Cell-to-Pack vs. Modular Battery Architecture: The Trade-Offs Defining EV Design

Inside the teardown bays of automotive engineering firms, a fundamental shift in electric vehicle design has become visible to those who dismantle them. For the first decade of mass-market electrification, automakers built battery packs like nesting dolls: individual cells were grouped into protective modules, and those modules were bolted into a larger pack enclosure. This modular architecture provided a straightforward, scalable way to power a vehicle, but it came with a steep physical cost. The casings, busbars, and internal wiring required to house the modules consumed valuable space, leaving the average volumetric cell-to-pack efficiency at just 31.5 percent.[3]

That modularity, however, served a distinct mechanical purpose. The intermediate module layer provided structural support for the fragile cells, aggregated their electrical output, and offered an extra layer of thermal separation to prevent a localized fault from spreading across the entire pack. Crucially, it also meant that if a specific cluster of cells degraded prematurely or sustained damage, a technician could theoretically unbolt the enclosure, isolate the faulty module, and replace it without discarding the rest of the battery.[3]

But as the electric vehicle market matured, the pressure to extract more range from the same vehicle footprint intensified. To achieve those gains, engineers began eliminating the middle layer entirely. The resulting architecture, known as Cell-to-Pack (CTP), mounts the active battery cells directly into the pack structure. By stripping out the redundant hardware, the outer enclosure is forced to take on the mechanical and thermal duties previously handled by the individual modules.[3]

The volumetric gains achieved by this streamlined approach are substantial. Without the dead space created by module casings, automakers can increase the volumetric efficiency of the pack to 59 percent or higher. This allows engineers to pack significantly more active lithium-ion material into the exact same physical envelope. The resulting vehicles are lighter, cheaper to manufacture at scale, and capable of traveling much further on a single charge.[3]

To maximize these benefits, some automakers have taken the concept a step further, evolving CTP into fully structural battery packs. In these designs, the cells are often bonded directly to the vehicle's chassis using high-strength structural adhesives. The energy storage system is no longer just a fuel tank; it becomes a load-bearing component of the vehicle's frame, increasing torsional rigidity and improving crash safety while further reducing the total part count.[2][3]

The downstream consequences of that structural integration, however, are now surfacing in salvage yards and insurance claims. Because the cells are permanently glued into the vehicle's frame, assessing or repairing a structural battery pack after a collision is exceptionally difficult. Matthew Avery, research director at automotive risk intelligence company Thatcham Research, noted the contradiction in early 2023: "We're buying electric cars for sustainability reasons. But an EV isn't very sustainable if you've got to throw the battery away after a minor collision."[1]

When a structural pack sustains even minor damage, insurers are increasingly forced to write off the entire vehicle, regardless of its mileage. The economics of these write-offs are stark. A replacement battery for a highly integrated vehicle like the Tesla Model 3 can cost up to $20,000, a figure that often exceeds the residual value of the car itself. Manufacturing expert Sandy Munro, known for his detailed vehicle teardowns, described the structural battery pack used in the Texas-built Model Y as having "zero repairability," noting that a damaged unit "is going straight to the grinder."[1][2]

This dynamic shifts the financial benefits of CTP architecture entirely to the front end of the vehicle's lifecycle. Automakers save money on assembly and materials, but consumers and insurers absorb the elevated risk of a total loss following a minor accident. The industry is now fracturing along these architectural lines. While some manufacturers are doubling down on structural integration to drive down production costs, automakers like Ford and General Motors have explicitly stated that they are designing their battery packs to retain modularity, preserving the ability to swap out individual components and sustain a viable second-hand market.[1][3]