thermocouples across the full load and ambient range
  • Outputs: Estimator error (°C) vs. physical reference; confirms the model is trustworthy enough to back BMS thermal limits

Balancing & Communications

• Passive / Active Balancing Functional Test
  • Procedure: Deliberately unbalance the module (via partial charge or discharge of selected cells); run the BMS balancing algorithm; track convergence
  • Outputs: Balancing current (mA) per channel; time to converge within target spread; bleed-resistor or active-balancer thermal check
  • Relevance: Confirms the BMS algorithm behaves correctly against the real cell population, not just a simulation
• Communications / Harness Validation
  • Procedure: Full module harness exercised under vibration and thermal cycling while communications integrity is monitored
  • Outputs: Bit-error rate / dropped-frame rate vs. condition; confirms harness routing and connector choice are robust in situ
• Sensor Fault Injection
  • Procedure: Deliberately introduce open-wire, short, and out-of-range conditions on voltage and temperature channels
  • Outputs: Confirms the BMS correctly flags the fault rather than reporting a plausible but wrong value

Thermal Tests

Module-level thermal testing is where cooling system design and cell-to-cell thermal coupling first become measurable. A single-cell thermal model cannot capture neighbour-to-neighbour heat transfer; a module can.

Thermal Imaging & Gradient Mapping

• IR Thermography Under Load
  • Procedure: Thermal camera coverage of the module surface across a representative duty cycle
  • Outputs: Surface temperature map; identifies the worst-case cell location for sensor placement and validates against predicted hotspots
• Thermal Gradient Mapping (Cell-to-Cell and Top-to-Bottom)
  • Procedure: Dense thermocouple grid across the cell population, including edge and centre cells
  • Outputs: Maximum cell-to-cell ΔT; identifies whether edge cells run cooler (more exposed surface) or hotter (further from the cooling interface) than centre cells

Cooling System Validation

• Coolant Flow Rate vs. ΔT
  • Procedure: Coolant inlet/outlet temperature and flow rate logged across the load profile
  • Outputs: Heat removed (W) vs. flow rate; confirms the cooling circuit design meets the heat generation rate established at cell level
• Cold Plate / Thermal Interface Resistance
  • Procedure: Differential temperature measured across the thermal interface material (TIM) between cell and cold plate at known heat flux
  • Outputs: Interface thermal resistance (K·cm²/W); a key term often missing from early-stage thermal models

Cell-to-Cell Thermal Coupling

• Thermal Coupling Characterisation
  • Procedure: A single cell is deliberately heated (resistive heater or controlled overcurrent) while neighbouring cell temperatures are logged
  • Outputs: Thermal coupling coefficient between adjacent cells; time constant for heat transfer to propagate through the module structure
  • Relevance: Direct input to thermal-propagation and thermal runaway models — this is non-destructive groundwork for the abuse testing in the next category

Mechanical Tests

Module-level mechanical testing repeats the vibration, shock, and structural themes from cell mechanical testing but shifts the focus from the cell body itself to the assembly that holds the cells together — end-plates, tie-rods, busbars, and the harness.

Vibration & Shock

• Sine Sweep & Random Vibration
  • Standard: IEC 62660-2 principles (cell-level); commonly scaled to module per ISO 19453-6 / SAE J2380-style profiles
  • Procedure: Module mounted to vibration table in its production fixture; swept sine and PSD random profiles applied in X, Y, Z
  • Outputs: Busbar and weld fatigue assessment; loose-fastener detection; pass/fail vs. voltage and communications integrity during test
  • Relevance: Busbars and welds are far more vibration-sensitive than the cells they connect — this is the primary failure mode module vibration testing is looking for
• Mechanical Shock
  • Procedure: Defined half-sine or trapezoidal shock pulse applied to the assembled module in each axis
  • Outputs: Post-shock continuity, IR, and capacity check; confirms no joint cracked or connector backed out

Structural Integrity

• Crush / Compression Test
  • Procedure: Quasi-static compressive load applied to the module structure (end-plates and enclosure), representative of pack-level crush loading
  • Outputs: Force-displacement curve; structural failure mode; whether cell venting or short circuit occurs before structural collapse
• Swelling & Stack Compression Force Over Life
  • Procedure: Load cell or pressure film within the module stack, monitored through charge/discharge cycling and through ageing
  • Outputs: Compression force vs. SoC and vs. cycle count; confirms end-plate and tie-rod design maintains adequate — but not excessive — stack pressure as cells swell with age
  • Relevance: Builds directly on cell-level pressure / swelling data, now in the actual mechanical constraint the cells will live in
• Fastener & Joint Torque Retention
  • Procedure: Fastener torque checked before and after vibration and thermal cycling
  • Outputs: Torque retention (%); identifies any joint prone to loosening in service

Environmental Tests

Module enclosures add a sealed boundary between the cells and the outside environment, so module-level environmental testing checks that boundary rather than repeating the cell-level tests above.

• Enclosure Sealing (IP Rating): dust and water ingress per IEC 60529, sized to the module’s mounting location (underbody, engine bay, indoor rack)
• Humidity / Damp Heat: steady-state or cyclic exposure per the IEC 60068-2 series, checking busbar and connector corrosion and insulation resistance
• Salt Spray / Corrosion: per IEC 60068-2-11 or ASTM B117, for modules mounted in coastal or de-icing-salt road environments
• Thermal Shock / Cycling: rapid ambient-temperature transitions to check for seal, potting, and interconnect fatigue distinct from the internal thermal cycling covered above

Ageing & Parallel Group Divergence

Cell-level ageing tells you how one cell degrades. Module-level ageing tells you whether cells within a parallel group degrade together — and if not, why not.

Module-Level Cycle & Calendar Ageing

• Module Cycle Ageing
  • Procedure: Module cycled between defined voltage limits at a reference C-rate; periodic reference performance tests (capacity, DCIR)
  • Outputs: Module-level capacity fade and resistance growth curves; compared against the cell-level ageing model to quantify any module-specific acceleration
• Module Calendar Ageing
  • Procedure: Module stored at fixed SoC and temperature; periodic capacity and resistance checks
  • Outputs: Calendar fade at module level; checks whether parallel-group circulating current accelerates fade relative to isolated cells

Parallel Group Divergence

• Cell-to-Cell Divergence Within a Parallel Group
  • Procedure: Individual cell currents and, where instrumented, individual cell voltages within a parallel group tracked across ageing
  • Outputs: Spread in capacity and resistance between cells in the same group over life; identification of a “weak cell” pulling disproportionate current from its neighbours
  • Relevance: This is one of the central reasons to test at module rather than cell level — divergence within a parallel group is invisible to single-cell testing by definition
• Balancing Efficacy Over Life
  • Procedure: Re-run the balancing functional test (see BMS Sensing above) at intervals through the ageing programme
  • Outputs: Confirms balancing current and convergence time remain adequate as cell impedance and capacity spread grow with age

Abuse & Thermal Runaway Propagation

A module is the smallest assembly at which thermal runaway propagation — rather than a single cell’s runaway behaviour — can be meaningfully studied, because neighbouring cells, busbars, and any module-level barriers or vent paths are physically present. This is one of the most safety-critical reasons to test at this level rather than jumping from cell data straight to a full pack build.

Trigger Methods

• Nail Penetration Trigger
  • Procedure: A single target cell within the populated module is triggered into internal short circuit via nail penetration, following the same method as cell-level nail testing, while neighbouring cells remain instrumented and untouched
  • Outputs: Whether and how fast the runaway propagates to adjacent cells
• Internal Heater Trigger
  • Procedure: An embedded heater element forces a single target cell into runaway without mechanical penetration of neighbours
  • Outputs: Cleaner trigger-cell isolation than nail penetration; preferred where the propagation path itself, rather than the trigger mechanism, is the variable of interest
• Overcharge Trigger
  • Procedure: Target cell or parallel group overcharged beyond Vmax until thermal event occurs, representative of a BMS sensing or balancing failure scenario

Propagation Characterisation

• Cell-to-Cell Propagation Timing
  • Procedure: Thermocouples on every cell in the module log temperature through the event
  • Outputs: Time from trigger-cell venting to each neighbouring cell’s own runaway onset; propagation front speed through the module
  • Relevance: Directly informs spacing, barrier material, and vent path design before committing to a pack-level architecture
• Vent Gas Behaviour in a Confined Module
  • Procedure: Gas composition and pressure monitored within the (typically enclosed) module structure during the event
  • Outputs: Peak pressure; gas accumulation; whether the module’s own vent path is adequate or whether gas pools and re-ignites
  • Relevance: See Thermal Runaway in High-Voltage Systems for how this connects to system-level isolation and safety strategy
• Barrier & Divider Efficacy
  • Procedure: Repeat propagation testing with candidate inter-cell barrier materials (mica, ceramic-fibre, phase-change) in place
  • Outputs: Comparative propagation delay or full arrest; this is the practical design-iteration loop module-level TR testing exists to support, since it’s far cheaper to iterate barrier material at module scale than at full pack scale

Module-level propagation sits between the single-cell propagation mechanisms covered on Thermal Propagation and the pack/rack containment strategies covered on Pack Thermal Runaway and its Propagation Resistance Testing page. UL 9540A structures exactly this progression as a four-tier cell → module → unit → installation test sequence, escalating from a single-cell trigger to a full-installation fire test; the module tier’s job is to establish whether propagation is contained cell-to-cell within the module before it reaches neighbouring modules.

Electrical Abuse at Module Level

• External Short Circuit
  • Procedure: Defined low resistance applied across module terminals
  • Outputs: Peak current, busbar and joint temperature response; confirms fusing or contactor protection acts before joint damage occurs
• Forced Discharge / Reverse Charge
  • Procedure: A weaker cell within a series step is deliberately driven into reverse charge by the remaining stronger cells in the string
  • Relevance: A pack-level failure mode (see cell-level over-discharge abuse) that first becomes physically reproducible once real cells are connected in series, i.e. at module level

Legislative / Certification Testing

Few standards are written exclusively “for modules” — most formally target the cell or the full pack/system. In practice, though, module-level testing is where much of this certification work is actually developed and de-risked, either because a standard explicitly includes a module tier, or because pack/system procedures are commonly scaled down during the development programme.

Standards With an Explicit Module Tier

• UL 9540A (Test Method for Evaluating Thermal Runaway Fire Propagation in BESS)
  • Scope: Defines four escalating test levels — cell, module, unit, and installation. The module-level test forces the same population of cells used in production into thermal runaway under a smoke-collection hood, measuring heat release, gas composition, and propagation behaviour
  • Relevance: If module-level testing meets the performance criteria, the test programme can in some cases avoid the cost of unit-level testing — making good module-level data directly valuable for certification, not just development
• USABC / DOE Battery Test Procedures Manual
  • Scope: Explicitly written to scale across cell, module, and full battery system, using the same drive-cycle and characterisation profiles (e.g. the FUDS power profile) at whichever level is under test
  • Relevance: A long-standing reference for structuring a performance and life-testing programme consistently from module data through to system-level prediction

Pack / System Standards Commonly Scaled to Module Level

• ISO 12405 Series (Performance, Reliability, Safety)
  • Scope: Formally targets lithium-ion traction battery packs and systems (cell-level testing is referred to IEC 62660); in practice, the performance and safety procedures in Parts 3 and 4 are frequently run on representative modules during development, ahead of full pack availability
• GB 38031-2025 — Thermal Propagation Test
  • Scope: The headline requirement — no fire or explosion for at least 120 minutes following thermal runaway, effective from July 2026 — is formally a system-level pass/fail criterion
  • Relevance: See GB 38031 for the full requirement. Module-level propagation testing (see above) is where the barrier, spacing, and vent-path design intended to meet this system-level criterion is actually developed and proven out
• UNECE R100 (Safety Requirements for REESS)
  • Scope: Vibration, thermal shock, mechanical shock, fire resistance, external short circuit, overcharge, and over-discharge requirements for the rechargeable energy storage system
  • Relevance: Several of its individual test methods are directly applicable, unmodified, at module scale during development

See also: Pack Testing, Abuse Testing, Cell Temperature Gradient, Battery Module: Manufacturing, Assembly and Test Process Flow