battery-bike.md

Here's a comprehensive overview of ebike battery designs — especially relevant for your Omera frame project.

Cell Types: 21700 vs 18650

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

Battery Placement Approaches

Downtube Integrated (Most Relevant for Omera)

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

Downtube External Mount

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

Triangle Bag/Pack

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

In-Tube Stealth

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

What This Means for Omera

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?

Spot on. This is a critical design point — the bike lives in Oslo weather, battery goes inside, frame stays out in the rain. The frame-side connector will get wet.

Three layers of protection

1. Fin geometry = self-draining


text
Rain enters open DT top
        ↓ ↓ ↓
    ┌─┐  ┌─┐  ┌─┐
    │ │  │ │  │ │   ← water runs DOWN
    │█│  │█│  │█│     the fin surfaces
    │ │  │ │  │ │
    └─┘  └─┘  └─┘
        ↓ ↓ ↓
    drains out bottom of DT

2. Contact material = corrosion-proof

Gold-plated contacts are standard for outdoor battery connectors — they don't oxidize, corrode, or build resistance even after years of rain exposure. Even if water touches them, nothing happens because:alibaba+1

Gold doesn't corrode.
36V DC at open circuit won't arc or short through rainwater across a 3–4mm gap between pins.
When battery is removed, there's no voltage on the frame-side pads at all — they're just passive copper/gold surfaces.

3. Optional: drain + dummy cap

For extra Oslo winter protection:

Feature | Purpose -- | -- Drain hole at bottom of DT cavity | Water never sits, always exits [bedeconnector] Snap-on dummy cap (like Giant does) | Plastic cover that clips over the fin area when battery is out. Takes 1 second [forums.electricbikereview] Dielectric grease on contacts | Thin coating repels moisture, doesn't affect conductivity [alibaba]

What this means for the design

The frame-side fin connector needs:

Housing: glass-filled nylon or PETG — UV and water resistant.
Contact pads: gold-plated phosphor bronze — welded/soldered to wires, rain-proof by material.
Open bottom: fins don't sit in a closed box — the bottom is open so water drains through.
No electronics: frame side is 100% passive — just metal pads on plastic fins. Nothing to break, nothing to short.

The battery side is always dry (it's inside with you when charging), so standard pogo pins + PCB are fine there.

Want me to model the full interleaved fin connector now — both the frame-side fin piece and the battery-side slot piece?

Spot on. This is a critical design point — the bike lives in Oslo weather, battery goes inside, frame stays out in the rain. The frame-side connector **will get wet**.