Cell-to-Pack (CTP): What You Gain and Lose by Eliminating the Module

What’s the Story: What Remains and What Disappears When You Remove the Module

Cell-to-pack (CTP) is a structure that skips the intermediate module stage and fills the empty space inside the pack with cells; in exchange, it gives up module-level serviceability and some design freedom. Space-efficiency figures have risen steadily in company announcements, but what is lost has been discussed relatively less. This article sets out the gains and losses of CTP together, using public materials and patent claims.

Starting with the industry context: the battery pack originally had a basic three-tier structure of “cell → module → pack.” The reason automakers and battery companies are moving to a CTP structure that omits the module is simple — to reduce empty space. CATL announced a volume utilization of over 50% when it introduced the module-less pack (CTP) for the first time in 2019, and BYD, unveiling the Blade Battery on March 29, 2020, stated that pack space utilization was more than 50% higher than conventional LFP block batteries.

The tension is that these numbers are mostly concentrated on the “gain” side. The module frame was not simply empty space; it was the management unit that bundled cells, restrained swelling, and let only the faulty part be removed. This article answers three questions: by what principle does CTP gain space, how much is gained and lost according to published figures, and in what form are rights to the CTP structure claimed?

Basic Principle: Removing the Divider Boxes and Shelving Long Books Directly

The module stage bundles cells for easier management, but the frame itself takes up volume and weight. CTP integrates cells directly into the pack structure to cut this waste. As module frames, inter-module connection parts, and the beams that held modules in place are reduced, more cells can fit inside the same pack envelope.

As an analogy, the three-tier structure is like dividing books into small divider boxes and then placing those boxes on a shelf. Shelf space is wasted by the box walls and the gaps between boxes. CTP removes the boxes and places the books directly on the shelf, and a design that makes cells long, like BYD’s Blade, is like making books as long as the shelf is wide so the books themselves also act as dividers. In return, the convenience of pulling out and replacing a whole box disappears, and removing a single book can mean handling the entire shelf.

In this analogy, CTP’s gains and losses are two sides of the same coin.

• What you gain: Higher volume utilization, fewer parts, more capacity within the same envelope
• What you lose: The module as a unit of separation and replacement for maintenance, and the buffer structure that contained cell swelling and heat within the module
• What shifts: The structural and thermal-management functions the module performed move to the pack case and to the cells themselves

Comparison: The Three-Tier Structure vs. CTP, and the Published Figures

We first keep the original structure comparison table as is, then summarize the CTP figures companies have announced by generation.

Structure	Composition	Space efficiency
Conventional (3-tier)	Cell → Module (frame + part of BMS) → Pack	Module frame takes up space
CTP	Cell → Pack (module stage omitted)	Higher volume utilization

What this table says: The difference between the two structures is whether the module frame and part of the BMS attached to it are present, and CTP’s space gain comes precisely from the volume that frame occupied.

Item	Announced	Space-related figure	Energy density (Wh/kg)
CATL first-generation CTP	2019	Volume utilization over 50%	Not confirmed in public materials
BYD Blade Battery (LFP)	2020-03-29	Pack space utilization up over 50% vs. conventional LFP block batteries	Not confirmed in public materials
CATL CTP 3.0 “Qilin”	2022-06-23	Volume utilization 72%	Ternary system 255 / LFP system 160
US11183729B2 specification	Priority 2019-01-09	Prior art about 40% or lower → 60% or more (e.g., 80%)	N/A

What this table says: By CATL’s announcements, volume utilization rose from over 50% in 2019 to 72% in 2022, and BYD’s patent specification puts conventional structures at about 40% or lower. However, “volume utilization” and “improvement in space utilization” are defined differently, so figures cannot be compared directly across companies.

Unlike the figures on the gain side, there is almost no published quantitative data on the loss side. Space efficiency and energy density rise, but the serviceability that came from separating and replacing by module drops. Even when a problem occurs in a single cell, a pack-level response is often needed rather than a module-level replacement, so service and maintenance strategy has to change along with it.

Insight: CTP’s real shift is not “the disappearance of structural members” but “the transfer of structural functions.” CATL stated that in the Qilin battery it integrated the internal crossbeam, liquid-cooling plate, and thermal pad into a single multifunctional elastic interlayer, and BYD highlighted a structure that arranges long cells directly in the pack. In both cases, the stiffness and thermal-management roles the module played were not eliminated but moved to other parts or to the cells themselves. So when evaluating a CTP pack, one should first ask “who took on the removed functions” rather than “what was removed,” and this becomes the starting point for discussions of serviceability and safety.

Patent Perspective: Claiming “Cell Dimensions and Arrangement,” Not “No Module”

The CTP concept itself is unlikely to be the subject of broad rights, because the idea of removing the module alone is hard to build a claim on. In practice, granted rights often narrow their scope with specific features such as cell shape, dimensions, and arrangement. As a representative case, we checked BYD’s US11183729B2 on Google Patents.

Bibliographic data: US11183729B2 (title “Battery pack, vehicle and energy storage device,” applicant and current assignee BYD Co Ltd) has a priority date of January 9, 2019, a filing date of June 21, 2019, and a grant date of November 23, 2021; its legal status on Google Patents is Active, with an anticipated expiration of June 21, 2039. The priority basis is shown as Chinese application CN110165117A.

Gist of claim 1 (restated): There is a pack housing with mutually perpendicular first and second directions, its dimension in the first direction is 600mm or more, and rectangular cell accommodating units are formed inside the housing. Multiple rectangular cells are arranged in the second direction and each extends in the first direction, with one cell per accommodating unit in the first direction. The cell’s length L, thickness D, and height H satisfy L>H, L>D, 600mm≤L≤2,500mm, and 23≤L/D≤208.

This claim does not directly claim the negative feature “there is no module.” Instead, its core is an arrangement in which a single long cell spans the accommodating unit in the first direction, together with cell dimension ratios. The specification’s description of conventional volume utilization as about 40% or lower and the improved structure as 60% or more also reads as a statement supporting the effect of these dimensional and arrangement limitations.

Design-around points: The boundary of the claim scope lies in the numerical limitations. A design with a cell length under 600mm, an L/D below 23 or above 208, or two or more cells placed end to end in the first direction may fall outside the literal scope of claim 1. A CTP approach that arranges short prismatic cells in multiple rows is, even if equally “module-less,” different in composition from this claim.

Design-around scenario (counterargument): Numerically limited claims tend to be read narrowly under the doctrine of equivalents, but a design that sets cell length just below the boundary, such as 590mm, may become the subject of a dispute over the technical significance of the limiting value. Also, other claims in the same family or granted Chinese and European patents may have different numerical ranges, so ending an FTO (freedom-to-operate) review after looking only at US claim 1 may leave gaps. Conversely, from the patentee’s side, the configuration of arranging long cells directly in a pack may face validity challenges in comparison with prior art.

Limitations and Outlook

CTP is a structural change that gains space by removing the module — the “intermediate stage for management convenience” — but in exchange gives up that convenience, namely the maintenance unit. It is not a question to be judged by energy density alone. Published figures show volume utilization rising with each generation, but loss-side indicators such as repair cost, replacement unit, and reusability remain hard to confirm from public materials.

The optimistic outlook is that CTP will lead to integration with the vehicle body, as in cell-to-body (CTB) or cell-to-chassis (CTC), raising space efficiency further. There is a counter-scenario as well. As integration increases, the replacement unit after an accident or defect grows to the pack or the body structure, and if this is reflected in insurance premiums, repair costs, and resale value, the total cost of ownership for consumers may offset the energy-density gain. The practice of detaching used batteries module by module for reuse may also be constrained.

The strategic alternative is to treat CTP not as a binary of “module or no module” but as a question of “designing the maintenance unit.” Even without modules, pairing a fastening structure that allows partial replacement of cell blocks with a BMS design that preserves cell-level diagnostic information can find a balance between space gain and serviceability, and this may also become a filing area that does not overlap with existing CTP patents centered on dimensional limitations.

Practical action: Staff planning a CTP pack should first compare their cell dimensions (length L, thickness D) and L/D ratio against the numerical limitations of US11183729B2 (600-2,500mm, 23-208), and, together with that result, add “minimum replacement unit in case of failure” as a required item in the design specification.

Items requiring verification

• Primary sources for detailed figures on CATL’s first-generation CTP (2019), such as parts reduction and production-efficiency gains
• Differences between companies’ definitions of “volume utilization” and “space utilization” (how cell volume / pack volume is calculated)
• Numerical claim ranges and legal status in the US11183729B2 family (CN, EP, etc.)
• Service cases of cell-level or partial replacement in CTP packs, and repair-cost comparison data

Sources

1. CATL press release, CATL launches CTP 3.0 battery “Qilin,” achieves the highest integration level in the world (2022-06-23)
2. BYD press release (PR Newswire), BYD’s New Blade Battery Set to Redefine EV Safety Standards (2020-03-29)
3. Google Patents, US11183729B2 – Battery pack, vehicle and energy storage device (BYD Co Ltd)

Related Reading — Why Do Pouch Cells Seem to Swell So Much? — The Structural Reasons for Swelling · Electrode Tab Design — The Hidden Variable That Determines Internal Resistance · The 4680 Cylindrical Cell: The Name “Tabless” and the Actual Structure Are Different