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?
yeah. each battery tube just has a flashlight cap. just like a maglight. you screw it in. Tighten it with a coin so it goes flush. can be all plastic I suppose? silicon ring in the plastic cap would keep it in place even under vibration over long time. also works as a weather seal. you only access this hatchet every 5 years or so when you change bats. its not something you open every day. we could add a thin teflon like tape inside each bat tube, that would reduce rattling, but not make the bats so tight that they would not slide out with gravity when needing to be replaced. wiring could go into the center of the cage in a long hole. I added a render of the scad you made earlier. I think we could iterate that design. just add screw-in caps and seal the bottom and add the pads there. and add a hole in the center that connects it all. the spring caps would connect to touchpoint just like a maglight. injection molding could have screw cap moulding part. that unscrew when mold is done. so groves are made when molding. the rest is just cylinder that goes with the mold cap sort of.