Here's a comprehensive overview of ebike battery designs — especially relevant for your Omera frame project.
The industry has largely moved to 21700 cells for new builds. They're 50% larger in volume than 18650s but pack up to 45% more energy density, meaning more range with fewer cells and better heat management. em3ev
| Spec | 18650 | 21700 |
|---|---|---|
| Diameter | 18mm | 21mm |
| Height | 65mm | 70mm |
| Max capacity | ~3,500 mAh | ~5,000 mAh |
| Weight | ~48g | ~70g |
| Best for | Compact builds, legacy packs | New builds, higher range batterydesign |
The battery pack sits inside an oversized downtube, typically accommodating 40–60 cells. This is the cleanest look — the battery is invisible. Modern frames like Giant's EnergyPak use a removable cartridge that slides in/out through the bottom of the DT. This is why we discussed going to a 50mm DT for Omera — a standard 42mm DT can't fit 21700 cells (21mm diameter × 2 rows = 42mm bare minimum, no room for BMS/wiring). myvelo
Battery bolts to the outside of the downtube using rivnuts or braze-on bosses. Common for conversions. Looks less clean but simpler to engineer. A 48V 25Ah pack with 65× 21700 cells comes pre-shaped in a downtube profile. yosepower.co
A custom-shaped battery fills the front triangle, held by a frame bag or custom cradle. Maximizes capacity (85+ cells possible) but looks bulky. Good for range-focused builds. twowheellifestyle
Cells are packed directly inside the frame tubes — split across top tube, seat tube, and down tube. Extremely stealthy but hard to service, limited capacity (~20 cells), and structurally risky since you're drilling access holes. forums.electricbikereview
For a clean fixed-gear-style ebike, the best approach is downtube integrated with a removable cartridge:
- DT outer diameter: 55–60mm (up from Detroit's 42mm) to fit 21700 cells in a 2-wide or 3-wide arrangement
- Configuration: 13S3P (48V, ~15Ah, 720Wh) = 39× 21700 cells — fits in a ~55mm × 400mm downtube cavity
- BMS: 40A continuous, UL 2271 certified for US/California compliance bev-intl
- Access: Bottom-load at BB junction or side-load panel
- Weight: ~2.8kg for the pack alone
The 2026 California regulation (SB 1271) now requires UL 2849 or UL 2271 certification on all ebike batteries, so designing around a certified battery pack from the start avoids costly redesigns later. bev-intl
Want me to calculate the exact cell layout dimensions for the Omera downtube, or look into specific off-the-shelf battery packs that could fit?
The difference is +72Wh (20% more energy) for one extra layer of cells.
Range Comparison
The 4×6 gives you half a day more between charges but eats 70mm of the space you just identified for hinge, drainage, gusset, seals, tool compartment, and cable routing. It also bumps the voltage from 36V to 43.2V, meaning you'd need a different controller — the PSW 250W you already specced is 36V.[ppl-ai-file-upload.s3.amazonaws]
4×5 at 36V is the right call. The range is already more than enough for daily use, it matches the controller and motor you've already specced, and that 130mm of free space is worth more than an extra 14km of eco-mode range you'll rarely need.
Range Comparison
The Real-World Take
For a typical Oslo commute (~8km each way, 16km round trip), both configs last multiple days between charges — 2.8 days for 4×5 vs 3.4 days for 4×6. You're charging every 2-3 days either way. [myvelo](https://myvelo.net/blogs/ebike-blog/most-powerful-e-bike-battery-how-much-power-fits-in-the-bike-frame)
What You Trade for +72Wh
The 4×6 gives you half a day more between charges but eats 70mm of the space you just identified for hinge, drainage, gusset, seals, tool compartment, and cable routing. It also bumps the voltage from 36V to 43.2V, meaning you'd need a different controller — the PSW 250W you already specced is 36V. [ppl-ai-file-upload.s3.amazonaws](https://ppl-ai-file-upload.s3.amazonaws.com/web/direct-files/attachments/139085844/b9fdad49-79fd-48bb-bcb5-b3f33bc64277/emotor.pdf)
4×5 at 36V is the right call. The range is already more than enough for daily use, it matches the controller and motor you've already specced, and that 130mm of free space is worth more than an extra 14km of eco-mode range you'll rarely need.