The inside of a modern wearable is not a rectangle. A smartwatch movement, a hearing-aid shell, and an AR glasses temple all have irregular volumes — tapered corners, space reserved for antenna tuning, regions occupied by flex-circuit routing. Rectangular cells leave those odd volumes empty and waste expensive enclosure real estate. Stepped and L-shaped geometries recover that volume, but they introduce manufacturing constraints that determine what you can and cannot do.
What "stepped" means in a pouch cell
A stepped cell has two or more discrete thickness regions within a single electrode stack. Picture a standard rectangular pouch cell, then imagine removing a rectangular slice from one corner — the remaining shape is an "L". Now imagine removing a notch from the centre of one edge — that is a "U" or "notched" cell. The electrode stack must follow the outer geometry: thinner regions have fewer electrode layers, so their local capacity density is lower than the thicker regions.
The key distinction from a simple curved cell is that stepped cells have discrete thickness transitions — flat regions at different z-heights — rather than a smooth curvature. This means the separator and electrodes must be cut or folded to accommodate the transition, which introduces specific manufacturing constraints.
L-shaped cells
L-shaped cells are the most common non-rectangular geometry in production today. They are manufactured by building a complete rectangular electrode stack and then trimming and folding one section of it to reduce thickness in the trimmed region. Alternatively, the stack is built in two separate sections that share a common pouch enclosure.
The capacity is concentrated in the thicker arm of the L. The thinner arm provides additional mAh at a lower capacity-per-volume density, and serves the secondary purpose of filling an otherwise-empty corner of the enclosure. A smartwatch that uses an L-shaped cell typically places the thicker arm under the watch face and the thinner arm in the strap hinge region, gaining 15–25% additional total capacity compared to the largest rectangular cell that would fit in the watch face alone.
Manufacturing constraints for L-shaped cells:
- Minimum width of either arm: ≥ 6 mm (to accommodate electrode tab and sealing margins)
- Transition corner radius: ≥ 2 mm (sharper corners stress the separator fold)
- Maximum step ratio (thicker arm thickness / thinner arm thickness): 3:1 or less for stable electrode contact
- Tab placement: must be on the thicker arm or at the L corner; tabs on the thin arm only create high current density at the transition
U-shaped and T-shaped cells
U-shaped cells have a notch cut from the centre of one long edge, creating two "legs" joined by a bridge. The notch accommodates a component that must live in the enclosure centre — a motor hub in a smartwatch, a button mechanism in a mouse, or a speaker driver in an earbud case. T-shaped cells are similar but with the notch asymmetrically placed.
These geometries are more mechanically fragile than L-shaped cells because the bridge between the two legs is a stress concentration point. Internal pressure from normal swelling can cause delamination at the bridge if the bridge width is less than 8 mm. For production volumes below 50,000 pcs/month, the cost of the custom electrode cutting tooling often makes U-shaped cells uneconomical compared to using two smaller rectangular cells with a combined capacity target.
Capacity and form-factor trade-offs per geometry
| Geometry | Typical capacity gain vs. best-fit rectangle | Minimum production volume for cost-effectiveness | Key mechanical risk |
|---|---|---|---|
| L-shaped | 15–30% | 20,000 pcs/month | Tab placement at transition |
| U-shaped / notched | 10–20% (net of notch) | 50,000 pcs/month | Bridge delamination |
| T-shaped | 8–18% | 50,000 pcs/month | Bridge + asymmetric internal pressure |
| Stepped (3+ levels) | 20–35% | 30,000 pcs/month | Electrode fold cracking at step transitions |
Where tabs can go
Tab placement in non-rectangular cells is more constrained than in standard cells. For any shaped cell, valid tab locations are:
- The short edge of the thicker region (standard)
- The long edge of the thicker region (if space at the short edge is used for another component)
- The L-corner, with the tab running along the inside of the corner bend (requires a custom tab fold in the pouch)
Invalid tab locations include: any edge of the thin arm, the bridge section of a U-shaped cell, and within 4 mm of any fold or corner. Tabs placed near structural stress points fail at the weld under vibration testing.
Design handoff requirements
When requesting a custom-shaped cell from a supplier, provide:
- 3D STEP file of the available battery envelope (not the product enclosure — the specific volume reserved for the battery)
- Maximum cell thickness per region
- Tab exit direction and maximum tab length
- Minimum capacity requirement at end of life (cycle 500, 25 °C)
- Charge and discharge C-rate requirements
A supplier who receives this information can return a feasibility sketch within 48 hours and a formal geometry proposal within a week. This is the correct starting point for a custom-shaped battery program, and it is free — do not commit to tooling costs until you have seen and approved the geometry proposal.