Yes — multiple cells in-line along the collar is a much better architecture.
Recommended battery topology:
- Use parallel cells: 1S2P or 1S3P, not series.
- That keeps the pack at the same safe single-cell Li-ion voltage range for the existing charger/SIM800L design.
- It increases capacity while allowing cells to be placed end-to-end around the collar curve.
- Each cell should have protection/fusing or a protected pack design, because parallel Li-ion cells need safe balancing and fault protection.
Runtime impact:
- 1× 3500 mAh cell: ~20.8 mA average available for 7 days — borderline.
- 2× 3500 mAh in parallel: ~41.7 mA average available — much safer.
- 3× 3500 mAh in parallel: ~62.5 mA average available — strong margin.
- With the earlier conservative estimate around 20.6 mA average, 2 cells should comfortably meet 7 days, assuming decent cellular/GPS signal and proper sleep firmware.
Design recommendation:
- Use 1S2P 18650 cells as the baseline.
- Keep 5-minute location updates as default.
- Keep SIM800L/GPS sleep modes mandatory.
- Mechanically, distribute the cells along the collar and connect them with flexible wiring or a rigid-flex harness; do not rely on the PCB itself to bend sharply around cylindrical cells.
I still can’t save this into the project because your access is comment-only, but the electrical direction is now clear: 1S2P 18650 pack + 5-minute reporting is the practical baseline.