Before a single module is assembled, before a BMS is programmed, before a cooling channel is designed — a battery engineer must choose a cell format. That choice is not purely about energy density or cost per kWh. It determines how thermal management is implemented, whether modules are needed, what welding process is used in manufacturing, how the pack responds to a cell failure, and how repairability is handled in the field.
The three formats in production use — cylindrical, prismatic, and pouch — represent genuinely different engineering philosophies with different strengths and weaknesses. Understanding the trade-offs explains why Tesla, BYD, Hyundai, and Tata have made different choices with each building a rational case for their decision.
- Three cell formats dominate EV packs: cylindrical (Tesla 4680/21700), prismatic (rigid aluminium-housed rectangular cells), and pouch (flexible laminate-packaged cells).
- Cylindrical: highest energy density possible, good mechanical rigidity, easiest thermal management (side-surface cooling), but requires thousands of precision welds per pack.
- Prismatic: simpler module assembly, good energy density, rigid housing resists crush but adds weight. Industry mainstream for Chinese and Korean OEMs.
- Pouch: highest possible gravimetric energy density (no metal can weight), but requires compression management (controlled swelling) and tab connection complexity.
- Cell-to-Pack (CTP) eliminates the module layer in both prismatic (BYD Blade) and cylindrical (Tesla structural battery) designs — improving pack-level energy density at the cost of cell-level serviceability.
Cylindrical Cells: The Format That Started Commercial EVs
The cylindrical cell is the oldest and most mature lithium-ion format. A cylindrical cell consists of a jellyroll of electrodes (anode foil, separator, cathode foil) wound into a cylinder, inserted into a metal can, sealed with a crimp cap, and pressure-tested. The can provides mechanical rigidity and electrical contact.
The 18650 legacy: The 18650 format (18mm diameter, 65mm length) was developed for laptop batteries in the 1990s. By 2008, it was available at high volume, low cost, and with well-understood quality characteristics. Tesla used 6,831 of them in the original Roadster. The Panasonic-Tesla Model S used 7,104–8,256 cells (depending on battery version). This was viable because Tesla invested in the robotics and automation required to make thousands of laser-spot-welds per pack cost-effective.
21700 improvement: By 2017, Tesla and Panasonic moved to the 21700 format (21mm × 70mm) for the Model 3. Larger diameter = more volume = more energy per cell. Approximately 50% more energy per cell than the 18650, reducing cell count and weld count per pack.
The 4680: Tabless Architecture
Conventional cylindrical cells have metal tabs — small strips of foil that connect the jellyroll's anode and cathode current collectors to the cell's terminal caps. These tabs create a current constriction: all current entering or leaving the cell passes through a small tab area, which limits fast-charging rates and creates localised heat.
Tesla's 4680 eliminates discrete tabs. The electrode foils are laser-patterned with notches that allow the full width of the anode foil to connect to the negative cap and the full width of the cathode foil to connect to the positive cap. Current can enter and leave the cell uniformly across the full electrode width — reducing internal resistance and enabling higher charge/discharge rates.
Tabless design in the 4680 reduces the ohmic path length for current from approximately L/2 (half the electrode length) to approximately L/20 — a 10× reduction. This dramatically reduces internal resistance per cell and therefore heat generation at high current. For fast charging, lower internal resistance means the cell can accept more current before thermal limits are reached, enabling 250 kW+ charging rates without excessive cell temperature rise.
Prismatic Cells: The Module-Assembly-Friendly Format
Prismatic cells use a flat, rectangular form factor with a rigid aluminium housing. The electrodes are either wound (like a cylindrical cell, pressed flat) or stacked (individual electrode sheets layered sequentially). The housing has a pressure relief valve (safety vent) and terminals on the top face.
Why prismatic dominates the Chinese and Korean EV industry: Prismatic cells are easier to assemble into modules. A module is simply a stack of cells clamped between end-plates with busbars connecting terminal-to-terminal — the rectangular geometry enables tight, efficient packing. A prismatic module can be assembled in minutes with relatively low automation complexity. The same 18650 Tesla pack that requires 60,000 laser welds would be built with a few hundred busbar bolts or laser welds in a prismatic equivalent.
The CATL modular approach: CATL supplies large prismatic NMC cells to most major OEMs outside Tesla — BMW, Volkswagen, Hyundai, Honda. A typical CATL Gen3 cell is 148mm × 91mm × 26.5mm and stores 280–330 Wh. A 96S1P NMC pack using 96 such cells stores approximately 27 kWh — a single layer of 96 cells in one compact module structure.
LFP in prismatic format: LFP chemistry (lithium iron phosphate) is exclusively manufactured in prismatic format at production scale (not cylindrical or pouch). The Tata Nexon EV Max, CATL-supplied LFP packs, and BYD Blade Battery are all prismatic LFP. LFP's lower energy density (compared to NMC) is partially offset at pack level by CTP architecture, which eliminates the space penalty of module housings.
LFP chemistry is intrinsically safer than NMC because the iron-phosphate cathode does not release oxygen during thermal runaway — there is no oxygen to feed a fire. This allows LFP packs to use less sophisticated thermal runaway containment barriers between cells, enabling more compact module designs. The BYD Blade Battery's arrangement of thin adjacent cells with minimal gap between them is only practical in LFP — an equivalent NMC blade design would require more inter-cell spacing for thermal runaway propagation prevention.
Pouch Cells: Maximum Energy Density, Maximum Complexity
Pouch cells replace the metal can with a flexible aluminium-laminate pouch. The electrodes are stacked (not wound), and the pouch is heat-sealed after filling with electrolyte. Terminals (tabs) protrude from the seal edge.
The energy density advantage: Eliminating the metal can removes dead weight. A cylindrical cell is approximately 15–20% dead weight (can + cap + vents + negative terminal spring). A pouch cell can be 5–8% dead weight — mostly the laminate film. This enables higher gravimetric energy density (Wh/kg) for the same chemistry. For applications where weight is critical (aviation, high-performance vehicles), pouch is attractive.
The swelling problem: Pouch cells swell during cycling. Graphite anodes expand approximately 10% volumetrically when fully lithiated. The flexible pouch accommodates this, but the expansion must be managed in the module design. Without controlled compression, the cell swells unevenly, creating pressure gradients across the electrode stack that cause localised degradation.
Stiff aluminium end-plates at each end of the module constrain cell swelling. End-plate thickness and material determine how much compressive force is available throughout the cell's life.
Foam or spring compression pads between cells accommodate variation in individual cell expansion. Typical compression targets: 0.1–0.3 MPa initial pressure, designed to remain positive (never tensile) at end of life as cells shrink slightly after many cycles.
Thermal pads between cells and the cooling plate provide both thermal path and additional compression. Selected for thermal conductivity (1–3 W/m·K) and compressibility.
Pouch cell tabs (aluminium for cathode, copper for anode) are laser-welded or ultrasonic-welded to busbars. Tab connections are the most common failure mode in pouch modules — weld quality is critical.
Each cell tab connection point is also the voltage measurement point for the BMS. Harness routing and connector design affect BMS signal quality.
Korean OEM choice for pouch: LG Energy Solution, SK On, and Samsung SDI — the three major Korean cell suppliers — all produce pouch cells as their primary format. LG's NMC pouch cells power the Hyundai Ioniq 5, Kia EV6, Chevrolet Bolt, and Jaguar I-Pace. The Korean supply chain has deeply optimised pouch manufacturing, making it cost-competitive with prismatic at scale.
| Property | Cylindrical | Prismatic | Pouch |
|---|---|---|---|
| Gravimetric energy density | High | Moderate | Highest |
| Volumetric energy density | Highest | Moderate | High |
| Mechanical rigidity | High (metal can) | High (Al housing) | Low (flexible) |
| Thermal management | Good (side cooling) | Moderate (face cooling) | Good (face cooling) |
| Swelling management needed | No | No | Yes (compression) |
| Module assembly complexity | High (many welds) | Low | Medium |
| Cell-to-Pack possible | Yes (Tesla structural) | Yes (BYD Blade) | Limited |
| Repair/replacement | Difficult (welded) | Easier | Moderate |
| Leading manufacturers | Tesla/Panasonic | CATL, BYD | LG, SK On, Samsung |
| Indian EV usage | Limited | Tata, BYD, Ola | MG ZS EV, Ioniq 5 |
Cell-to-Pack: Eliminating the Module Layer
Conventional pack architecture has three layers: cell → module → pack. CTP removes the module layer, arraying cells directly in the pack housing. The space and weight saved by eliminating module housings, end-plates, and inter-module connectors improves pack-level energy density by 15–50% depending on implementation.
BYD Blade Battery (prismatic CTP): Long thin prismatic LFP cells span the full width of the pack housing. Each cell's aluminium case contributes to the structural rigidity of the pack housing. The cells are bonded to the pack floor and end-plates with structural adhesive. Cooling is via channels in the pack floor contacting the cell bases. This design achieves ~450 Wh/L at pack level from LFP cells — competitive with NMC modular packs — by eliminating module structural overhead.
Tesla 4680 Structural Battery (cylindrical CTP): 4680 cells are embedded in a structural adhesive that fills the pack housing. The cells, adhesive matrix, and pack housing form a composite structure that is bonded directly into the vehicle floor — the pack is the vehicle floor, contributing to body torsional stiffness. A traditional pack housing tray is eliminated. This reduces total vehicle weight but makes cell-level repair impossible — the structural battery is not a replaceable module but a structural vehicle component.
CTP designs create a serviceability trade-off that matters for long-term ownership cost. In a modular pack, a failed module can be replaced for ₹30,000–80,000 (typical module cost). In a CTP pack, a failed region may require complete pack replacement. For Indian market EVs where after-warranty cost of ownership is a significant purchasing concern, check whether the manufacturer's service structure supports module-level (or at least pack-section-level) replacement in their CTP or near-CTP designs — some designs allow "cluster" replacement rather than full pack, preserving some serviceability advantage.
How Cell Format Affects Indian Market EVs
The cell format landscape in Indian EVs reflects the supplier relationships OEMs established globally:
- Tata Motors (Nexon EV, Tiago EV, Punch EV): CATL prismatic LFP cells. Chosen for safety (LFP thermal stability is a clear advantage in the Indian context where fast charging and ambient temperature are stressors) and CATL's production scale reducing cost.
- BYD (Atto 3): BYD Blade Battery prismatic LFP — BYD's in-house cell, vertically integrated, demonstrating the CTP architecture in an Indian production vehicle.
- MG (ZS EV, Comet EV): CATL prismatic NMC/LFP depending on variant. SAIC-MG sourced from CATL as their primary supplier.
- Hyundai/Kia (Ioniq 5, EV6): SK On prismatic-format pouch NMC cells. The high energy density supports long range in a relatively compact pack.
- Ola Electric (S1 Pro): NMC cells, supplier undisclosed, in a prismatic or pouch arrangement integrated into the scooter frame.
- Cylindrical, prismatic, and pouch formats each involve different trade-offs between energy density, assembly complexity, thermal management, and serviceability — no format is universally superior. The right choice depends on the vehicle's performance targets, the OEM's manufacturing capabilities, and the cell chemistry used.
- CTP (Cell-to-Pack) architecture eliminates modules to achieve higher pack-level energy density, but reduces cell-level serviceability. BYD Blade Battery and Tesla 4680 structural battery are the leading CTP implementations.
- The 4680 cylindrical format's tabless architecture reduces internal resistance by approximately 10× compared to tabbed cells, enabling faster charging and lower heat generation — the key innovation driving Tesla's move away from 21700.
- LFP chemistry is only produced at commercial scale in prismatic format. Its thermal safety advantages (no oxygen release in thermal runaway) allow more compact arrangements and explain why LFP prismatic dominates budget-to-mid-range Indian EVs.
- Pouch cell packs require controlled compression management (0.1–0.3 MPa) to prevent uneven swelling from degrading performance over the cell's life. This adds module design complexity that cylindrical and prismatic formats do not face.
Part of the bms-design Series
Frequently Asked Questions
Why does Tesla use cylindrical cells while most other EV makers use prismatic or pouch?
What is the BYD Blade Battery and how is it different from a standard prismatic cell?
Why do pouch cells swell and how does the pack design manage this?
What is the 4680 cell and why is it significant?
Which cell formats are used in Indian EV models?
References
- Hossain, E. and Faruque, H.M.R. — A Comprehensive Review on the Battery Packaging Technology for EVs, Energies, 2019
- Ziegler, J.I. and Trancik, J.E. — Re-examining rates of lithium-ion battery technology improvement, Energy & Environmental Science, 2021
- Liu, Y., Zhang, R., Wang, J. and Wang, Y. — Current and future lithium-ion battery manufacturing, iScience, 2021
- Deng, D. — Li-ion batteries: basics, progress, and challenges, Energy Science & Engineering, 2015