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 65×55 oval is a smart call — it fits 4 cells per layer in a 2×2 square arrangement, which doubles what a 55mm circle can do.
Why 65×55 Oval Works
The inner cavity is 60×50mm after wall + liner. Four 21700 cells (21.5mm effective diameter each) pack into a tight 2×2 square with solid clearances: em3ev
The tube is only 10mm wider than a 55mm circle but the same 55mm height — so from the side profile it looks identical to a normal frame. The extra width just blends into the visual mass when viewed from riding position. myvelo
Battery Specs at 48V
With 480mm usable DT length (6 layers × 70mm), 4 cells/layer gives 24 total cells:
The interesting trade-off here: 13S1P at 48V only uses 13 of 24 cells, leaving 11 unused (wasted space). But 10S2P at 36V uses 20 of 24 and gives you 360Wh — that's 50% more energy than the 48V config, and 36V is the EU-legal sweet spot for pedelec compliance. bev-intl
Also Consider: 60×55 Oval
This also fits 4 cells in the same 2×2 layout — it's 5mm narrower than 65×55 while keeping all four cells. The clearances are tighter (right gap drops to ~6.3mm) but still viable. If you want to keep the DT as slim as possible while still getting 4 cells, 60×55 is the minimum oval that works.
Orientation Note
65w×55h (landscape oval) is better than 55w×65h (portrait) for a downtube because: