Lithium Polymer Pouch Cell (NMC & LFP)

Custom Cell Sizing and Design Constraints

Pouch cells offer the most flexibility of any lithium-ion format — you specify the dimensions, and the factory cuts the electrode sheets and laminate pouch accordingly. That flexibility comes with real engineering constraints that affect cost, lead time, and reliability.

Minimum thickness is ~3mm. Below 3mm, the electrode stack (anode / separator / cathode layers) cannot be wound or stacked with enough mechanical stability to maintain uniform pressure across the cell face. Cells thinner than 3mm exhibit early capacity fade because electrode contact pressure is inconsistent. If your wearable device or PCB design calls for a 2mm cell, you need a solid-state cell — which is a different supply chain and cost structure entirely.

Aspect ratio limit: length:width ≤ 4:1. A 100mm × 20mm cell (5:1 ratio) will have uneven electrolyte distribution and poor lithium-ion diffusion uniformity across the electrode width. The practical limit for reliable cells is 4:1. For oddly shaped devices, the correct approach is a battery pack with two smaller cells rather than a single extreme-ratio cell.

Tab position and welding compatibility. Pouch cells use two dissimilar tabs: aluminum (positive terminal) and nickel (negative terminal). Tab position — same-end or opposite-end — must match your PCB layout. More importantly, your PCB’s contact method must be compatible with tab material. Nickel tabs can be spot-welded, soldered, or laser-welded. Aluminum tabs cannot be soldered with standard tin-lead or SAC305 solder — they require ultrasonic welding or laser welding to the PCB pad. Specify tab material and welding method before tooling.

NRE cost for a custom pouch size. Electrode die tooling and formation fixtures for a new cell model run $3,000–8,000, depending on cell dimensions and number of electrodes per stack. This is a one-time cost — ongoing production runs use the same tooling. The NRE is separate from the per-cell cost and is typically quoted line-item. For wearables buyers running low-volume custom hardware, sharing the NRE between 2,000–5,000 units is the realistic minimum to make custom sizing economically viable. Catalog sizes (common dimensions stocked by factories like Grepow and HIGEE) have no NRE and lower MOQ — evaluate whether a catalog cell fits your device before committing to custom tooling.

Swelling Management: What the Datasheet Does Not Tell You

All pouch cells swell over their cycle life. NMC pouch cells expand 8–15% in thickness between fresh and end-of-life due to two mechanisms: SEI (solid electrolyte interphase) layer growth on the anode surface, and gas generation from electrolyte decomposition. This is not a defect — it is an inherent property of the chemistry. Your mechanical design must accommodate it.

Spring-loaded vs rigid enclosure. The correct approach is a spring-loaded compression plate or foam gasket that maintains constant, controlled pressure on the cell face throughout cycle life (typically 5–15 psi). This pressure keeps the electrode stack in uniform contact, slowing capacity fade and improving cycle life. A rigid enclosure — metal case or hard plastic with no compliance — restricts swelling. Restricted swelling does not stop the gas generation; it increases internal pressure. Consequences: separator deformation under sustained pressure load, electrolyte redistribution to the cell edges, accelerated capacity fade, and in worst cases, seam failure at the pouch heat seal. A rigid enclosure is not a design shortcut; it is a cycle life penalty.

Specify maximum allowable swell at end-of-life. In your cell specification (sent to the factory before tooling), include a line: “Maximum thickness increase at end of rated cycle life (80% capacity retention): X mm.” A reasonable value for an NMC pouch cell rated 500 cycles is 1.5–2.0mm additional thickness. Factories that skip this specification will not design the electrode stack to meet a swell limit — they will optimize for rated capacity only.

Formation gas venting. During the first 3–5 charge/discharge cycles (called formation), the SEI layer forms and releases gas. Reputable pouch cell manufacturers perform formation at the factory, degas the cell (puncture the pouch, remove gas, reseal under vacuum), and then do final capacity grading. Do not accept cells that have not completed factory formation and degassing — field swelling in the first 10 cycles will be severe. When auditing a supplier for power electronics products, verify that the formation and degassing step is in their documented production process, not skipped for cost reduction.

UN 38.3 and IATA PI966 Classification

Pouch cells are classified as lithium-ion batteries under IATA DGR regardless of form factor. Getting the classification right at the time of first import saves significant delays at customs and with freight forwarders.

Energy per cell determines the IATA section. A common wearable cell: 3.7V × 200mAh = 0.74Wh — this is well within IATA PI966 Section II (<100Wh per battery, <20Wh per cell). A drone cell: 3.7V × 5,000mAh = 18.5Wh per cell — still Section II, but approaching the limit. A cell at 3.7V × 2,000mAh = 7.4Wh is a clear Section II shipment. Section I (≥20Wh per cell or ≥100Wh per battery) requires Class 9 Dangerous Goods declaration, IATA-certified outer packaging, and carrier pre-approval — meaningfully more expensive to ship.

UN 38.3 test report is cell-model specific. The test must be performed on the exact cell model number being shipped — a UN 38.3 report issued for “NMC pouch cell 3.7V 2000mAh” without a specific model number does not satisfy IATA requirements. Request the test report for your specific cell model before booking air freight. The eight required tests (altitude simulation, thermal, vibration, shock, external short circuit, impact/crush, overcharge, forced discharge) are performed once per model; subsequent shipments reference the existing report.

UN 38.3 cell test vs battery pack test. The cell-level UN 38.3 report covers the bare cell. If you are shipping assembled battery packs (cells + PCM + case), the pack itself also requires a UN 38.3 report — the cell report alone is not sufficient for pack-level shipments. The pack test can be performed by a third-party lab in China (SGS, Bureau Veritas, and Intertek all have labs in Shenzhen and Dongguan).

State-of-charge for air freight: 30%. IATA requires that lithium-ion cells shipped by air be at or below 30% SOC (state of charge). Factories that ship fully charged cells are non-compliant. Specify this requirement in your purchase order. Our inspection service includes SOC verification as part of pre-shipment checks for battery cell orders.

Packaging requirements. Each cell must be individually protected against short circuit — typically a plastic bag or cell-level sleeve. Outer packaging must pass a 1.2m drop test (UN 4G fiberboard box). Each outer package must carry the IATA Lithium Battery Mark (UN 3480 for cells, UN 3481 for cells contained in equipment).

Chinese Supplier Landscape: Who Makes Custom Pouch Cells

The Chinese custom pouch cell market splits into tier-1 volume manufacturers and specialty/mid-tier factories. Selecting the right tier depends on your volume, delivery timeline, and technical requirements.

ATL (Amperex Technology Limited, Ningde). The largest lithium-polymer pouch cell manufacturer in China by volume, a primary supplier to Apple, Huawei, and major laptop brands. ATL does take OEM orders, but their practical minimum meaningful engagement is 50,000+ cells per model with multi-year purchase commitments. For most hardware startups and mid-size OEMs, ATL’s sales process and MOQ make them inaccessible at early stage.

CATL pouch division. CATL is better known for prismatic cells and cylindrical cells for EVs, but they have a pouch cell division. Similar high-MOQ constraints as ATL for standalone pouch cell orders. Typically accessed by buyers who are already CATL customers for other product lines.

Grepow (Grepow Battery Co., Shenzhen). Specialty manufacturer focused on high C-rate, shaped, and non-standard pouch cells. Strong track record in drone batteries (25C–50C continuous), wearables (ultra-thin cells, curved cells), and RC applications. MOQ for custom sizes starts at 1,000–2,000 cells. Lead time for custom tooling is 30–45 days. Grepow provides detailed formation capacity distribution data per batch and supports third-party inspection without friction — relevant for buyers who need quality inspection access during production.

HIGEE Energy. Mid-tier manufacturer with broader standard catalog of pouch sizes. Good option for buyers needing catalog-size cells (no NRE) or modest-volume custom sizes (2,000–5,000 cells). HIGEE’s quality documentation is generally complete — formation reports, 100% OCV sorting, and internal resistance distribution data are standard deliverables.

Ganfeng Lithium (Ganfeng LiEnergy division). Primarily a lithium materials company, but their battery division makes pouch cells with strong electrolyte quality given their raw material vertical integration. Less common for consumer electronics OEM; more common for industrial and energy storage applications.

What to require from any custom pouch cell supplier:

• Formation capacity distribution report for the specific batch — histogram showing the full capacity distribution, not just a minimum specification. Reject batches where the distribution tail is wide (standard deviation >2% of nominal capacity indicates inconsistent electrode coating).
• 100% OCV sorting at the factory — all cells measured for open-circuit voltage post-formation and graded. Cells with OCV below the lower control limit are rejected before shipment.
• Internal resistance distribution — measured at 1kHz AC. Tight IR distribution (standard deviation <5mΩ for a typical consumer cell) indicates consistent electrolyte fill and electrode contact.
• Tab weld pull test — nickel tab to electrode current collector weld must withstand ≥15N pull force without delamination. This is the most common failure mode in low-quality pouch cells under vibration.

Our sourcing service qualifies custom pouch cell manufacturers against these criteria before recommending them for your project. For new supplier relationships, a factory audit covering formation equipment, degassing process, and quality control documentation is strongly recommended before committing to tooling NRE.