Cell to Pack vs Module Based Battery Architecture
Short answer: Cell-to-pack (CTP) architecture removes the module layer, placing cells directly into the pack enclosure. This improves volumetric energy density by 10–20% and reduces part count, but increases design complexity and limits repairability. Module-based design retains discrete, replaceable units, offering faster integration, multi-source procurement and a clear second-life pathway.
The choice between CTP and module-based construction is one of the more consequential decisions in pack architecture. Neither approach is universally superior; each trades one set of engineering constraints for another. What follows is a structured comparison across the axes that matter most to a system integrator.
What each architecture actually means
Cell-to-pack (CTP)
In a CTP battery, individual prismatic or cylindrical cells are bonded, compressed or otherwise fixed directly inside the structural pack enclosure. The intermediate module — with its own housing, endplates, busbars and thermal interface — is removed. The pack enclosure itself carries mechanical and thermal loads that the module previously handled.
CTP designs have been adopted in high-volume passenger EV programmes where every kilogram and cubic centimetre of pack volume is budgeted tightly. The approach is demanding: the pack enclosure must be engineered around the specific cell geometry, which binds the design to a single cell format.
Module-based architecture
A module-based battery pack is assembled from discrete, pre-tested battery modules — each a self-contained unit with its own housing, busbars, thermal interface and cell group. Standardised formats such as the VDA 355 (approximately 355 × 151 × 108 mm) and the MEB 590 (approximately 590 mm long) allow modules from different cell suppliers to occupy the same mechanical envelope. The pack integrator defines the series/parallel arrangement; the module supplier handles the cell-level assembly and testing.
Battery modules in VDA and MEB formats sit at the core of this approach and are the entry point for most commercial and industrial integrators working outside high-volume OEM programmes.
Direct comparison
| Criterion | Cell-to-pack (CTP) | Module-based |
|---|---|---|
| Volumetric energy density | Higher — module housing eliminated; gains of ~10–20% reported in automotive programmes | Lower — module enclosures, endplates and inter-module gaps consume volume |
| Gravimetric energy density | Marginally higher for same reason | Marginally lower |
| Pack engineering complexity | High — thermal management, cell retention and BMS wiring all designed from cell level | Lower — module is a tested sub-assembly; integrator works at module level |
| Time to first prototype | Long — full pack tooling and cell integration from scratch | Short — modules procurable from stock; pack frame is the primary custom element |
| Repairability | Low — individual cell replacement is impractical in most CTP designs | High — a faulty module can be swapped in the field without disturbing the rest of the pack |
| Cell supplier flexibility | Low — cell geometry is designed into the pack structure | High — standardised formats allow drop-in substitution across suppliers |
| Minimum order economics | Favours very high volumes; tooling cost is substantial | Suited to lower volumes; modules procurable to order |
| Second-life suitability | Poor — pack must be disassembled to recover cells | Good — modules removed intact, tested and redeployed in stationary storage |
| Compliance documentation | Burden falls entirely on pack builder | Module-level test data (UN 38.3, IEC 62619) often available from module supplier |
| Thermal management | Integrated into pack floor or lid; no module-level interface | Module thermal interface is defined; integrator connects to module base or side |
Where CTP architecture earns its place
CTP is justified when volumetric efficiency is the dominant constraint and volume is high enough to amortise the tooling and validation cost. Passenger EV platforms built in tens of thousands of units per year, where every 5 Wh/L improvement translates to meaningful range gain, are the natural home for CTP. The engineering investment is real: thermal runaway containment, structural load paths and electrical isolation must all be engineered at the cell level rather than delegated to the module.
For a system integrator building commercial vehicles, industrial equipment or stationary storage in quantities below that threshold, the CTP efficiency gains are unlikely to offset the integration cost and schedule risk.
Where module-based design holds the practical advantage
Module-based design gives the integrator several things that CTP cannot.
Procurement flexibility. Standardised formats decouple the pack frame design from any single cell supplier. If a cell line is delayed or discontinued, a mechanically equivalent module from a different supplier fits without retooling. Prismatic LFP and NCM cells are available in configurations matched to VDA and MEB module formats precisely because this interchangeability has commercial value.
Time to market. A pack built from procurable modules can reach prototype stage in weeks rather than months. The module is a validated sub-assembly; the integrator's engineering effort focuses on the pack enclosure, thermal loop and BMS — not on cell-level stack management.
Serviceability. In commercial vehicle and industrial applications, maintenance access matters. A module-based pack can be partially rebuilt in the field. A CTP pack, if a cell group fails, typically requires full pack replacement or factory-level intervention.
Second-life pathway. The IEC 62619 standard (iec.ch) covers safety requirements for stationary lithium battery systems; modules removed from automotive service and redeployed in stationary storage benefit from documentation that is already at the module level. The European Commission's Battery Regulation 2023/1542 (EUR-Lex, europa.eu) introduces traceability and second-life requirements that align with the module-as-unit model: a module has an identity and a documented history in a way that a cell bonded into a CTP structure does not.
The growing interest in second-life applications for automotive module formats — particularly the MEB 590, as described in the next-generation MEB platform battery module — reflects this structural advantage.
Compliance support. Module suppliers with established product lines carry test documentation — UN 38.3 transport certification, cell-level safety data — that the integrator inherits. This is not a minor point for a team managing CE marking or preparing a technical file under the Battery Regulation.
Making the decision
The right architecture depends on three questions:
- What is the production volume? Below roughly a few thousand packs per year, CTP tooling and validation costs are difficult to recover.
- How important is field serviceability? Transport, marine and industrial applications with long service lives and distributed operation favour module-based designs.
- Is second-life or residual-value recovery part of the business model? If yes, the module is the natural unit of value; CTP makes that recovery structurally harder.
For integrators working across multiple programmes or geographies, the procurement and scheduling flexibility of module-based design typically outweighs the density advantage of CTP. Complete module systems are available in configurations suited to both traction and stationary applications.
Frequently asked questions
What is the energy density advantage of cell-to-pack over module-based battery design?
CTP designs typically achieve a volumetric energy density improvement of approximately 10–20% compared with equivalent module-based packs, because the module enclosure, endplates and inter-module gaps are eliminated. The exact gain depends on cell format and module housing design, and varies between manufacturers.
Can a module-based pack be repaired in the field?
Yes. In a module-based pack, a failed module can be identified, removed and replaced without disturbing the remaining modules. This is one of the primary practical advantages over CTP architecture, where cell-level failure typically requires factory-level intervention or full pack replacement.
Why does standardised module format matter for procurement?
A standardised format such as VDA 355 or MEB 590 means that modules from different cell suppliers share the same mechanical envelope, cooling interface and busbar geometry. The pack frame design is therefore not locked to a single supplier, which reduces supply-chain risk and allows specification changes without retooling.
How does the EU Battery Regulation 2023/1542 affect the choice of architecture?
The Battery Regulation introduces traceability, state-of-health reporting and second-life requirements. Module-based designs align more naturally with these requirements because the module is a discrete, identifiable unit with its own documented test history. CTP designs place the compliance burden entirely at the pack level, with no intermediate traceable unit.
Is cell-to-pack suitable for stationary energy storage?
CTP is rarely used in stationary storage. The density advantage that justifies CTP in passenger EVs is less relevant when space is not the primary constraint, and the reduced repairability and second-life compatibility are significant disadvantages for systems expected to operate for 15–20 years.
Discuss your architecture requirements
If you are evaluating pack architecture for a current programme, contact Avantis Energy to discuss module selection, format compatibility and technical documentation for your application.