Cell format decides shape, heat, cycle life and shipping compliance on a power bank project — not just the capacity on the label.
Three cell formats cover almost every power bank on the market: the 18650 cylinder, the larger 21700 cylinder, and the flat lithium-polymer pouch. The choice is not about capacity on the label — it decides the shape of the product, how hot it runs, how long it lasts, and what it costs to certify and ship.
| 18650 | 21700 | Li-Po pouch | |
|---|---|---|---|
| Typical capacity | 2.0–3.5 Ah | 4.0–5.0 Ah | Set by the pouch size |
| Shape freedom | Fixed cylinder | Fixed cylinder | Any flat profile |
| Energy density | Baseline | Higher per cell | Lower per volume |
| Thermal behaviour | Good, metal can spreads heat | Good, fewer cells to manage | Needs deliberate thermal design |
| Cycle life | Longest of the three | Long | Shorter, and swelling is visible |
| Best for | Rugged, high-cycle, cost-led | High capacity in fewer cells | Slim, pocket, magnetic and credit-card shapes |
Shape comes first. If the product has to be slim, magnetic, or shaped like a card, pouch cells are the only option — you cannot get there with cylinders. If the product is a brick-shaped rugged unit, cylinders give you more capacity per yuan and longer cycle life.
Capacity target comes second. A 20 000 mAh brick built from 18650 cells needs six or seven of them; the same capacity in 21700 needs four. Fewer cells means fewer welds, a simpler BMS and less balancing work — which is why 21700 has become the default for larger banks.
Cost comes third, and it is not just cell cost. Pouch packs need a rigid frame, a thermal pad and more careful assembly. Cylinders need holders and nickel strip welding. The pack-level cost often narrows the gap that the cell-level price suggests.
A "20 000 mAh" bank is rated at cell voltage, around 3.7 V. Delivered at 5 V through a boost converter, with conversion losses, the usable figure is roughly 60–65% of that number. Print both, or expect returns.
Pouch cells gas slightly over their life and visibly when abused. Without a rigid frame and a defined expansion gap, a swollen pack pushes against the housing and, in the worst case, against the PCB. This is the main reason cheap slim banks fail in the second year.
Cylindrical packs are usually built as parallel groups in series. Cells in a group must be matched for capacity and internal resistance, or the strong cell carries the weak one and the pack ages unevenly. Ask whether incoming cells are graded, and ask for the grading report.
UN 38.3 test summaries, IEC 62133, and — depending on the market — UL 2056 or CB. Air freight limits watt-hours per package regardless of format, so a very large bank changes your logistics, not just your BOM.
18650 cylindrical cells, in general. They run cooler and the metal can tolerates more abuse than a pouch.
Not honestly. Slim products are physically limited; a very thin 20 000 mAh claim usually means the number is cell capacity and the delivered figure is far lower.
For larger banks, usually. For small ones, the physically bigger cell may not fit the housing you want.
They fail differently — by swelling rather than by losing capacity quietly. Good frames and thermal design make them perfectly reliable.
Often yes, if they are graded and documented. Send the datasheet and we will confirm whether the pack design and BMS suit them.
It depends on housing tooling more than on cells. See MOQ and pricing for how the two interact.
Sampling on an existing platform is a matter of weeks; a new housing with tooling runs longer. Related: power bank ODM.
Yes — UN 38.3 documentation and the market-specific certifications are part of the deliverable. See safety and shipping.