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?
Good call on moving to screws for the scaffold-to-cover attachment — simpler and more durable than rails. Let me break down the two electrical systems you need.
USB-C Charging System (65W / 45W Apple)
Your pack is 20 cells (4×5). The series/parallel config determines what charging hardware you need, but assuming 10S2P(36V nominal, 42V full — the most common ebike voltage for 250W):
The pogo pins mount on a small PCB at one end of the scaffold. When you slide the battery in, the spring-loaded pins compress against flat pads fixed in the frame — self-aligning, vibration-tolerant, and weatherproof when mated.scribd+1
Parts summary for the battery end
On the insertion end of the scaffold you'll have a small PCB or bracket holding:
USB-C panel-mount port (user-facing, accessible from outside)
PD trigger board + boost converter (tiny, fits behind the USB-C port)
2 power pogo pins + 2–4 signal pogo pins (facing into the frame dock)
On the frame side (fixed permanently):
Matching flat contact pads (PCB or gold-plated copper strips)
Wiring from pads → motor controller, lights, display
The wiring from the BMS and USB-C board runs through the central 6mm wire hole in the scaffold to reach the pogo pin PCB at the end.
Before I spec exact parts, do you have a preferred series/parallel configuration for the 20 cells? 10S2P (36V) is most common for legal 250W ebikes in Norway, but 13S (~48V) is another option if the motor supports it.
USB-C Charging System (65W / 45W Apple)
Your pack is 20 cells (4×5). The series/parallel config determines what charging hardware you need, but assuming 10S2P (36V nominal, 42V full — the most common ebike voltage for 250W):
The signal chain is: USB-C charger → PD trigger (20V) → boost converter (42V) → BMS → cells. [bigyanproject](https://bigyanproject.com/en/type-c-module/pd65w-usb-c-fast-charging-module)
Why this works with Apple 45W
Apple's 45W charger outputs 20V @ 2.25A via USB-C PD. The PD trigger board negotiates whatever the charger offers. At 45W you'd charge at ~1A into the pack (after boost losses), which is perfectly fine for a pack you charge overnight. [electrek](https://electrek.co/2022/06/23/electric-bike-usb-c-battery-chargers/)
Battery-to-Frame Connector (the dock)
When you slide the battery into the frame, you need electrical contact. The cleanest solution is spring-loaded pogo pins on one end:
The pogo pins mount on a small PCB at one end of the scaffold. When you slide the battery in, the spring-loaded pins compress against flat pads fixed in the frame — self-aligning, vibration-tolerant, and weatherproof when mated. connectors-cables
Parts summary for the battery end
On the insertion end of the scaffold you'll have a small PCB or bracket holding:
On the frame side (fixed permanently):
The wiring from the BMS and USB-C board runs through the central 6mm wire hole in the scaffold to reach the pogo pin PCB at the end.
Before I spec exact parts, do you have a preferred series/parallel configuration for the 20 cells? 10S2P (36V) is most common for legal 250W ebikes in Norway, but 13S (~48V) is another option if the motor supports it.