The battery module is the intermediate assembly between individual cells and the complete battery pack. In a traditional Cell-Module-Pack (CMP) architecture, cells are grouped into modules first, and modules are then assembled into the pack. This additional layer adds cost and mass — but it also provides mechanical structure, a serviceable unit for field repair, thermal management integration, and a manageable sub-assembly for manufacturing and quality control.
The module is not a permanent fixture of battery design. The industry trend towards Cell-to-Pack (C2P) and Cell-to-Body (C2B) architectures eliminates the module layer entirely in some applications, integrating cells directly into the pack or vehicle structure. But in the majority of industrial, commercial, BESS, and non-automotive EV applications, a modular architecture remains the engineering and commercial default — and for good reason. When a cell fails in a C2P pack, the entire pack may need replacement. When a cell fails in a modular pack, the module is swapped. That difference in serviceability has direct implications for total cost of ownership, warranty exposure, and end-of-life flexibility.
Understanding module design is therefore not just relevant to the engineer building traditional packs — it is the foundation for understanding what is gained and lost when the module layer is removed.
Full Index
A complete, always up to date index of every page in the Module section, organised by hierarchy. Click any heading to expand it.
What a Module Contains
A battery module is a self-contained sub-assembly of cells with everything needed to connect, sense, and protect them as a group. A complete module typically includes:
Cells in series and parallel — the cell count and configuration (S×P) within the module is determined by the pack voltage architecture, the cell format, and the target module energy. Prismatic and pouch modules often have all cells in parallel within the module (P groups) and modules in series to build pack voltage. Cylindrical modules may have cells in both series and parallel within a single module.
Electrical interconnects (busbars) — the primary conductors connecting cell terminals within the module. Sized from the continuous current requirement; designed for low resistance, thermal management, and mechanical durability at the cell-to-busbar joint.
Voltage sense harness — the low-voltage wiring that connects each cell group voltage measurement point to the Cell Sensor Circuit (CSC). A separate wire per cell group (each parallel group in series) is required, meaning a 12S module needs 13 voltage measurement wires. This harness is a significant source of manufacturing cost and potential field reliability issues.
Temperature sensors — NTC thermistors or PT100 sensors placed between cells at the expected thermal hotspots. Typically one sensor per 4–8 cells; placement is determined by the thermal model to ensure the hottest cell in the module is always represented.
Cell Sensor Circuit (CSC) — the local electronics board that digitises cell voltage and temperature measurements and communicates them to the BMS master via a digital bus (typically isoSPI or CAN). In a Master-Slave BMS architecture, the CSC is the slave module.
Mechanical structure — the housing that locates all cells in their correct position, provides the structural rigidity of the module as a shipping and handling unit, and in the case of pouch and prismatic cells, applies and maintains the compression pressure required for cell performance and lifetime.
Heating/cooling interface — the thermal pathway from cell surfaces to the cooling circuit. For liquid-cooled modules, this is typically a cooling plate on the base or side of the module, with thermal interface material (TIM) between the cell surfaces and the plate. The module structure must maintain consistent TIM contact as cells swell over their lifetime.
→ Module design — detailed breakdown of each component
Format-Specific Module Design
The module architecture differs significantly between cell formats — the mechanical, thermal, and electrical design challenges are fundamentally different for cylindrical, prismatic, and pouch cells.
Cylindrical Cell Modules
Cylindrical cells are self-supporting — the metal can provides structural rigidity without external compression. This simplifies module design significantly. Cells are typically held in a plastic carrier or end-plate assembly, with busbars connecting cell terminals at one or both ends.
The key design decisions for cylindrical modules are:
Single-side vs double-side busbars — in a double-side design, positive terminals are connected on the top and negative terminals (the cell can) are connected at the base. This requires busbars on both faces, complicating the cooling path. In a single-side design, the cell geometry is used so that both positive and negative terminals are accessible from the top, leaving the cell base entirely clear for a cooling plate. Single-side designs are more manufacturing-friendly and enable better cooling, but require a specific module layout.
Cooling position — for 21700 and 4680 cells, base cooling (cooling plate under all cell bases) provides approximately 12% better heat transfer than side cooling for the same temperature gradient. The larger cross-sectional area at the base and the in-plane thermal conductivity of the jellyroll both favour base cooling.
Cell-to-busbar joining — laser welding is the dominant technique for cylindrical cells. The small terminal area at the top of the cell requires precise alignment and controlled energy input to avoid damaging the cell during joining. Weld quality directly affects cell-to-busbar resistance and long-term reliability.
→ Single side 21700 busbars → Base vs side cooling — cylindrical cells → Cell joining techniques
Prismatic Cell Modules
Prismatic cells have large flat faces that can transmit force — enabling the module compression system to use the cells themselves as structural members within the cell stack.
The defining mechanical challenge is compression management. Prismatic cells expand during charging and contract during discharge (breathing), and swell permanently over their lifetime — a cell can be 10–20% thicker at end of life than when new. The module structure must accommodate this dimensional change while maintaining the contact pressure on the cell faces that the cell chemistry requires for performance and lifetime. End plates, compression pads, and tie rods (or adhesive) are the common approaches.
Thermal management is typically via a cooling plate on the base or large face of the module, with TIM between the cell and plate. The large cell-to-plate contact area gives prismatic modules good thermal performance without immersion cooling.
Compression pads between cells serve a dual thermal and mechanical function — they provide compliance for cell breathing and swelling while also maintaining thermal contact. Selecting a compression pad that retains its spring force over thousands of thermal cycles and permanent cell deformation is an important materials decision.
→ Compression pads and cell expansion → Options for thermal interface materials
Pouch Cell Modules
Pouch cells are the most mechanically demanding format at module level. The flexible laminate pouch provides no structural support — every cell must be held, supported, and compressed by the module structure. Pouch cells have the highest energy density at cell level, but that advantage is partially offset by the structural overhead of the module.
Compression management is critical for pouch cells. Insufficient compression causes delamination of the electrode layers and loss of ionic contact over cycling. Excessive compression crushes the cell and can cause tab damage or electrolyte displacement. The compression system must maintain the correct pressure range throughout the lifetime of the module across the full cell breathing and swelling envelope.
Edge cooling is the preferred thermal approach for pouch cells — a cooling plate contacts the cell edge, conducting heat via the in-plane thermal conductivity of the electrode layers. The in-plane conductivity is 10–100× higher than through-plane, making edge cooling far more effective per unit contact area despite the smaller contact surface.
→ Pouch cell cooling → Pouch cell design and edge cooling
Cell-to-Cell Variation and Module Performance
Individual cells from the same production batch are never perfectly identical. Capacity, internal resistance, self-discharge rate, and open circuit voltage all vary within a statistical distribution. In a module, these variations matter because cells connected in series share the same current — the weakest cell in the string sets the limit for the entire string.
Capacity variation — the lowest-capacity cell in a series string determines the usable energy of the string. A string of 12 cells where 11 have 60Ah capacity and one has 57Ah has an effective capacity of 57Ah — the 5% capacity deficit in one cell reduces the whole string by 5%.
Resistance variation — cells with higher resistance generate more heat at the same current, running hotter than their neighbours. Over thousands of cycles this thermal divergence causes differential ageing — the hotter cell degrades faster, further increasing its resistance, in a slowly self-reinforcing cycle.
These effects compound over the module’s lifetime. A module that is well-matched at the beginning of life will diverge over time; a module that starts with significant variation will diverge faster. This is why cell matching before module assembly, and cell balancing during operation, both matter.
How it’s managed:
- Cell matching — sorting cells by capacity and internal resistance before assembly and grouping similar cells into the same module
- Cell balancing — the BMS monitors cell group voltages and equalises SoC differences during charging, typically via passive (dissipative) balancing through a resistor switched across each cell group
- Digital twin modelling — predicting the current, temperature, and SoH of each cell in the module from accessible measurements, to identify diverging cells before they become a performance or safety issue
→ Cell to cell variation → Digital twin of a battery module → Cell balancing
The Module Production Process
Manufacturing a module involves seven distinct stages, each with its own quality control requirements:
1. Incoming cell inspection — each cell is electrically tested (OCV, capacity, DCIR) and visually inspected before entering the module build. Cells that fall outside specification are rejected before they enter the assembly. This is the only point at which cell-level quality can be verified before it becomes a module-level problem.
2. Pre-assembly — cells are sorted by measured capacity and resistance for matching; any protective films, labels, or cell carriers are fitted; cells are placed in the module carrier or housing in the correct orientation.
3. Cell stack tightening / loading with end plates — cells are compressed to the target initial pressure and constrained by end plates, side plates, or tie rods. The initial compression must account for cell breathing during the first charge cycle and the long-term swelling trajectory over the module’s lifetime.
4. Cell interconnection — busbars are attached to cell terminals by laser welding, ultrasonic welding, or bolting depending on cell format and terminal material. Every joint is the lowest-risk point in the module for resistance increase over lifetime — joint quality here directly affects long-term module performance.
5. CSC mounting — the Cell Sensor Circuit board is fitted and connected to the voltage sense harness and temperature sensors. Connections are verified electrically.
6. Loading into module housing — the cell stack and CSC assembly are loaded into the outer module housing, which provides structural protection, thermal pathway, and the interface geometry for pack assembly.
7. End-of-line testing — the completed module undergoes electrical testing (capacity, DCIR, cell voltage balance), thermal testing where required, and dimensional verification. This is the quality gate before the module enters pack assembly.
→ Module production process — detailed flow
Module vs Cell-to-Pack: The Trade-off
The module layer is disappearing from some applications and persisting in others. Understanding why requires understanding what the module provides and what it costs.
What the module provides:
- A serviceable unit — the smallest replaceable assembly in the field
- Manufacturing modularity — a common building block that can be built and tested independently before pack assembly
- Cell containment — physical barriers between cell groups that limit thermal propagation
- Quality control — a testable sub-assembly with defined acceptance criteria before it enters pack assembly
- Supply chain flexibility — modules can be sourced from multiple suppliers and are more easily shipped than complete packs
What the module costs:
- Mass — module housings, end plates, and inter-module connections add mass that does not store energy; typical cell-to-pack mass ratio is 60–65% in CMP vs 75–80% in C2P
- Volume — module structures consume space; cell-to-pack volume ratio improves by 10–15% with C2P
- Cost — module components, assembly labour, and module-level testing add per-kWh cost
- Thermal resistance — additional material interfaces between cells and cooling system increase thermal resistance in some designs
When the module makes sense:
- Applications requiring in-field serviceability (BESS, industrial vehicles, non-automotive EVs)
- Applications where module standardisation across multiple products reduces per-unit cost
- High-voltage packs where modules provide natural electrical subdivision for safety management
- Applications where pack-level thermal runaway containment benefits from inter-module barriers
When C2P makes sense:
- High-volume passenger EV applications where energy density and unit cost dominate
- Designs using blade cells (LFP) which are structurally self-supporting at pack level
- Applications where the BMS can manage cell-level quality without module-level testing gates
→ Cell to Pack (C2P) — architecture and trade-offs → Pack generations → Module benchmarking
Module Benchmarking
The module benchmarking section assembles performance and architecture data from real production modules — energy density (Wh/kg and Wh/L), cell chemistry and format, cell count, voltage, dimensions, mass, and cooling approach. It provides the calibration data that an engineer needs before committing to a module architecture.
Notable production modules covered include those from CATL, LG Energy Solution, Samsung SDI, Panasonic, SK On, BYD, and Tier 1 module suppliers.
→ Module benchmarking — full database → Pack benchmarking — for context on where module sits in the full pack
Related Sections
The module connects the cell and the pack — it sits at the interface of every other section:
- Battery Cell — cell format, chemistry, and dimensional specification determine module architecture
- Battery Pack — module count, orientation, and inter-module connections define the pack architecture
- BMS — the CSC on each module is the slave in the Master-Slave BMS architecture; harness design is primarily a module-level problem
- Thermal — module cooling architecture, TIM selection, and cell-to-cell temperature uniformity are thermal design problems
- Electrical — busbar sizing, cell joining, and voltage sense harness design are electrical design problems at module level
- Mechanical — cell compression, module housing structure, vibration and shock loads, and cell expansion management are mechanical design problems
- Safety — inter-module barriers contribute to thermal propagation containment; module-level testing validates abuse behaviour
- Manufacturing — module assembly process, cell matching, and end-of-line testing are manufacturing topics
- Testing — module-level electrical, mechanical, and thermal testing are intermediate validation steps before pack integration
- Lifetime — cell-to-cell variation and its amplification over lifetime is a module-level design and modelling challenge
- Costs — module cost is a significant fraction of pack cost; the module vs C2P decision is partly a cost engineering decision
→ Module Definitions & Glossary
See also: Cell Design, Electrode