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downtube electronics.md

I've now thoroughly reviewed both PDFs. Let me give you a clear summary.

USB Charging & Battery Protection: Already in the Design

Yes, both USB charging and battery protection are already covered in your battery architecture — they're part of the self-contained battery pack design you've been developing. Here's the breakdown:

USB-C Charging ✅

The battery pack includes a full USB-C PD charging system built into the electronics bay at the bottom of the scaffold: ppl-ai-file-upload.s3.amazonaws

  • USB-C panel-mount port (IP67, on the side of the battery end cap)
  • PD trigger board (ZY12PDN or STUSB4531, ~31×15×4mm) — negotiates 20V from any charger 45W–240W automatically
  • Boost converter (XL6019, 20V→42V CC/CV) — steps up to pack charge voltage
  • Total electronics fit in ~60×45×25mm, tucked into the electronics bay

The key design point: all electronics live on the battery, not in the frame. You pull the battery out, plug your MacBook charger into the USB-C port on the side, charge on the kitchen table. The frame side is 100% passive — just flat copper pads. ppl-ai-file-upload.s3.amazonaws

Battery Protection (BMS) ✅

A 10S BMS is included in the electronics bay, handling: ppl-ai-file-upload.s3.amazonaws

  • Overcurrent, overvoltage, undervoltage protection
  • Short-circuit and temperature cutoff
  • Cell balancing during charge
  • Per-cell fusing was deliberately skipped — at 3.5A per cell in 2P, the BMS alone handles everything adequately

Battery-to-Frame Connector ✅

The connector design settled on an elevated pad island + recessed pogo pin dock approach: ppl-ai-file-upload.s3.amazonaws

  • Frame side: Elevated flat gold pads on a raised "speed bump" — water sheets off, no pooling
  • Battery side: 6-pin pogo block recessed deep inside a dock cavity — finger-safe by geometry (IP2X-style)
  • 2 power pins + signal/thermistor/comm pins

Downtube Oval Size: ~50×65 Is Flexible

Your notes confirm the DT dimensions are not locked down yet, and you've been exploring both round and oval options: ppl-ai-file-upload.s3.amazonaws

Discussed Size Context
50mm round Original spec for 2×2 grid of 21700 cells
55–60mm Referenced for fitting 21700s with BMS/wiring room
43×59mm oval The 1-2-1 diamond layout (narrower, taller)
~50×65 (your current thinking) Wider than the diamond, gives more lateral room

The 50×65 oval gives you more cross-sectional area than the 43×59 diamond and better lateral stiffness for pedaling loads. The tall axis (65mm) resists braking/vertical loads while the narrow axis (50mm) keeps the frame slim between your knees. Since you're building in aluminum (not steel), you have freedom to specify custom oval tubing from the fabricator. ppl-ai-file-upload.s3.amazonaws

The scaffold cage parameters (currently cagew = 53, cageh = 48 in the OpenSCAD file) would need updating to match whatever oval you settle on. With a 50×65 outer and ~2mm wall, you'd have roughly 46×61mm inner bore — plenty of room for the 2×2 cell grid (~43×43mm footprint) plus wiring channel and air gaps. ppl-ai-file-upload.s3.amazonaws

Bottom line: your electronics stack is already specced and costed at $22–38 total for BMS + USB-C PD + boost converter. The DT oval just needs to be finalized around whatever gives comfortable clearance for the scaffold + 1mm air gap on each side. ppl-ai-file-upload.s3.amazonaws

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eonist commented Mar 14, 2026

You're right — I was wrong about the path. Let me reconsider.

What's Actually Inside the Steerer

The carbon fork steerer tube is not a clear conduit. Inside it you have:

  • Star nut (or compression plug on carbon) — pressed into the steerer, blocks the bore
  • Top cap bolt — runs down through the top cap into the star nut/plug
  • That's the entire preload mechanism — the steerer bore is occupied

The brake hose goes in the space between the steerer outer wall and the head tube inner wall — not inside the steerer itself. That's the cable routing channel you're describing. [ppl-ai-file-upload.s3.amazonaws](https://ppl-ai-file-upload.s3.amazonaws.com/web/direct-files/attachments/139085844/51f251cf-3b46-47fd-9df4-4b92d46f5b41/frame-build.pdf)

So Can the Wire Get Out?

Actually yes — it just doesn't go through the steerer. The antenna wire exits at the edge of the top cap, not down the center:

    ┌───── top cap (nylon, antenna inside) ─────┐
    │          bolt hole (center)                │
    └──┬────────────────────────────────────┬────┘
       │                                    │
       │  tiny coax exits at rim of cap     │
       │  drops into gap between            │
       │  steerer and head tube             │
       ▼                                    
    Same space where brake hose lives
    → into DT at HT junction

The coax (~1.3mm diameter) just sneaks out at the seam between the top cap edge and the top spacer. It's the same gap where water already gets in — it's not sealed. Then it drops into the same routing channel as your brake hose and front light wire, between the steerer and head tube wall. [ppl-ai-file-upload.s3.amazonaws](https://ppl-ai-file-upload.s3.amazonaws.com/web/direct-files/attachments/139085844/51f251cf-3b46-47fd-9df4-4b92d46f5b41/frame-build.pdf)

Is That Clean Enough?

Honestly — it's a visible wire running from the top cap edge down into the headset area. It's thin, and you could color-match it black, but it's not invisible. You'd see a hair-thin black wire at the stem.

If that bothers you, the bottle cage bolt on the seat tube might still be the cleaner option despite the manufacturing challenge — or you could simply accept that BLE range through aluminium will be reduced but not zero, and skip the external antenna entirely for v1. How critical is the GPS/BLE tracking for the first prototype?

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eonist commented Mar 14, 2026

I guess we could design a small plastic extrusion that would go in a hole under the top tube. that would be stealth. its kind of cool to have these subtle purppousful design elements, that are not easy to discover. we could shape it to the frame circular shape. and use 4 screws to fasten it to the alu frame. weather seal it and that's that. it would face down. do you think that's bad for gps?

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eonist commented Mar 14, 2026

Here's what the industry actually does — and it's revealing:

How Other E-Bikes Solve It

VanMoof S3/S5

Everything is integrated into their custom plastic top tube housing. VanMoof designed the frame from scratch with non-metal sections specifically for antennas. The top tube has plastic panels that are RF-transparent — GPS, BLE, and cellular antennas sit behind plastic, not metal. But VanMoof designed an entire proprietary frame around this. Not applicable to a welded alu frame. help.vanmoof

Cowboy

Similar approach — the top tube is an integrated plastic cockpit with display, BLE, and GPS all behind a plastic window at the handlebars. Again, designed from scratch with RF windows built into the frame architecture. youtube

Bosch ConnectModule

This is the most relevant one. It mounts inside the frame near the motor and on aluminium frames it actually works by finding a thin spot or getting signal through gaps. A user on a Trek PowerFly (aluminium frame) put it inside the frame on top of the motor, below the battery and reported "GPS/Mobile connection appears to be working flawlessly even inside the aluminium frame". The module uses a 400mm cable to position itself where signal leaks through. [reddit](https://www.reddit.com/r/ebikes/comments/16zi96n/bosch_connect_module_tight_cover/)

Tracefy (Dutch, market leader)

They cracked this specifically. Their newest unit has a separate external antenna — the GPS module goes inside the metal frame, and just a tiny antenna (they call it "not much bigger than a pinhead") gets placed behind any plastic part on the bike. That's their whole innovation — decoupling the antenna from the unit. tracefy

The Takeaway

The industry answer is: find existing plastic on the bike and put the antenna behind it. Nobody drills antenna holes in aluminium frames.

On Omera, your existing plastic/rubber parts include:

  • Rubber USB-C flap on the DT — already exists, could embed a chip antenna in it
  • Nylon battery dock area — if any nylon is exposed at the bottom of the DT
  • Brake lever housing — plastic, on the handlebars, clear sky view
  • Saddle — plastic base, already discussed
  • Handlebar grips/tape — wrap a flex antenna under bar tape

Or most interesting: the Bosch/Trek experience suggests that at your frame's joints and cable exit points, enough RF leaks through gaps that it might just work from inside. You have cable pass-throughs at the BB and gaps at the head tube. Those aren't sealed RF-tight. reddit

The smartest move might be: build v1 with GPS+BLE inside the electronics module, test actual signal levels through the frame's natural gaps, and only engineer an external antenna solution if it doesn't work. The Bosch users say it often does.

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eonist commented Mar 14, 2026

For daily use, you can absolutely set-and-forget via the app over BLE. Most riders find one assist level they like and leave it there. The KT controller stores the last setting in memory — it doesn't reset when you power cycle. [ppl-ai-file-upload.s3.amazonaws](https://ppl-ai-file-upload.s3.amazonaws.com/web/direct-files/attachments/139085844/f4be1598-1d22-4aea-a953-6b306655fec4/emotor-3.pdf)

But there's one edge case: hills. In Oslo you'll want to bump assist up for Ekeberg or Tøyen climbs and back down on flats. Pulling out your phone every time is annoying and unsafe while riding.

Options

The Torque Sensor Changes This

With the Bikone torque sensor you specced, the answer is actually you probably don't need buttons at all. The torque sensor gives proportional assist — pedal harder, get more help. Pedal easy, get less. The "assist level" just becomes a multiplier that you set once in the app ("eco" vs "sport") and the torque sensor handles everything dynamically from there. [ppl-ai-file-upload.s3.amazonaws](https://ppl-ai-file-upload.s3.amazonaws.com/web/direct-files/attachments/139085844/51f251cf-3b46-47fd-9df4-4b92d46f5b41/frame-build.pdf)

That's how Cowboy and VanMoof work — no handlebar controls, no buttons, just ride. The torque sensor is the interface.

Skip the buttons. Set the multiplier in the app. The torque sensor does the rest.

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eonist commented Mar 14, 2026

Using a spare pogo for the antenna feed is a good idea and works electrically.

Using a pogo pin as RF feed

  • A thin coax from the GPS/BLE module ends at the battery-side pogo pad (center conductor to pad, shield to local ground plane).

  • The matching frame-side pogo pin connects down to the antenna element mounted in your small plastic console (lock + USB-C piece that already protrudes from the alu).[ppl-ai-file-upload.s3.amazonaws]​

  • Electrically it’s just a single RF feed-through contact. Pogo contact resistance is low enough at GNSS/BLE power levels, and RF sees it as a short line segment.

So your 4+1 pins become:

Pin | Use -- | --

Antenna in the plastic console

That console is:

  • Plastic, protruding from the alu = perfect RF window.

  • “Size of 3 nickels in a row” = ~60×20mm → huge for a small chip or flex antenna.

  • Fixed to the frame; the pogo just brings RF up to it, so the antenna never moves.

Mount a tiny chip or flex antenna on the inside face of that console, as close to the outside surface as possible. The pogo-fed trace from the pin to the antenna is only a few cm, easy to keep 50Ω-ish on a tiny PCB.

Why this is nice

  • No extra frame holes — the console already exists in your design.

  • Antenna sits in plastic, outside the alu Faraday cage.

  • Only one extra pogo used; connector stays compact.

  • GPS and BLE both handled by the same combined antenna, through that one feed line.

This is a very clean integration.

Using a spare pogo for the antenna feed is a **good idea** and works electrically.

Using a pogo pin as RF feed

So your 4+1 pins become:

Pin Use
1 V+ (power)
2 GND
3 UART TX
4 UART RX
5 RF feed to antenna in plastic console

Antenna in the plastic console

That console is:

  • Plastic, protruding from the alu = perfect RF window.
  • “Size of 3 nickels in a row” = ~60×20mm → huge for a small chip or flex antenna.
  • Fixed to the frame; the pogo just brings RF up to it, so the antenna never moves.

Mount a tiny chip or flex antenna on the inside face of that console, as close to the outside surface as possible. The pogo-fed trace from the pin to the antenna is only a few cm, easy to keep 50Ω-ish on a tiny PCB.

Why this is nice

  • No extra frame holes — the console already exists in your design.
  • Antenna sits in plastic, outside the alu Faraday cage.
  • Only one extra pogo used; connector stays compact.
  • GPS and BLE both handled by the same combined antenna, through that one feed line.

This is a very clean integration.

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eonist commented Mar 14, 2026

That’s a really elegant way to frame it: one intentional plastic island that tells the whole tech story, and everything else stays pure bike.

Concentrating all “ebike-ness” there works well with your architecture:

Visually: one small, sculpted plastic island on an otherwise seamless alu frame. Functionally: it’s the IO hub for everything — power in, data out, RF out. That’s a very strong, coherent design move.

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eonist commented Mar 14, 2026

Skipping LEDs is totally reasonable here.

If you ever miss quick at-a-glance feedback, you can add LEDs in v2 without touching the core frame or battery architecture.

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eonist commented Mar 14, 2026

Here are a few things that haven't come up yet but matter:

Power On/Off

How does the bike wake up? There's no button, no display. The KT needs to be told to start. Options:

  • BLE wake — nRF52840 runs in ultra-low-power sleep (3µA), phone app sends a wake command, nRF52840 switches a MOSFET that connects battery to KT. This is how Cowboy does it.
  • Motion wake — a cheap accelerometer on the nRF52840 board detects the bike moving, auto-powers the system. Auto-sleeps after 5 min stationary.
  • Torque sensor wake — first pedal stroke triggers a signal, nRF52840 catches it and powers on. Most natural.

You need to decide this. It's a small design detail but affects how the whole system feels day-to-day.

Brake Cutoff

EU law (EN 15194) requires motor cutoff when braking. Your Shimano MT200 hydraulic brakes need brake sensors — small magnetic switches that plug into the KT controller. When you pull the lever, motor cuts instantly. These are ~$3 each, tiny, and wire into the KT's existing brake sensor input. Don't forget to spec them. ppl-ai-file-upload.s3.amazonaws

Front/Rear Lights

Norway requires lights for riding. Are you doing:

  • Integrated (powered by the main battery, wired internally) — stealthier, but more wiring
  • Standard removable lights — simpler, user's problem

If integrated, the nRF52840 could control them and the wiring routes through the frame alongside existing cables. [ppl-ai-file-upload.s3.amazonaws](https://ppl-ai-file-upload.s3.amazonaws.com/web/direct-files/attachments/139085844/51f251cf-3b46-47fd-9df4-4b92d46f5b41/frame-build.pdf)

Speed Limiter

The KT T06S needs to be programmed to cut assist at 25 km/h for EU legal compliance (EN 15194). This is a software setting in the KT configuration — easy, but you need to flash it before delivery. [ppl-ai-file-upload.s3.amazonaws](https://ppl-ai-file-upload.s3.amazonaws.com/web/direct-files/attachments/139085844/51f251cf-3b46-47fd-9df4-4b92d46f5b41/frame-build.pdf)

On/Off is the Big One

The other items are straightforward. But the power-on UX defines how it feels to use the bike every day. I'd go with BLE wake + motion sleep — open the app, bike wakes up, ride. Stop for 5 minutes, it sleeps. No buttons, no switches, fits your zero-interface philosophy.

What's your instinct on that?

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eonist commented Mar 14, 2026

Answers in order:

1. Torque sensor wake

Totally viable. The Bikone BB torque sensor outputs a UART stream whenever it has power and sees axle movement. For wake: [ppl-ai-file-upload.s3.amazonaws](https://ppl-ai-file-upload.s3.amazonaws.com/web/direct-files/attachments/139085844/51f251cf-3b46-47fd-9df4-4b92d46f5b41/frame-build.pdf)

  • nRF52840 + torque sensor are powered directly from the battery (through a small LDO), always-on but ultra-low power.
  • When the nRF52840 sees valid torque/cadence data (first pedal stroke), it closes a MOSFET that powers the KT controller.
  • KT boots in ~1s; assist ramps in smoothly after the first few crank degrees.

So: no button, no phone needed. You just start pedaling and the system wakes. [ppl-ai-file-upload.s3.amazonaws](https://ppl-ai-file-upload.s3.amazonaws.com/web/direct-files/attachments/139085844/51f251cf-3b46-47fd-9df4-4b92d46f5b41/frame-build.pdf)

2. Brake sensors — do you really need them?

From a pure ride-behaviour angle: you’re right. With a torque sensor, as soon as you stop pedaling, torque = 0, the controller cuts motor power. You also typically reduce torque before you start braking, so the motor is already fading out.

The reasons brake sensors still exist:

That said: plenty of legal EU EPACs ship without dedicated brake sensors when using high-quality torque systems; they rely on instant torque cut + speed cutoff. So you can reasonably skip them for v1 and revisit if a cert lab flags it.

3. Lights

If you don’t sell lights, you’re right: Norwegian law puts the obligation on the rider to have lights in use, not on the manufacturer to ship them on the bike. So skipping integrated lights is fine; you’re just selling a bike that’s “light-ready” (user adds clip-on lights). [cyclingweekly](https://www.cyclingweekly.com/group-tests/find-your-stolen-bike-with-a-gps-tracker-165579)

4. Speed limit & “race mode”

KT behaviour depends on settings, not just voltage:

For legal EU bikes you’d flash / program:

For “private track” / off-road profiles you can create alternate KT configs:

  • Same hardware, different parameter set:
    • Speed limit 32–35 km/h (or “off”, depending on KT firmware)
    • Possibly higher current if you ever go to a beefier controller / battery.

You then have two options:

  • Dealer-only flashing: you flash the legal profile by default; for private use / race track, a shop (or the user, explicitly warned) can reflash via the KT display/programming cable.
  • Hidden app switch (v2 idea): nRF52840 acts as “mode gatekeeper” — it stores two parameter sets and only uploads the “race” set when a hidden app action + legal disclaimer is confirmed. For v1, simpler to keep it as a manual flash.

So: yes, you can set 25 km/h for normal bikes, and have a documented alternate config for private/track use. Just keep the out-of-the-box profile strictly 25 km/h to stay on the right side of EN 15194.

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eonist commented Mar 14, 2026

They’re very simple: they’re just “motor kill” switches on the brake levers that signal the controller.

Basic idea

  • Each brake has a tiny switch or sensor associated with the lever.
  • When the lever is not pulled, the switch is in its normal state.
  • When you pull the lever, the switch changes state and sends a signal to the controller.
  • The controller sees that signal and immediately cuts power to the motor, regardless of what the torque sensor or PAS is saying. reallygoodebikes

Electrically

On a KT-style controller you usually have a 2‑wire brake input per lever:

For hydraulic brakes like your MT200s, you’d typically add an external magnet + sensor kit:

  • A small magnet on the lever blade.
  • A reed or Hall sensor on the lever body.
  • Lever at rest: magnet close to sensor → one state.
  • Lever pulled: magnet moves away → sensor flips state → controller cuts power. reallygoodebikes

With a good torque sensor, this is belt‑and‑suspenders: torque already drops to zero when you stop pedaling, but the brake sensor guarantees instant cut-off even if the torque signal were noisy or laggy.

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eonist commented Mar 14, 2026

Yes — two routes:

Levers With Built-In Sensors

A few brands make hydraulic brake levers with the cut-off sensor integrated:

  • Magura MT5e — high-end, integrated Hall sensor in the lever body, 2-pin Higo connector. Just works, no magnets to fall off. Pricey though (~€100–150 per brake). pedelecs.co
  • Budget e-bike specific levers — brands like Yukti, Zillan, GT03 sell cheap hydraulic lever sets (~$30–50 per set) with sensors built in, but the brake quality is mediocre compared to Shimano. aliexpress

Add-On Sensors for Your MT200s

Since you've already specced the Shimano MT200s, which are solid brakes, the cheaper and simpler path is bolt-on sensors:

  • MT200-specific sensor kit — a tiny sensor body clips onto the MT200 lever housing, magnet goes under the lever blade. Purpose-made to fit the MT200 shape. ~£8–15 per set. ebikesolution.co
  • Generic magnetic sensor — magnet + Hall sensor, stick-on with 3M tape or zip-ties. Universal fit, ~$3–5 per pair. Works but the magnets can fall off over time. reddit

What I'd Do for Omera

Keep the MT200s and get the MT200-specific sensor kit — it's designed to clip cleanly onto that exact lever without tape or zip-ties. Two wires run from the levers to the KT controller's brake input. About $10 total and it looks factory. [youtube](https://www.youtube.com/watch?v=mnP-nsL0YmM)

Or, given your torque sensor setup: skip them entirely for now, test the bike, and add them later if you feel the motor doesn't cut fast enough when braking. The sensor kit is a 10-minute install — no reason to over-commit on v1.

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eonist commented Mar 14, 2026

Most don’t, especially the clean, modern ones you’re inspired by.

What other e-bikes do

  • Torque-sensor city/commuter bikes (Cowboy, VanMoof, etc.) usually do not run extra brake-cut wires to the bars. They rely on:
    • fast torque-sensor shutdown when you stop pedaling, and
    • speed limiting + motor ramp-down in firmware. help.vanmoof
  • Bosch/Yamaha/Shimano mid-drive systems also generally don’t have visible brake wires; cut-off is handled by pedal/torque sensing and internal logic rather than external brake switches. ebike24

Extra brake wiring is mostly seen on:

  • Cheap conversion kits and older hub-motor systems where control is cadence-only and laggy, so they need a hard brake kill. forums.electricbikereview
  • DIY builds using generic KT kits where safety is ensured by simple brake switches.

Given you’re:

  • Using a BB torque sensor (fast, proportional cutoff),
  • Keeping power low (250W),
  • Aiming for a super-clean cockpit,

you’re aligned with the “modern integrated” camp. It’s reasonable to skip bar brake sensors in v1 and rely on torque-based cutoff, and only add dedicated sensors later if real-world testing shows you need them.

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eonist commented Mar 14, 2026

okay then. ill sketch the downtube electronic module tomorrow. and then we can continue with more scad modeling. it will be fun to put things together. I think we can nail most of the downtube modeling tomorrow. after that is done the real fun begins. designing and modeling the frame in more detail. then adding bike parts and see it come alive in 3d renders.

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eonist commented Mar 14, 2026

Sounds like a great plan. With the KT + torque wake, 4+1 pogo pins, plastic console antenna, and no bar buttons or brake wires, the architecture is solid enough to model.

When you sketch the DT electronics module, prioritize:

  • KT box placement and cradle geometry
  • Air hulls and how the module keys into the DT
  • The plastic console volume and antenna feed pogo

Bring those sketches or dimensions tomorrow and we can turn them into SCAD primitives and constraints, then fit everything against your current DT oval.

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eonist commented Mar 14, 2026

Here's what typically comes in a Bafang G020 kit from GreenBikeKit or Varstrom:

Standard Kit Contents

Item | Useful for Omera? -- | -- Motor wheel (laced to rim) | ✅ The core — but you'll want it laced to your rim (700c) 1T4 cable harness | ⚠️ This is the main wiring loom — splits one connector into 4: display, throttle, brake, PAS. You won't use most of these Motor controller (KT style) | ✅ Your KT T06S Controller box + mounting hardware | ❌ You're mounting inside the DT PAS cadence sensor | ❌ You're using Bikone torque sensor instead Thumb throttle | ❌ No throttle, EU legal 2× eBrake levers or sensors | ❌ Skipping for v1 Display | ❌ No display, app only Headlight | ❌ No integrated lights Speed sensor + magnet | ⚠️ You might actually need this — the KT needs a speed input for the 25 km/h cutoff Motor extension cable (9-pin Higo) | ✅ Useful — extends motor wire length for internal routing through chainstay Cable ties, labels, screws | ✅ Free stuff

[varstrom]​[youtube]​

What's Actually Useful

Out of a ~$80–100 full kit, you really only need:

  • Motor wheel (or motor only, lace yourself)

  • KT controller (bare)

  • Motor extension cable — the stock motor cable is short (~80cm). Running it internally through the chainstay to the DT electronics module, you'll likely need the extension[aliexpress]​

  • Speed sensor — unless the Bikone torque sensor also outputs speed to the KT (it outputs cadence + torque, but the KT may need a separate wheel speed signal for the 25 km/h limit)[youtube]​

The Smart Move

Buy the motor wheel only + KT controller only + extension cable as separate items from GreenBikeKit or AliExpress. Don't buy the full kit — you'd be paying for a throttle, display, PAS sensor, brake levers, and harness you'll throw in a drawer.ebikerepairfactory+1

The 1T4 cable harness is worth looking at though — even if you don't use most branches, it gives you the weatherproof Higo connectors that mate to the KT. You can cut off the branches you don't need and use the main trunk as your internal wiring loom.[youtube]​

One thing to double-check: does the Bikone torque sensor feed wheel speed to the KT, or do you still need the magnetic speed sensor on the rear wheel? That determines whether you need one more small sensor mounted at the chainstay.

Here's what typically comes in a **Bafang G020 kit** from GreenBikeKit or Varstrom:

Standard Kit Contents

Item Useful for Omera?
Motor wheel (laced to rim) ✅ The core — but you'll want it laced to your rim (700c)
1T4 cable harness ⚠️ This is the main wiring loom — splits one connector into 4: display, throttle, brake, PAS. You won't use most of these
Motor controller (KT style) ✅ Your KT T06S
Controller box + mounting hardware ❌ You're mounting inside the DT
PAS cadence sensor ❌ You're using Bikone torque sensor instead
Thumb throttle ❌ No throttle, EU legal
2× eBrake levers or sensors ❌ Skipping for v1
Display ❌ No display, app only
Headlight ❌ No integrated lights
Speed sensor + magnet ⚠️ You might actually need this — the KT needs a speed input for the 25 km/h cutoff
Motor extension cable (9-pin Higo) ✅ Useful — extends motor wire length for internal routing through chainstay
Cable ties, labels, screws ✅ Free stuff

[varstrom](https://varstrom.com/products/bafang-500w-48v-front-hub)

What's Actually Useful

Out of a ~$80–100 full kit, you really only need:

  • Motor wheel (or motor only, lace yourself)
  • KT controller (bare)
  • Motor extension cable — the stock motor cable is short (~80cm). Running it internally through the chainstay to the DT electronics module, you'll likely need the extension aliexpress
  • Speed sensor — unless the Bikone torque sensor also outputs speed to the KT (it outputs cadence + torque, but the KT may need a separate wheel speed signal for the 25 km/h limit) [youtube](https://www.youtube.com/watch?v=Embjdj5ezfc)

The Smart Move

Buy the motor wheel only + KT controller only + extension cable as separate items from GreenBikeKit or AliExpress. Don't buy the full kit — you'd be paying for a throttle, display, PAS sensor, brake levers, and harness you'll throw in a drawer. ebikerepairfactory

The 1T4 cable harness is worth looking at though — even if you don't use most branches, it gives you the weatherproof Higo connectors that mate to the KT. You can cut off the branches you don't need and use the main trunk as your internal wiring loom. [youtube](https://www.youtube.com/watch?v=Embjdj5ezfc)

One thing to double-check: does the Bikone torque sensor feed wheel speed to the KT, or do you still need the magnetic speed sensor on the rear wheel? That determines whether you need one more small sensor mounted at the chainstay.

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eonist commented Mar 14, 2026

Short answer: the G020 hub already has a speed sensor inside, and the Bikone BB torque sensor only gives torque/cadence, not wheel speed.

Motor: G020

Owners and Bafang docs confirm the RM G020 has an internal Hall-based speed sensor that outputs speed via the motor cable to the controller/display. You don’t need an external spoke magnet sensor for speed; the KT can read speed directly from the motor’s Hall speed line. endless-sphere

So:

  • Speed limit at 25 km/h = handled by KT using the G020’s internal speed pulses.
  • No separate frame‑mounted speed sensor required.

Bikone torque sensor

Bikone’s BB torque sensor outputs torque, cadence, crank angle, etc. over UART / digital / pulses, but it is explicitly a rider input sensor, not a wheel speed sensor. It does not replace the motor speed sensor; it just tells the KT how hard and how fast you’re pedaling. bikone

So your final picture:

  • Wheel speed: from G020’s internal sensor → KT.
  • Rider input (torque/cadence): from Bikone BB → KT.
  • No extra magnets or speed sensors needed on the frame.

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eonist commented Mar 15, 2026

The Dance bike uses an underside downtube mount with a removable battery that clicks in from below. Here's how that compares to your current topside/internal approach for Omera.[help.dance]​

Underside vs Topside DT Battery

Factor | Underside (Dance style) | Topside / Internal (Omera current) -- | -- | -- Center of gravity | Lower — best possible CoG on the frame leoguarbikes+1 | Slightly higher, but still central and low Road debris exposure | Directly exposed to rocks, water, mud spray from front wheel [youtube]​ | Protected by the DT tube itself Visual stealth | Hidden when viewed from the side, but visible from below | Fully hidden inside the tube — looks like a normal bike [upway]​ Water/dirt ingress | Battery latch and contacts face downward — worst case for moisture [reddit]​ | Sealed inside the tube, much better Impact protection | Vulnerable to speed bumps, curbs, potholes [youtube]​ | The DT aluminum shell protects everything Removal for charging | Easy slide-out from below [help.dance]​ | Requires your pogo dock or side-entry — slightly less convenient Frame structural impact | DT stays solid, battery hangs on external rails | Hollow DT needs reinforcement to maintain stiffness [upway]​ Aerodynamics | Creates drag underneath | Clean tube profile Battery capacity | Not constrained by tube diameter — can go wider/taller | Limited by DT cross-section

Why Dance Does It This Way

Dance is a subscription rental bike — their #1 priority is that users can pop the battery out in 2 seconds with no tools for charging indoors. The underside mount with a spring latch achieves that. They accept the dirt/impact trade-off because their fleet gets regular maintenance.reddit+1

Why Topside/Internal Is Better for Omera

Your design already commits to the battery inside the oversized oval DT, which gives you:

  • Best protection from Norwegian winter weather, road salt, and debris[leoguarbikes]​

  • Stealth appearance — the bike looks like a normal fixie[upway]​

  • Structural integration — the battery pack becomes part of the frame's mass, not a cantilevered load hanging off rails

  • Electronics co-location — the KT controller, Bikone interface, BT/GPS module, and battery all live in one sealed volume

The main downside is charging convenience. But you've already solved that with the pogo dock + side console approach — you charge in-frame or pop the console to access connectors. For a personal bike (not a rental fleet), that's perfectly fine.

One Hybrid Idea Worth Considering

If you ever want quick battery swaps (e.g., for long rides), you could design the DT with a bottom hatch — a CNC'd aluminum panel on the underside held by two quarter-turn fasteners. Battery slides out downward, same as Dance, but the hatch seals flush when closed. This gives you the protection of internal mounting plus the convenience of underside removal. But that's a v2 consideration — for v1, your current internal pogo-charge approach is cleaner and simpler.

The Dance bike uses an **underside downtube mount with a removable battery that clicks in from below**. Here's how that compares to your current topside/internal approach for Omera. [[help.dance](https://help.dance.co/en/articles/5902466)](https://help.dance.co/en/articles/5902466)

Underside vs Topside DT Battery

Factor Underside (Dance style) Topside / Internal (Omera current)
Center of gravity Lower — best possible CoG on the frame leoguarbikes Slightly higher, but still central and low
Road debris exposure Directly exposed to rocks, water, mud spray from front wheel [youtube](https://www.youtube.com/watch?v=B19FPDKLiZ8) Protected by the DT tube itself
Visual stealth Hidden when viewed from the side, but visible from below Fully hidden inside the tube — looks like a normal bike [upway](https://upway.co/blogs/news/e-bike-battery-locations-comparison)
Water/dirt ingress Battery latch and contacts face downward — worst case for moisture [reddit](https://www.reddit.com/r/ebikes/comments/za2j6k/why_do_ebike_companies_put_the_battery_at_the/) Sealed inside the tube, much better
Impact protection Vulnerable to speed bumps, curbs, potholes [youtube](https://www.youtube.com/watch?v=B19FPDKLiZ8) The DT aluminum shell protects everything
Removal for charging Easy slide-out from below [help.dance](https://help.dance.co/en/articles/5902466) Requires your pogo dock or side-entry — slightly less convenient
Frame structural impact DT stays solid, battery hangs on external rails Hollow DT needs reinforcement to maintain stiffness [upway](https://upway.co/blogs/news/e-bike-battery-locations-comparison)
Aerodynamics Creates drag underneath Clean tube profile
Battery capacity Not constrained by tube diameter — can go wider/taller Limited by DT cross-section

Why Dance Does It This Way

Dance is a subscription rental bike — their #1 priority is that users can pop the battery out in 2 seconds with no tools for charging indoors. The underside mount with a spring latch achieves that. They accept the dirt/impact trade-off because their fleet gets regular maintenance. reddit

Why Topside/Internal Is Better for Omera

Your design already commits to the battery inside the oversized oval DT, which gives you:

The main downside is charging convenience. But you've already solved that with the pogo dock + side console approach — you charge in-frame or pop the console to access connectors. For a personal bike (not a rental fleet), that's perfectly fine.

One Hybrid Idea Worth Considering

If you ever want quick battery swaps (e.g., for long rides), you could design the DT with a bottom hatch — a CNC'd aluminum panel on the underside held by two quarter-turn fasteners. Battery slides out downward, same as Dance, but the hatch seals flush when closed. This gives you the protection of internal mounting plus the convenience of underside removal. But that's a v2 consideration — for v1, your current internal pogo-charge approach is cleaner and simpler.

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eonist commented Mar 15, 2026

The short answer: safety liability, industry lobbying, and planned obsolescence. There are both legitimate engineering reasons and cynical business reasons.

Legitimate Engineering Challenges

  • Cell matching matters. A BMS balances cells in series groups, and mixing old degraded cells with fresh ones causes the weakest cell to limit the whole pack. If one cell has 80% health and you swap in a 100% cell, the BMS still cuts off based on the weakest link — or worse, the mismatch causes overcharge on the weak cell during balancing. marsantsx
  • Spot-welded nickel strips. Standard 18650/21700 packs are spot-welded into series/parallel groups. This is a permanent bond — you can't just pull a cell out without desoldering or cutting the strip. Connectorized holders (like your spring/clip design) add contact resistance and bulk. [repair](https://repair.eu/news/disposable-e-bikes-the-problem-with-unrepairable-batteries/)
  • BMS calibration. The BMS tracks cell impedance and state-of-charge over time per cell group. Swapping individual cells resets those assumptions and can confuse the balancing algorithm. [gebbattery](https://www.gebbattery.com/news/can-i-upgrade-or-replace-the-bms-in-my-electri-85457155.html)

Industry / Business Reasons

Why Omera's Approach Is Different

Your design sidesteps most of these problems:

  • You're the manufacturer and the user — no liability chain to worry about
  • You can design the BMS to expect cell swaps — track per-slot impedance, flag degraded cells, and rebalance after a swap
  • Spring/clip holders instead of spot welds — accepts the small contact resistance trade-off for serviceability
  • Standard 21700 cells — you buy Samsung 50E or Molicel P42A from any supplier, not a proprietary pack
  • Cell-level monitoring lets you replace only the degraded cell(s) instead of the whole pack — extending total pack life by years [repair](https://repair.eu/news/disposable-e-bikes-the-problem-with-unrepairable-batteries/)

The battery repair shops across Europe (like Daurema in Brussels) already do exactly this — they open sealed packs, test individual cells, and swap the dead ones. They just have to fight the spot welds and glue to do it. You're designing the pack to make that process trivial from day one. [repair](https://repair.eu/news/disposable-e-bikes-the-problem-with-unrepairable-batteries/)

The real reason the industry doesn't do it: they don't want to. A modular cell pack is a solved engineering problem — it's just not in their financial interest. [bike-eu](https://www.bike-eu.com/51466/eu-commission-dismisses-e-bike-battery-single-cell-replacement-in-amendment)

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eonist commented Mar 15, 2026

You're right to push back — it's not a big issue at your current levels. Let me do the actual math.

The Real Numbers

Connection type | Resistance per contact | Source -- | -- | -- Spot-welded nickel strip | 0.3–0.5 mΩ | [thinkrobotics]​ Good BeCu spring holder | 10–15 mΩ per terminal | [alibaba]​ Cheap stamped spring holder | 20–40 mΩ | [alibaba]​ Copper contact holder (recent test) | ~10.7 mΩ | [youtube]​

So worst case, a quality holder adds ~15 mΩ per cell compared to a spot weld. Now let's see what that means for Omera's pack.

Power Loss in Practice

Your pack is a 13S2P configuration (48V, 250W motor). At full load:

  • Max continuous current: ~5–6A from the pack (250W ÷ 48V)

  • Per cell in a 2P group: ~2.5–3A each

Power lost per contact at 3A with 15 mΩ: P = I² × R = 9 × 0.015 = 0.135W

With 26 cells × 2 contacts each = 52 contacts total: 52 × 0.135W = ~7W total

On a 250W system, that's about 2.8% loss — comparable to the loss in the wiring and BMS MOSFETs anyway. You'd barely notice it in range.[alibaba]​

And that's the absolute worst case at full sustained load. At typical cruising assist (~100–150W), the current is half that, and since losses scale with I², the real-world loss drops to under 1%.

Why the Industry Argument Is Overblown

The "contact resistance" argument only matters for high-drain applications — 20A+ per cell for power tools or drones. For an e-bike pulling 3A per cell, it's negligible. The real reasons the industry spot-welds are:[reddit]​

  • Manufacturing speed — spot welding is faster on a production line than placing cells in holders[evlithium]​

  • Vibration resistance — spot welds can't rattle loose, springs theoretically can (but proper BeCu holders with 8–14N insertion force handle this fine)[alibaba]​

  • It prevents user servicing — which is the actual goal[bike-eu]​

Your "Replace All At Once" Policy

This is the correct approach and it completely eliminates the cell-matching argument. If users swap all 26 cells simultaneously:

  • All cells are same age, same chemistry, same capacity

  • BMS starts fresh with a balanced pack

  • No weak-cell / strong-cell mismatch possible

  • Users buy a standard cell (Samsung 50E, Molicel P42A) from any supplier

You just specify in the manual: "Replace all cells simultaneously with identical cells. Mixing old and new cells voids warranty." This is no different from how every flashlight, remote control, and AA-powered device has worked for decades.

The EU Battery Regulation actually supports your right-to-repair angle, even though the commission recently caved to industry pressure on single-cell replacement. A pack designed for full-set user replacement is still fully compliant and far more sustainable than a sealed glued brick that goes to landfill after 3 years.repair+1

You're right to push back — it's **not a big issue** at your current levels. Let me do the actual math.

The Real Numbers

Connection type Resistance per contact Source
Spot-welded nickel strip 0.3–0.5 mΩ [thinkrobotics](https://thinkrobotics.com/blogs/learn/diy-spot-welder-for-18650-batteries-complete-build-guide-for-perfect-battery-packs)
Good BeCu spring holder 10–15 mΩ per terminal [alibaba](https://www.alibaba.com/product-insights/how-to-choose-the-best-21700-battery-holder-a-complete-buying-guide.html)
Cheap stamped spring holder 20–40 mΩ [alibaba](https://www.alibaba.com/product-insights/how-to-choose-the-best-21700-battery-holder-a-complete-buying-guide.html)
Copper contact holder (recent test) ~10.7 mΩ [youtube](https://www.youtube.com/watch?v=PI1vEBie0BI)

So worst case, a quality holder adds ~15 mΩ per cell compared to a spot weld. Now let's see what that means for Omera's pack.

Power Loss in Practice

Your pack is a 13S2P configuration (48V, 250W motor). At full load:

  • Max continuous current: ~5–6A from the pack (250W ÷ 48V)
  • Per cell in a 2P group: ~2.5–3A each

Power lost per contact at 3A with 15 mΩ: P = I² × R = 9 × 0.015 = 0.135W

With 26 cells × 2 contacts each = 52 contacts total: 52 × 0.135W = ~7W total

On a 250W system, that's about 2.8% loss — comparable to the loss in the wiring and BMS MOSFETs anyway. You'd barely notice it in range. [alibaba](https://www.alibaba.com/product-insights/how-to-choose-the-best-21700-battery-holder-a-complete-buying-guide.html)

And that's the absolute worst case at full sustained load. At typical cruising assist (~100–150W), the current is half that, and since losses scale with I², the real-world loss drops to under 1%.

Why the Industry Argument Is Overblown

The "contact resistance" argument only matters for high-drain applications — 20A+ per cell for power tools or drones. For an e-bike pulling 3A per cell, it's negligible. The real reasons the industry spot-welds are: [reddit](https://www.reddit.com/r/batteries/comments/1e8x6j3/power_losses_from_21700_holder_vs_nickel/)

Your "Replace All At Once" Policy

This is the correct approach and it completely eliminates the cell-matching argument. If users swap all 26 cells simultaneously:

  • All cells are same age, same chemistry, same capacity
  • BMS starts fresh with a balanced pack
  • No weak-cell / strong-cell mismatch possible
  • Users buy a standard cell (Samsung 50E, Molicel P42A) from any supplier

You just specify in the manual: "Replace all cells simultaneously with identical cells. Mixing old and new cells voids warranty." This is no different from how every flashlight, remote control, and AA-powered device has worked for decades.

The EU Battery Regulation actually supports your right-to-repair angle, even though the commission recently caved to industry pressure on single-cell replacement. A pack designed for full-set user replacement is still fully compliant and far more sustainable than a sealed glued brick that goes to landfill after 3 years. repair

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eonist commented Mar 15, 2026

Good news — there's a Darfon battery that comes very close to your 65×55mm target. Here's how the integrated in-tube batteries on the market compare:

In-Tube Battery Dimensions

Battery | Dimensions (L × W × H) | Capacity | Fits 65×55? | System -- | -- | -- | -- | -- Darfon E2C0D | 412 × 58 × 52mm | 400 Wh | ✅ Yes — 58×52 fits inside 65×55 | Shimano EP801/EP6 [darfonenergy]​ Darfon (14Ah) | 358.7 × 78.2 × 63.8mm | ~500 Wh | ❌ 78mm too wide | Shimano [darfon]​ Darfon (17.5Ah) | 427 × 78.2 × 63.8mm | 630 Wh | ❌ 78mm too wide | Shimano [darfon]​ Bosch PowerTube 500 | 349 × 83.7 × 65mm | 500 Wh | ❌ 84mm way too wide | Bosch [ebike24]​ Bosch PowerTube 625 | 426.5 × 83.7 × 65mm | 625 Wh | ❌ 84mm too wide | Bosch [ebike24]​ E-Bike Vision Intube | 349 × 83.7 × 65mm | 461 Wh | ❌ 84mm too wide | Bosch [ebike24]​ Shimano BT-E8035-L | ~similar to Darfon 78mm class | 504 Wh | ❌ Too wide | Shimano [alutech-cycles]​

The Darfon E2C0D Is Interesting

At 58 × 52mm cross-section, this is by far the slimmest in-tube battery on the market. It fits a Shimano EP801/EP6 system and uses CANBus/UART communication. At 400 Wh and only 1.9 kg, it's remarkably compact. The trade-off is it's a sealed, non-serviceable Shimano-specific pack — the exact opposite of your philosophy.[darfonenergy]​

But it proves something important: 400 Wh in a 58×52mm cross-section over ~412mm length is physically achievablewith current cell technology.

What This Means for Omera

Your 65×55mm oval DT cross-section is actually more generous than Darfon's smallest battery. You have roughly:

  • 65 × 55mm internal oval ≈ ~2,800 mm² cross-section

  • Darfon E2C0D at 58 × 52mm ≈ ~2,370 mm² cross-section

You have about 18% more cross-sectional area than the smallest commercial in-tube battery. With your spring-clip 21700 cell holders in a 13S2P configuration, you're in the same ballpark as Darfon's 400 Wh — except your pack is user-serviceable, runs at 48V instead of 36V, and uses standard cells anyone can buy.

None of these off-the-shelf batteries would work for Omera anyway — they're all locked to Bosch or Shimano motor systems via proprietary BMS communication. Your KT controller speaks a simpler protocol. But the Darfon E2C0D validates that your tube dimensions aren't crazy — the industry is already shipping batteries in that envelope.

Good news — there's a Darfon battery that comes very close to your 65×55mm target. Here's how the integrated in-tube batteries on the market compare:

In-Tube Battery Dimensions

Battery Dimensions (L × W × H) Capacity Fits 65×55? System
Darfon E2C0D 412 × 58 × 52mm 400 Wh ✅ Yes — 58×52 fits inside 65×55 Shimano EP801/EP6 [darfonenergy](https://www.darfonenergy.com/en/product/24051015255825/24120614163384)
Darfon (14Ah) 358.7 × 78.2 × 63.8mm ~500 Wh ❌ 78mm too wide Shimano [darfon](https://www.darfon.com/en/product/18071309102090/18071615475913)
Darfon (17.5Ah) 427 × 78.2 × 63.8mm 630 Wh ❌ 78mm too wide Shimano [darfon](https://www.darfon.com/en/product/18071309102090/18071615475913)
Bosch PowerTube 500 349 × 83.7 × 65mm 500 Wh ❌ 84mm way too wide Bosch [ebike24](https://www.ebike24.com/ebike-vision-powerpack-intube-battery-461-511-wh-bosch-powertube-alternative)
Bosch PowerTube 625 426.5 × 83.7 × 65mm 625 Wh ❌ 84mm too wide Bosch [ebike24](https://www.ebike24.com/ebike-vision-powerpack-intube-battery-640-wh-bosch-powertube-alternative)
E-Bike Vision Intube 349 × 83.7 × 65mm 461 Wh ❌ 84mm too wide Bosch [ebike24](https://www.ebike24.com/ebike-vision-powerpack-intube-battery-461-511-wh-bosch-powertube-alternative)
Shimano BT-E8035-L ~similar to Darfon 78mm class 504 Wh ❌ Too wide Shimano [alutech-cycles](https://alutech-cycles.com/Shimano-STEPS-BT-E8035L-LONG-VERSION-battery-504Wh-integratet)

The Darfon E2C0D Is Interesting

At 58 × 52mm cross-section, this is by far the slimmest in-tube battery on the market. It fits a Shimano EP801/EP6 system and uses CANBus/UART communication. At 400 Wh and only 1.9 kg, it's remarkably compact. The trade-off is it's a sealed, non-serviceable Shimano-specific pack — the exact opposite of your philosophy. [darfonenergy](https://www.darfonenergy.com/en/product/24051015255825/24120614163384)

But it proves something important: 400 Wh in a 58×52mm cross-section over ~412mm length is physically achievable with current cell technology.

What This Means for Omera

Your 65×55mm oval DT cross-section is actually more generous than Darfon's smallest battery. You have roughly:

  • 65 × 55mm internal oval ≈ ~2,800 mm² cross-section
  • Darfon E2C0D at 58 × 52mm ≈ ~2,370 mm² cross-section

You have about 18% more cross-sectional area than the smallest commercial in-tube battery. With your spring-clip 21700 cell holders in a 13S2P configuration, you're in the same ballpark as Darfon's 400 Wh — except your pack is user-serviceable, runs at 48V instead of 36V, and uses standard cells anyone can buy.

None of these off-the-shelf batteries would work for Omera anyway — they're all locked to Bosch or Shimano motor systems via proprietary BMS communication. Your KT controller speaks a simpler protocol. But the Darfon E2C0D validates that your tube dimensions aren't crazy — the industry is already shipping batteries in that envelope.

@eonist
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eonist commented Mar 15, 2026

Good news — there's a Darfon battery that comes very close to your 65×55mm target. Here's how the integrated in-tube batteries on the market compare:

In-Tube Battery Dimensions

Battery | Dimensions (L × W × H) | Capacity | Fits 65×55? | System -- | -- | -- | -- | -- Darfon E2C0D | 412 × 58 × 52mm | 400 Wh | ✅ Yes — 58×52 fits inside 65×55 | Shimano EP801/EP6 [darfonenergy]​ Darfon (14Ah) | 358.7 × 78.2 × 63.8mm | ~500 Wh | ❌ 78mm too wide | Shimano [darfon]​ Darfon (17.5Ah) | 427 × 78.2 × 63.8mm | 630 Wh | ❌ 78mm too wide | Shimano [darfon]​ Bosch PowerTube 500 | 349 × 83.7 × 65mm | 500 Wh | ❌ 84mm way too wide | Bosch [ebike24]​ Bosch PowerTube 625 | 426.5 × 83.7 × 65mm | 625 Wh | ❌ 84mm too wide | Bosch [ebike24]​ E-Bike Vision Intube | 349 × 83.7 × 65mm | 461 Wh | ❌ 84mm too wide | Bosch [ebike24]​ Shimano BT-E8035-L | ~similar to Darfon 78mm class | 504 Wh | ❌ Too wide | Shimano [alutech-cycles]​

The Darfon E2C0D Is Interesting

At 58 × 52mm cross-section, this is by far the slimmest in-tube battery on the market. It fits a Shimano EP801/EP6 system and uses CANBus/UART communication. At 400 Wh and only 1.9 kg, it's remarkably compact. The trade-off is it's a sealed, non-serviceable Shimano-specific pack — the exact opposite of your philosophy.[darfonenergy]​

But it proves something important: 400 Wh in a 58×52mm cross-section over ~412mm length is physically achievablewith current cell technology.

What This Means for Omera

Your 65×55mm oval DT cross-section is actually more generous than Darfon's smallest battery. You have roughly:

  • 65 × 55mm internal oval ≈ ~2,800 mm² cross-section

  • Darfon E2C0D at 58 × 52mm ≈ ~2,370 mm² cross-section

You have about 18% more cross-sectional area than the smallest commercial in-tube battery. With your spring-clip 21700 cell holders in a 13S2P configuration, you're in the same ballpark as Darfon's 400 Wh — except your pack is user-serviceable, runs at 48V instead of 36V, and uses standard cells anyone can buy.

None of these off-the-shelf batteries would work for Omera anyway — they're all locked to Bosch or Shimano motor systems via proprietary BMS communication. Your KT controller speaks a simpler protocol. But the Darfon E2C0D validates that your tube dimensions aren't crazy — the industry is already shipping batteries in that envelope.

Good news — there's a Darfon battery that comes very close to your 65×55mm target. Here's how the integrated in-tube batteries on the market compare:

In-Tube Battery Dimensions

Battery Dimensions (L × W × H) Capacity Fits 65×55? System
Darfon E2C0D 412 × 58 × 52mm 400 Wh ✅ Yes — 58×52 fits inside 65×55 Shimano EP801/EP6 [darfonenergy](https://www.darfonenergy.com/en/product/24051015255825/24120614163384)
Darfon (14Ah) 358.7 × 78.2 × 63.8mm ~500 Wh ❌ 78mm too wide Shimano [darfon](https://www.darfon.com/en/product/18071309102090/18071615475913)
Darfon (17.5Ah) 427 × 78.2 × 63.8mm 630 Wh ❌ 78mm too wide Shimano [darfon](https://www.darfon.com/en/product/18071309102090/18071615475913)
Bosch PowerTube 500 349 × 83.7 × 65mm 500 Wh ❌ 84mm way too wide Bosch [ebike24](https://www.ebike24.com/ebike-vision-powerpack-intube-battery-461-511-wh-bosch-powertube-alternative)
Bosch PowerTube 625 426.5 × 83.7 × 65mm 625 Wh ❌ 84mm too wide Bosch [ebike24](https://www.ebike24.com/ebike-vision-powerpack-intube-battery-640-wh-bosch-powertube-alternative)
E-Bike Vision Intube 349 × 83.7 × 65mm 461 Wh ❌ 84mm too wide Bosch [ebike24](https://www.ebike24.com/ebike-vision-powerpack-intube-battery-461-511-wh-bosch-powertube-alternative)
Shimano BT-E8035-L ~similar to Darfon 78mm class 504 Wh ❌ Too wide Shimano [alutech-cycles](https://alutech-cycles.com/Shimano-STEPS-BT-E8035L-LONG-VERSION-battery-504Wh-integratet)

The Darfon E2C0D Is Interesting

At 58 × 52mm cross-section, this is by far the slimmest in-tube battery on the market. It fits a Shimano EP801/EP6 system and uses CANBus/UART communication. At 400 Wh and only 1.9 kg, it's remarkably compact. The trade-off is it's a sealed, non-serviceable Shimano-specific pack — the exact opposite of your philosophy. [darfonenergy](https://www.darfonenergy.com/en/product/24051015255825/24120614163384)

But it proves something important: 400 Wh in a 58×52mm cross-section over ~412mm length is physically achievable with current cell technology.

What This Means for Omera

Your 65×55mm oval DT cross-section is actually more generous than Darfon's smallest battery. You have roughly:

  • 65 × 55mm internal oval ≈ ~2,800 mm² cross-section
  • Darfon E2C0D at 58 × 52mm ≈ ~2,370 mm² cross-section

You have about 18% more cross-sectional area than the smallest commercial in-tube battery. With your spring-clip 21700 cell holders in a 13S2P configuration, you're in the same ballpark as Darfon's 400 Wh — except your pack is user-serviceable, runs at 48V instead of 36V, and uses standard cells anyone can buy.

None of these off-the-shelf batteries would work for Omera anyway — they're all locked to Bosch or Shimano motor systems via proprietary BMS communication. Your KT controller speaks a simpler protocol. But the Darfon E2C0D validates that your tube dimensions aren't crazy — the industry is already shipping batteries in that envelope.

@eonist
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eonist commented Mar 15, 2026

The honest answer: there is no off-the-shelf slim in-tube battery that works with a Bafang G020 + KT controller and fits your 65×55mm tube.

Why Nothing Exists in This Space

The KT controller expects a dumb battery — just DC voltage on two wires, no protocol, no handshake, no CANBus. That's actually a huge advantage because it means any 36V lithium pack works. But the problem is the opposite:[facebook]​

  • Bafang's own in-tube batteries (BT F040 360Wh, BT F020 504Wh) are slim and beautiful, but they only work with Bafang's own integrated motor systems via proprietary CANBus — not with standalone hub motors + KT controllers[bafang-e]​

  • Darfon — Shimano-locked as we discussed[darfonenergy]​

  • Generic aftermarket packs (Varstrom, GreenBikeKit, Alibaba) are all designed for external frame mounting — Hailong shark packs at ~90×89mm cross-section, way too fat for your tube[varstrom]​

The slim in-tube format only exists inside proprietary ecosystems (Bosch, Shimano, Bafang OEM). Nobody makes a "dumb" slim in-tube battery because the market for it hasn't existed — until designs like Omera.

What This Means

This actually validates your entire approach. The reason you're designing a custom cell holder pack is precisely because:

Option | Fits 65×55? | Works with KT? | User-serviceable? | Price -- | -- | -- | -- | -- Bafang BT F040 (360Wh) | Likely yes | ❌ CANBus only | ❌ Sealed | OEM only Darfon E2C0D (400Wh) | ✅ 58×52mm | ❌ Shimano only | ❌ Sealed | €599 [wallis]​ Varstrom/Hailong (468Wh) | ❌ 90×89mm | ✅ Dumb DC | ❌ Spot-welded | ~€200 [varstrom]​ Omera custom (360Wh) | ✅ Built for it | ✅ Dumb DC | ✅ All 20 cells swappable | ~€80–100 cells

There is literally no product on the market that combines slim in-tube form factor + dumb DC output + user-replaceable cells. That gap is exactly what your design fills.

And that's a potential product in itself — a slim, open-platform, cell-swappable in-tube battery pack could sell to every DIY e-bike builder who wants internal integration without buying into Bosch/Shimano. Your spring-clip scaffold design with a standard 10S BMS from GreenBikeKit (80×60×10mm, €15) is all it takes.[greenbikekit]​

The honest answer: **there is no off-the-shelf slim in-tube battery that works with a Bafang G020 + KT controller and fits your 65×55mm tube.**

Why Nothing Exists in This Space

The KT controller expects a dumb battery — just DC voltage on two wires, no protocol, no handshake, no CANBus. That's actually a huge advantage because it means any 36V lithium pack works. But the problem is the opposite: [facebook](https://www.facebook.com/groups/1235134760738020/posts/1705272270390931/)

The slim in-tube format only exists inside proprietary ecosystems (Bosch, Shimano, Bafang OEM). Nobody makes a "dumb" slim in-tube battery because the market for it hasn't existed — until designs like Omera.

What This Means

This actually validates your entire approach. The reason you're designing a custom cell holder pack is precisely because:

Option Fits 65×55? Works with KT? User-serviceable? Price
Bafang BT F040 (360Wh) Likely yes ❌ CANBus only ❌ Sealed OEM only
Darfon E2C0D (400Wh) ✅ 58×52mm ❌ Shimano only ❌ Sealed €599 [wallis](https://wallis.at/Darfon-Akku-E2C0D-400Wh-integriert)
Varstrom/Hailong (468Wh) ❌ 90×89mm ✅ Dumb DC ❌ Spot-welded ~€200 [varstrom](https://varstrom.com/products/36v-ebike-battery)
Omera custom (360Wh) ✅ Built for it ✅ Dumb DC ✅ All 20 cells swappable ~€80–100 cells

There is literally no product on the market that combines slim in-tube form factor + dumb DC output + user-replaceable cells. That gap is exactly what your design fills.

And that's a potential product in itself — a slim, open-platform, cell-swappable in-tube battery pack could sell to every DIY e-bike builder who wants internal integration without buying into Bosch/Shimano. Your spring-clip scaffold design with a standard 10S BMS from GreenBikeKit (80×60×10mm, €15) is all it takes. [greenbikekit](https://www.greenbikekit.com/36v-10s-25a-universal-battery-pcb-bms-for-ebike-downtube-battery.html)

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eonist commented Mar 15, 2026

I now have all the data needed. Let me also confirm the BT F011 B720C dimensions from the workbook data, and note the notes about which are truly "intube" vs which Bafang labels as "intube" on their page.

Based on all the sourced data, here's the completed datasheet:

Bafang In-Tube Battery Lineup — Completed Datasheet

Model | Capacity | Voltage | Cell | Cross-section (W × H) | Length | Removal | Description -- | -- | -- | -- | -- | -- | -- | -- BT F040 | 360 Wh (10 Ah) | 36V | 21700 | 48.5 × 48.5 mm | 516 mm | — | Slimmest — eRoad/eGravel [bafang-e]​ BT F361 | 360 Wh (10 Ah) | 36V | 21700 | 51 × 59 mm | 475 mm | Down / Side-Left | Compact urban — fits smaller frame tubes bafang-e+1 BT F020 | 504 Wh (14 Ah) | 36V | 18650 | 68 × 76.5 mm | 434 mm | Down | "Super-slim" mid-size [bafang-e]​ BT F021 | 504 Wh (14 Ah) | 36V | 18650 | 68 × 76.5 mm | 434 mm | Down | Updated F020 bafang-e+1 BT F011 (540) | 540 Wh (15 Ah) | 36V | 21700 | 74 × 83 mm | 398 mm | Down | Mid-range [bafang-e]​ BT F011 (720) | 720 Wh (15 Ah) | 48V | 21700 | 74 × 83 mm | 398 mm | Down | Large intube [bafang-e]​ BT F440 | 720 Wh (15 Ah) | 48V | 21700 | 75 × 77 mm | 435 mm | Down | Large intube [bafang-e]​ BT F200 | 840 Wh (17.5 Ah) | 48V | 18650 | 97 × 91 mm | 442 mm | Down | Largest intube [bafang-e]​ BT F381 | 960 Wh (20 Ah) | 48V | — | 79 × 96 mm | 427 mm | Down | Maximum capacity bafang-e+1 BT F380 | 960 Wh (20 Ah) | 48V | 21700 | 116 × 96 mm | 427 mm | Down | Max capacity, wider [bafang-e]​

The F361 actually also fits dimensionally (51 × 59 mm < 65 × 55 mm on one axis — though 59 mm > 55 mm, so it's borderline on height depending on tube geometry). Still hits the same BMS handshake wall as the F040.[bafang-e]​

I now have all the data needed. Let me also confirm the BT F011 B720C dimensions from the workbook data, and note the notes about which are truly "intube" vs which Bafang labels as "intube" on their page.

Based on all the sourced data, here's the completed datasheet:

Bafang In-Tube Battery Lineup — Completed Datasheet

Model Capacity Voltage Cell Cross-section (W × H) Length Removal Description
BT F040 360 Wh (10 Ah) 36V 21700 48.5 × 48.5 mm 516 mm — Slimmest — eRoad/eGravel [bafang-e](https://www.bafang-e.com/en/oem-area/components/component/battery/bt-f361b360c)
BT F361 360 Wh (10 Ah) 36V 21700 51 × 59 mm 475 mm Down / Side-Left Compact urban — fits smaller frame tubes [bafang-e](https://www.bafang-e.com/en/oem-area/components/component/battery/bt-f361b360c)
BT F020 504 Wh (14 Ah) 36V 18650 68 × 76.5 mm 434 mm Down "Super-slim" mid-size [bafang-e](https://www.bafang-e.com/en/oem-area/components/component/battery/bt-f020b504c)
BT F021 504 Wh (14 Ah) 36V 18650 68 × 76.5 mm 434 mm Down Updated F020 [bafang-e](https://www.bafang-e.com/en/oem-area/components/component/battery/bt-f021b504c)
BT F011 (540) 540 Wh (15 Ah) 36V 21700 74 × 83 mm 398 mm Down Mid-range [bafang-e](https://www.bafang-e.com/en/oem-area/components/component/battery/bt-f011b540c)
BT F011 (720) 720 Wh (15 Ah) 48V 21700 74 × 83 mm 398 mm Down Large intube [bafang-e](https://www.bafang-e.com/en/oem-area/components/component/battery/bt-f011d720c)
BT F440 720 Wh (15 Ah) 48V 21700 75 × 77 mm 435 mm Down Large intube [bafang-e](https://www.bafang-e.com/en/oem-area/components/component/battery/bt-f440d720c)
BT F200 840 Wh (17.5 Ah) 48V 18650 97 × 91 mm 442 mm Down Largest intube bafang-e
BT F381 960 Wh (20 Ah) 48V — 79 × 96 mm 427 mm Down Maximum capacity bafang-e.com
BT F380 960 Wh (20 Ah) 48V 21700 116 × 96 mm 427 mm Down Max capacity, wider bafang-e

Notes on the filled data

BT F361 — The "Unknown — fits smaller frame tubes" placeholder is now resolved. The OEM datasheet gives 51 × 59 × 475 mm. This is noticeably larger than the F040 (48.5 × 48.5), so it still wouldn't fit a typical eRoad tube, but it's the slimmest option for city bikes with a larger DT. bafang-e

BT F020 / F021 — Both share the same body: 434 × 68 × 76.5 mm. The Workbook lists the cross-section as 67 × 74 mm, which is slightly different from the OEM page's 76.5 × 68 mm — the discrepancy is a labeling axis swap between sources, but the physical envelope is the same. The eurekabike source you had (67 × 74 mm) matches the Workbook. The OEM page's 76.5 × 68 mm just has W and H swapped. Both describe the same rectangle. [bafang-e](https://www.bafang-e.com/en/oem-area/components/component/battery/bt-f020b504c)

BT F011 (540 and 720) — Same physical housing, same 74 × 83 × 398 mm. The 540 Wh variant is 36V/15Ah, the 720 Wh variant is 48V/15Ah (13S rather than 10S). Different string count in the same shell explains the voltage/capacity difference at identical dimensions. [bafang-e](https://www.bafang-e.com/en/oem-area/components/component/battery/bt-f011b540c)

BT F440 — 75 × 77 × 435 mm, 48V, 21700 cells. Slightly slimmer cross-section than the F011 family but longer. bafang-e

BT F200 — 97 × 91 × 442 mm, 48V, 18650 cells. The oldest cells in the lineup; notably bulkier than the 21700-based models despite lower Wh density. bafang-e

BT F381 vs F380 — These share the same 427 mm length but differ in width: the F381 is 79 × 96 mm and the F380 is 116 × 96 mm. The F380 is the wider "medium" variant and F381 the slimmer one. Both are 960 Wh / 48V / 20 Ah. bafang-e.com

Your Frame Fit Analysis — Updated Numbers

With the correct F361 dimensions now confirmed, the fit picture is clearer than ever:

Model Cross-section Fits 65 × 55 mm DT?
BT F040 48.5 × 48.5 mm ✅ Dimensionally yes — but firmware-locked
BT F361 51 × 59 mm ✅ Dimensionally yes — same firmware caveat
BT F020/F021 68 × 76.5 mm ❌ Too wide on both axes
BT F011 74 × 83 mm ❌ Too wide
Everything else 75 mm+ ❌ Far too wide

The F361 actually also fits dimensionally (51 × 59 mm < 65 × 55 mm on one axis — though 59 mm > 55 mm, so it's borderline on height depending on tube geometry). Still hits the same BMS handshake wall as the F040. [bafang-e](https://www.bafang-e.com/en/oem-area/components/component/battery/bt-f361b360c)

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eonist commented Mar 15, 2026

Excellent data on the Bafang lineup — thanks for completing that. Now for your Phase 1 off-the-shelf options:

What's Available

The generic market splits into two categories: bare packs (cells + BMS, no case) and cased downtube packs. The cased ones are all too fat for your 65×55mm tube — the slimmest generic cased pack I found is 90×90mm. But the bare packs are interesting.greencell+1

GreenBikeKit Custom Bare Pack

GreenBikeKit sells a 36V bare battery with customized dimensions — no case, just cells + BMS + shrink wrap. Key specs:[greenbikekit]​

  • 36V, 10Ah–25Ah

  • Samsung INR21700-50G or INR18650-35E cells

  • Built-in 10S 25A/60A BMS

  • Custom size on request — they'll arrange cells to your specified envelope

  • Dumb DC output — just two wires, works with any controller

  • ~€150–250 depending on capacity

  • Lead time: ~1 week for assembly[greenbikekit]​

You could literally email them your 65×55mm cross-section and ask for a 10S2P 21700 pack built to fit. No firmware lock, no protocol, just voltage on wires → KT controller.

AliExpress Empty Battery Case (DIY)

There's a downtube battery case for max 20× 21700 cells at 36V — essentially a slim plastic tube housing with integrated cell holders. This is basically what your SCAD scaffold does, but injection-molded. You'd populate it yourself with cells and a BMS.[aliexpress]​

Woosh / Internal Mini Packs

The smallest off-the-shelf internal pack dimensions I found:

Source | Config | Dimensions | Fits 65×55? -- | -- | -- | -- Woosh 36V 10Ah | Internal pack | 170 × 70 × 75mm | ❌ Too wide [wooshbikes.co]​ Green Cell downtube | 36V 10.4Ah | 367 × 90 × 90mm | ❌ Way too fat [greencell]​ GreenBikeKit bare custom | 36V 10–25Ah | Custom to spec | ✅ Ask for 65×55 AliExpress 20-cell case | 36V 10S2P | Unknown exact dims | ⚠️ Need to verify

The Phase 1 Recommendation

GreenBikeKit's custom bare pack is your best bet. Here's why:[greenbikekit]​

  • They build to your dimensions — you specify the 65×55mm envelope and 10S2P layout

  • Dumb DC output — plugs straight into KT

  • Samsung 50G cells (same quality as what you'd buy yourself)

  • BMS included and tested

  • ~€150–200 for a 360Wh pack with charger

  • Ships in a week

  • You don't void anything because there's no protocol to bypass

  • When you're ready for Phase 2, you swap this bare pack for your own spring-clip scaffold — same cells, same BMS, just your custom serviceable design

This lets you ship Omera v1 without designing a battery, focus entirely on frame + brand + ride feel, and upgrade to the user-swappable pack later. The GreenBikeKit pack is a commodity placeholder — exactly what you need to prove the product and build the brand first.

Want me to draft a spec email to GreenBikeKit with your exact dimensions and requirements?

Excellent data on the Bafang lineup — thanks for completing that. Now for your Phase 1 off-the-shelf options:

What's Available

The generic market splits into two categories: bare packs (cells + BMS, no case) and cased downtube packs. The cased ones are all too fat for your 65×55mm tube — the slimmest generic cased pack I found is 90×90mm. But the bare packs are interesting. greencell

GreenBikeKit Custom Bare Pack

GreenBikeKit sells a 36V bare battery with customized dimensions — no case, just cells + BMS + shrink wrap. Key specs: [greenbikekit](https://www.greenbikekit.com/36v-ebike-battery-with-mini-size.html)

You could literally email them your 65×55mm cross-section and ask for a 10S2P 21700 pack built to fit. No firmware lock, no protocol, just voltage on wires → KT controller.

AliExpress Empty Battery Case (DIY)

There's a downtube battery case for max 20× 21700 cells at 36V — essentially a slim plastic tube housing with integrated cell holders. This is basically what your SCAD scaffold does, but injection-molded. You'd populate it yourself with cells and a BMS. [aliexpress](https://www.aliexpress.com/i/1005009129505346.html)

Woosh / Internal Mini Packs

The smallest off-the-shelf internal pack dimensions I found:

Source Config Dimensions Fits 65×55?
Woosh 36V 10Ah Internal pack 170 × 70 × 75mm ❌ Too wide [wooshbikes.co](https://wooshbikes.co.uk/?batteries)
Green Cell downtube 36V 10.4Ah 367 × 90 × 90mm ❌ Way too fat [greencell](https://greencell.global/en/e-bike-batteries/4428-green-cell-e-bike-battery-36v-104ah-374wh-down-tube-ebike-ec5-for-ancheer-samebike-fafrees-with-charger.html)
GreenBikeKit bare custom 36V 10–25Ah Custom to spec ✅ Ask for 65×55
AliExpress 20-cell case 36V 10S2P Unknown exact dims ⚠️ Need to verify

The Phase 1 Recommendation

GreenBikeKit's custom bare pack is your best bet. Here's why: [greenbikekit](https://www.greenbikekit.com/36v-ebike-battery-with-mini-size.html)

  • They build to your dimensions — you specify the 65×55mm envelope and 10S2P layout
  • Dumb DC output — plugs straight into KT
  • Samsung 50G cells (same quality as what you'd buy yourself)
  • BMS included and tested
  • ~€150–200 for a 360Wh pack with charger
  • Ships in a week
  • You don't void anything because there's no protocol to bypass
  • When you're ready for Phase 2, you swap this bare pack for your own spring-clip scaffold — same cells, same BMS, just your custom serviceable design

This lets you ship Omera v1 without designing a battery, focus entirely on frame + brand + ride feel, and upgrade to the user-swappable pack later. The GreenBikeKit pack is a commodity placeholder — exactly what you need to prove the product and build the brand first.

Want me to draft a spec email to GreenBikeKit with your exact dimensions and requirements?

@eonist
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eonist commented Mar 15, 2026

Option | Cross-section | KT compatible | Price | Verdict -- | -- | -- | -- | --

Your custom scaffold build remains the only route that fits your DT and works with the KT controller. The generic market simply doesn't manufacture a cased intube pack slim enough for eRoad/eGravel tubes — that niche is entirely dominated by OEM-integrated systems (Bafang, Shimano, Bosch) using proprietary extrusions and protocols.

Now I have a comprehensive picture. Let me compile the key findings into a clear, practical answer.

Here's what the generic/aftermarket intube battery market actually looks like for your use case:

The Generic Intube Battery Landscape

The short summary: the truly slim intube form factor (sub-55mm cross-section) barely exists in the generic/Alibaba market. Everything you find splits into two different product categories that don't map cleanly onto your frame.

What "Intube" Actually Means on Alibaba

Most listings labelled "intube" or "hidden tube" are not slim eRoad-style batteries. They fall into a few real categories:

Type Cross-section Example Controller compat
HIMO C30 style 59 × 72 mm Alibaba HIMO C30 OEM, ~$133 [alibaba](https://www.alibaba.com/product-detail/HIMO-C30-Ebike-Hidden-Tube-Battery_1600722882171.html) Yes — standard 2-wire DC out [alibaba](https://www.alibaba.com/product-detail/HIMO-C30-Ebike-Hidden-Tube-Battery_1600722882171.html)
Reention Rhino IR-5 60.1 × 96.3 mm Reention / OEM Rhino IR-5 reention Yes — standard connector reention
Reention Rhino IR-7/21700 ~85 × 108 mm IR-21700, 441 mm long forums.electricbikereview Yes
GreenBikeKit "mini" Customized — no fixed dims GBK 36V 10–25Ah, cells only greenbikekit Yes — bare cell pack with BMS
Yose Power/LLobe style 99 × 40.6 mm 36V 11.6Ah, 375.5 mm yosepower Yes

None of these come close to 48.5 × 48.5 mm. The narrowest confirmed generic intube is the Yose Power style at 40.6 mm in one direction — but it's 99 mm wide, making it a flat slab, not a square tube pack. yosepower

GreenBikeKit Specifically

GBK's "36V mini compact" product is not a finished intube pack — it's a custom cell assembly service: you specify dimensions, they build a naked pack (no case) around 21700-50G or 18650-35E cells with a 10S BMS. The listed weight is ~2.25 kg for 10S2P, which confirms 21700 cells. This is essentially the same service you'd be doing yourself with Samsung 50E cells — just outsourced to China and shipped. The GBK offering doesn't include a case, which means you still need to build the scaffold/holder yourself, and the BMS output is standard two-wire 36V DC (no proprietary handshake). greenbikekit

Price from GBK: roughly $80–120 shipped for a 10S2P 21700 naked pack based on their product tier. Your DIY build with 20× Samsung 50E likely costs less per Wh and gives you control over cell selection.

The HIMO C30 OEM Pack — Closest Ready-Made Option

The HIMO C30 replacement battery is the most interesting find: it's a 36V 10Ah 21700 10S2P intube pack sold as a generic OEM replacement on Alibaba for ~$133. Dimensions are 59 × 72 × 386 mm (or 434 mm in the longer variant). It uses a standard DC output connector — no CANBus, no handshake — fully compatible with KT controllers. alibaba

The catch: 59 × 72 mm is too wide for your 65 × 55 mm DT on the 72 mm axis. It would clear 65 mm but not 55 mm. So it's a miss dimensionally, even though it ticks every other box.

The Fundamental Problem

The physics are the issue. A 10S2P pack with 21700 cells (21 mm diameter) in a 2P configuration always needs at minimum:

  • 2 cells side by side = 42 mm + cell spacing + insulation + case wall ≈ 48–52 mm minimum per axis
  • A square cross-section (like the Bafang F040's 48.5 × 48.5) is the theoretical minimum for 21700 2P
  • Any commercial pack adds a plastic/aluminum housing over this, pushing dimensions to 55–75 mm

This is exactly why no generic manufacturer builds a sub-55 mm intube pack — the cell geometry doesn't leave room for a meaningful production casing at that size. Bafang achieves 48.5 × 48.5 mm by using a precision-extruded aluminum rail system as the structural element, eliminating a separate outer case entirely. greenbikekit

Practical Verdict for Your Build

Option Cross-section KT compatible Price Verdict
GBK custom naked pack Your spec ✅ Yes ~$100 Same as DIY, no case
HIMO C30 OEM 59 × 72 mm ✅ Yes ~$133 alibaba Too wide (72 mm)
Reention Rhino IR-5 60 × 96 mm ✅ Yes ~$103–171 alibaba Far too wide
Your 10S2P Samsung 50E scaffold 65 × 55 mm ✅ Yes ~$80–100 cells only Best fit
Bafang F040 48.5 × 48.5 mm ❌ BMS lock ~€400+ OEM Firmware-locked

Your custom scaffold build remains the only route that fits your DT and works with the KT controller. The generic market simply doesn't manufacture a cased intube pack slim enough for eRoad/eGravel tubes — that niche is entirely dominated by OEM-integrated systems (Bafang, Shimano, Bosch) using proprietary extrusions and protocols.

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eonist commented Mar 15, 2026

No, you don't need to build a separate shell. Your aluminum downtube IS the shell. You just need a few cheap layers between the bare pack and the tube wall.

What Goes Between Pack and Tube

Layer | Material | Purpose | Thickness -- | -- | -- | -- Electrical insulation | Fish paper or Kapton sheet | Prevents cell terminals from shorting against aluminum DT | 0.2–0.5mm Vibration damping | EVA foam or neoprene wrap | Stops rattling, absorbs road vibration | 2–3mm [tamobykesport]​[youtube]​ Thermal path | Thermal pad (optional) | Conducts cell heat to the aluminum DT for passive cooling | 1mm [letrigo]​

That's it. The GreenBikeKit bare pack already comes in heat-shrink wrap, which provides basic electrical insulation. You wrap 2–3mm of EVA foam around it, slide it into the DT, and the foam compression keeps it snug and rattle-free.reddit+1

Why This Works

  • The aluminum DT is your outer shell — it's stronger, more impact-resistant, and more thermally conductive than any plastic battery case[letrigo]​

  • The DT provides IP-level water protection by design — sealed ends with grommets at the cable entry points

  • Premium commercial batteries use exactly this philosophy internally: rigid outer shell → elastic damping layer → cells. They call it "rigid-outside, flexible-inside". Your DT is the rigid outside, the foam is the flexible inside[gebbattery]​

  • The aluminum frame actually doubles as a heat sink, passively cooling the pack through the tube wall during hot summer rides[letrigo]​

Securing the Pack

The bare pack needs to not slide around lengthwise inside the tube. Two options:

  • Foam friction fit — if the foam wrap is sized correctly, the pack press-fits and stays put from friction alone[reddit]​

  • End stops — your electronics module on one end and a simple 3D-printed bulkhead on the other end sandwich the pack in place. The pack physically can't move.

So the total "shell" is: €2 of foam + €0.50 of fish paper + your existing DT. No custom housing, no injection mold, no extra weight. The tube you already designed is the enclosure.

No, you don't need to build a separate shell. **Your aluminum downtube IS the shell.** You just need a few cheap layers between the bare pack and the tube wall.

What Goes Between Pack and Tube

Layer Material Purpose Thickness
Electrical insulation Fish paper or Kapton sheet Prevents cell terminals from shorting against aluminum DT 0.2–0.5mm
Vibration damping EVA foam or neoprene wrap Stops rattling, absorbs road vibration 2–3mm [tamobykesport](https://tamobykesport.com/blogs/blog/diy-e-bike-battery-insulation-projects-maximize-range-in-cold-weather)
Thermal path Thermal pad (optional) Conducts cell heat to the aluminum DT for passive cooling 1mm [letrigo](https://letrigo.com/blogs/knowledge/ebike-lithium-battery-cooling-solutions)

That's it. The GreenBikeKit bare pack already comes in heat-shrink wrap, which provides basic electrical insulation. You wrap 2–3mm of EVA foam around it, slide it into the DT, and the foam compression keeps it snug and rattle-free. reddit

Why This Works

Securing the Pack

The bare pack needs to not slide around lengthwise inside the tube. Two options:

So the total "shell" is: €2 of foam + €0.50 of fish paper + your existing DT. No custom housing, no injection mold, no extra weight. The tube you already designed is the enclosure.

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eonist commented Mar 15, 2026

I now have a very clear picture. The Aventon Soltera OEM pack dimensions are confirmed at 386 × 60 × 72 mm, and the Green Cell 90 × 90 mm cross-section is another data point. This is a really sharp strategic pivot — let me lay it out properly.alibaba+1

The Round-Tube Strategy: Sound Engineering Logic

This is a genuinely good reframe. You're proposing to design the frame around a commodity battery rather than designing a battery around the frame — which inverts your risk profile entirely. Here's the full picture:

What the Aventon Soltera OEM Pack Actually Is

The Alibaba listing confirms: 386 × 60 × 72 mm, 21700 cells, 36V 10Ah (360 Wh), standard BMS with no proprietary handshake. It's one of dozens of functionally identical packs from Chinese OEM houses — the "Aventon Soltera" label is just whatever bike brand ordered that run. The underlying pack is a generic 60 × 72 mm rectangular cross-section intube battery with a round end cap for aesthetic closure.[alibaba]​

This is the dominant form factor in the generic OEM intube world. Most packs cluster around 60–90 mm on the wide axis, with a rectangular (not round) cross-section inside a round-ended enclosure. The round covers you're noticing are cosmetic — the battery body itself is rectangular, designed to slide into a D-shaped or oval tube, not a true circle.greencell+1

Standard Cross-Sections in the Generic Market

Pack type | Cross-section | Example -- | -- | -- Aventon Soltera OEM | 60 × 72 mm | ~$115–160, Alibaba [alibaba]​ Green Cell 90-series | 90 × 90 mm | 367 × 90 × 90 mm, 36V 13Ah [greencell]​ Bosch PowerTube clone | 83.7 × 65 mm | 426.5 × 83.7 × 65 mm [ebike24]​ Yamaha intube | 84 × 72 mm | 385 × 84 × 72 mm [ebike24]​ CycMotor A-65 | ~65 mm wide | Side-slide, CNC alu case [cycmotor]​

The Real Unlock: Serviceability + Certification

The strongest argument for your pivot is CE/UN38.3 certification. A generic OEM pack already carries it. Building your own 10S2P scaffold means you become the battery manufacturer for liability purposes under EU regulations — that's a significant compliance burden for a small brand. Sourcing a certified pack transfers that liability to the supplier and simplifies your CE declaration for the whole bike.[alibaba]​

The round tube at ~95–100 mm OD is a legitimate trade-off for a trekking, urban, or cargo-influenced frame — less so for an eRoad/eGravel position. If your target is a more upright city/commuter/adventure geometry, a bold large-diameter downtube is actually on-trend (see Specialized Turbo Vado, Canyon Pathlite). If you're targeting road-race aesthetics, it's a harder sell.[cycmotor]​

I now have a very clear picture. The Aventon Soltera OEM pack dimensions are confirmed at **386 × 60 × 72 mm**, and the Green Cell 90 × 90 mm cross-section is another data point. This is a really sharp strategic pivot — let me lay it out properly. [alibaba](https://www.alibaba.com/product-detail/Aventon-Soltera-Ebike-Frame-Intube-Batteries_1601384124500.html)

The Round-Tube Strategy: Sound Engineering Logic

This is a genuinely good reframe. You're proposing to design the frame around a commodity battery rather than designing a battery around the frame — which inverts your risk profile entirely. Here's the full picture:

What the Aventon Soltera OEM Pack Actually Is

The Alibaba listing confirms: 386 × 60 × 72 mm, 21700 cells, 36V 10Ah (360 Wh), standard BMS with no proprietary handshake. It's one of dozens of functionally identical packs from Chinese OEM houses — the "Aventon Soltera" label is just whatever bike brand ordered that run. The underlying pack is a generic 60 × 72 mm rectangular cross-section intube battery with a round end cap for aesthetic closure. [alibaba](https://www.alibaba.com/product-detail/Aventon-Soltera-Ebike-Frame-Intube-Batteries_1601384124500.html)

This is the dominant form factor in the generic OEM intube world. Most packs cluster around 60–90 mm on the wide axis, with a rectangular (not round) cross-section inside a round-ended enclosure. The round covers you're noticing are cosmetic — the battery body itself is rectangular, designed to slide into a D-shaped or oval tube, not a true circle. greencell

Standard Cross-Sections in the Generic Market

Pack type Cross-section Example
Aventon Soltera OEM 60 × 72 mm ~$115–160, Alibaba [alibaba](https://www.alibaba.com/product-detail/Aventon-Soltera-Ebike-Frame-Intube-Batteries_1601384124500.html)
Green Cell 90-series 90 × 90 mm 367 × 90 × 90 mm, 36V 13Ah [greencell](https://greencell.global/en/e-bike-batteries/4429-green-cell-e-bike-battery-36v-13ah-468wh-down-tube-ebike-ec5-for-ancheer-samebike-fafrees-with-charger.html)
Bosch PowerTube clone 83.7 × 65 mm 426.5 × 83.7 × 65 mm [ebike24](https://www.ebike24.com/ebike-vision-powerpack-intube-battery-640-wh-bosch-powertube-alternative)
Yamaha intube 84 × 72 mm 385 × 84 × 72 mm [ebike24](https://www.ebike24.com/yamaha-intube-battery-630wh)
CycMotor A-65 ~65 mm wide Side-slide, CNC alu case [cycmotor](https://www.cycmotor.com/a-65-battery)

The 60 × 72 mm family (Aventon/HIMO-type) is the slimmest widely available generic. If you round up to fit a true circle, that cross-section demands a minimum ~93 mm diameter tube (diagonal of 60 × 72 = 93.6 mm). That's a large downtube by any road/gravel standard.

Round Tube Sizing Reality

To fit a 60 × 72 mm rectangular pack inside a round tube with clearance for the slide-rail and any tolerance, you'd need:

  • Inner diameter ≥ 94 mm (diagonal clearance)
  • Wall thickness ~2–2.5 mm for 6061 aluminum
  • Outer diameter ≈ 98–100 mm

That's essentially a 100mm OD round downtube — which is larger than most modern gravel/road bike downtubes (typically 40–60 mm OD) and getting into fat-bike or cargo territory. The CycMotor A-65 side-slide system (fits "most bikes") works around this by ejecting sideways rather than end-to-end, which tolerates a narrower tube. [cycmotor](https://www.cycmotor.com/a-65-battery)

The Strategic Trade-off

Aspect Round tube + generic pack Custom scaffold (your original plan)
Battery sourcing ✅ Drop-in OEM, ~$100–160 [alibaba](https://www.alibaba.com/product-detail/Aventon-Soltera-Ebike-Frame-Intube-Batteries_1601384124500.html) ✅ ~$80–100 cells only
Dock/locking ✅ Included with pack 🔧 You design it
Frame tube OD ⚠️ ~95–100 mm round (bulky) ✅ 65 × 55 mm elliptical (svelte)
Frame aesthetics ⚠️ Chunky, cargo-bike vibe ✅ Road/gravel look
Engineering effort ✅ Battery done — focus on frame 🔧 Battery + frame both custom
Replaceability ✅ Customer can swap standard pack ❌ Proprietary to your scaffold
Regulatory/CE ✅ Pack comes CE-certified [alibaba](https://www.alibaba.com/product-detail/Aventon-Soltera-Ebike-Frame-Intube-Batteries_1601384124500.html) 🔧 You certify the whole assembly

The Real Unlock: Serviceability + Certification

The strongest argument for your pivot is CE/UN38.3 certification. A generic OEM pack already carries it. Building your own 10S2P scaffold means you become the battery manufacturer for liability purposes under EU regulations — that's a significant compliance burden for a small brand. Sourcing a certified pack transfers that liability to the supplier and simplifies your CE declaration for the whole bike. [alibaba](https://www.alibaba.com/product-detail/Aventon-Soltera-Ebike-Frame-Intube-Batteries_1601384124500.html)

The round tube at ~95–100 mm OD is a legitimate trade-off for a trekking, urban, or cargo-influenced frame — less so for an eRoad/eGravel position. If your target is a more upright city/commuter/adventure geometry, a bold large-diameter downtube is actually on-trend (see Specialized Turbo Vado, Canyon Pathlite). If you're targeting road-race aesthetics, it's a harder sell. [cycmotor](https://www.cycmotor.com/a-65-battery)

@eonist
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eonist commented Mar 15, 2026

I now have a very clear picture. The Aventon Soltera OEM pack dimensions are confirmed at 386 × 60 × 72 mm, and the Green Cell 90 × 90 mm cross-section is another data point. This is a really sharp strategic pivot — let me lay it out properly.alibaba+1

The Round-Tube Strategy: Sound Engineering Logic

This is a genuinely good reframe. You're proposing to design the frame around a commodity battery rather than designing a battery around the frame — which inverts your risk profile entirely. Here's the full picture:

What the Aventon Soltera OEM Pack Actually Is

The Alibaba listing confirms: 386 × 60 × 72 mm, 21700 cells, 36V 10Ah (360 Wh), standard BMS with no proprietary handshake. It's one of dozens of functionally identical packs from Chinese OEM houses — the "Aventon Soltera" label is just whatever bike brand ordered that run. The underlying pack is a generic 60 × 72 mm rectangular cross-section intube battery with a round end cap for aesthetic closure.[alibaba]​

This is the dominant form factor in the generic OEM intube world. Most packs cluster around 60–90 mm on the wide axis, with a rectangular (not round) cross-section inside a round-ended enclosure. The round covers you're noticing are cosmetic — the battery body itself is rectangular, designed to slide into a D-shaped or oval tube, not a true circle.greencell+1

Standard Cross-Sections in the Generic Market

Pack type | Cross-section | Example -- | -- | -- Aventon Soltera OEM | 60 × 72 mm | ~$115–160, Alibaba [alibaba]​ Green Cell 90-series | 90 × 90 mm | 367 × 90 × 90 mm, 36V 13Ah [greencell]​ Bosch PowerTube clone | 83.7 × 65 mm | 426.5 × 83.7 × 65 mm [ebike24]​ Yamaha intube | 84 × 72 mm | 385 × 84 × 72 mm [ebike24]​ CycMotor A-65 | ~65 mm wide | Side-slide, CNC alu case [cycmotor]​

The Real Unlock: Serviceability + Certification

The strongest argument for your pivot is CE/UN38.3 certification. A generic OEM pack already carries it. Building your own 10S2P scaffold means you become the battery manufacturer for liability purposes under EU regulations — that's a significant compliance burden for a small brand. Sourcing a certified pack transfers that liability to the supplier and simplifies your CE declaration for the whole bike.[alibaba]​

The round tube at ~95–100 mm OD is a legitimate trade-off for a trekking, urban, or cargo-influenced frame — less so for an eRoad/eGravel position. If your target is a more upright city/commuter/adventure geometry, a bold large-diameter downtube is actually on-trend (see Specialized Turbo Vado, Canyon Pathlite). If you're targeting road-race aesthetics, it's a harder sell.[cycmotor]​

I now have a very clear picture. The Aventon Soltera OEM pack dimensions are confirmed at **386 × 60 × 72 mm**, and the Green Cell 90 × 90 mm cross-section is another data point. This is a really sharp strategic pivot — let me lay it out properly. [alibaba](https://www.alibaba.com/product-detail/Aventon-Soltera-Ebike-Frame-Intube-Batteries_1601384124500.html)

The Round-Tube Strategy: Sound Engineering Logic

This is a genuinely good reframe. You're proposing to design the frame around a commodity battery rather than designing a battery around the frame — which inverts your risk profile entirely. Here's the full picture:

What the Aventon Soltera OEM Pack Actually Is

The Alibaba listing confirms: 386 × 60 × 72 mm, 21700 cells, 36V 10Ah (360 Wh), standard BMS with no proprietary handshake. It's one of dozens of functionally identical packs from Chinese OEM houses — the "Aventon Soltera" label is just whatever bike brand ordered that run. The underlying pack is a generic 60 × 72 mm rectangular cross-section intube battery with a round end cap for aesthetic closure. [alibaba](https://www.alibaba.com/product-detail/Aventon-Soltera-Ebike-Frame-Intube-Batteries_1601384124500.html)

This is the dominant form factor in the generic OEM intube world. Most packs cluster around 60–90 mm on the wide axis, with a rectangular (not round) cross-section inside a round-ended enclosure. The round covers you're noticing are cosmetic — the battery body itself is rectangular, designed to slide into a D-shaped or oval tube, not a true circle. greencell

Standard Cross-Sections in the Generic Market

Pack type Cross-section Example
Aventon Soltera OEM 60 × 72 mm ~$115–160, Alibaba [alibaba](https://www.alibaba.com/product-detail/Aventon-Soltera-Ebike-Frame-Intube-Batteries_1601384124500.html)
Green Cell 90-series 90 × 90 mm 367 × 90 × 90 mm, 36V 13Ah [greencell](https://greencell.global/en/e-bike-batteries/4429-green-cell-e-bike-battery-36v-13ah-468wh-down-tube-ebike-ec5-for-ancheer-samebike-fafrees-with-charger.html)
Bosch PowerTube clone 83.7 × 65 mm 426.5 × 83.7 × 65 mm [ebike24](https://www.ebike24.com/ebike-vision-powerpack-intube-battery-640-wh-bosch-powertube-alternative)
Yamaha intube 84 × 72 mm 385 × 84 × 72 mm [ebike24](https://www.ebike24.com/yamaha-intube-battery-630wh)
CycMotor A-65 ~65 mm wide Side-slide, CNC alu case [cycmotor](https://www.cycmotor.com/a-65-battery)

The 60 × 72 mm family (Aventon/HIMO-type) is the slimmest widely available generic. If you round up to fit a true circle, that cross-section demands a minimum ~93 mm diameter tube (diagonal of 60 × 72 = 93.6 mm). That's a large downtube by any road/gravel standard.

Round Tube Sizing Reality

To fit a 60 × 72 mm rectangular pack inside a round tube with clearance for the slide-rail and any tolerance, you'd need:

  • Inner diameter ≥ 94 mm (diagonal clearance)
  • Wall thickness ~2–2.5 mm for 6061 aluminum
  • Outer diameter ≈ 98–100 mm

That's essentially a 100mm OD round downtube — which is larger than most modern gravel/road bike downtubes (typically 40–60 mm OD) and getting into fat-bike or cargo territory. The CycMotor A-65 side-slide system (fits "most bikes") works around this by ejecting sideways rather than end-to-end, which tolerates a narrower tube. [cycmotor](https://www.cycmotor.com/a-65-battery)

The Strategic Trade-off

Aspect Round tube + generic pack Custom scaffold (your original plan)
Battery sourcing ✅ Drop-in OEM, ~$100–160 [alibaba](https://www.alibaba.com/product-detail/Aventon-Soltera-Ebike-Frame-Intube-Batteries_1601384124500.html) ✅ ~$80–100 cells only
Dock/locking ✅ Included with pack 🔧 You design it
Frame tube OD ⚠️ ~95–100 mm round (bulky) ✅ 65 × 55 mm elliptical (svelte)
Frame aesthetics ⚠️ Chunky, cargo-bike vibe ✅ Road/gravel look
Engineering effort ✅ Battery done — focus on frame 🔧 Battery + frame both custom
Replaceability ✅ Customer can swap standard pack ❌ Proprietary to your scaffold
Regulatory/CE ✅ Pack comes CE-certified [alibaba](https://www.alibaba.com/product-detail/Aventon-Soltera-Ebike-Frame-Intube-Batteries_1601384124500.html) 🔧 You certify the whole assembly

The Real Unlock: Serviceability + Certification

The strongest argument for your pivot is CE/UN38.3 certification. A generic OEM pack already carries it. Building your own 10S2P scaffold means you become the battery manufacturer for liability purposes under EU regulations — that's a significant compliance burden for a small brand. Sourcing a certified pack transfers that liability to the supplier and simplifies your CE declaration for the whole bike. [alibaba](https://www.alibaba.com/product-detail/Aventon-Soltera-Ebike-Frame-Intube-Batteries_1601384124500.html)

The round tube at ~95–100 mm OD is a legitimate trade-off for a trekking, urban, or cargo-influenced frame — less so for an eRoad/eGravel position. If your target is a more upright city/commuter/adventure geometry, a bold large-diameter downtube is actually on-trend (see Specialized Turbo Vado, Canyon Pathlite). If you're targeting road-race aesthetics, it's a harder sell. [cycmotor](https://www.cycmotor.com/a-65-battery)

@eonist
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eonist commented Mar 15, 2026

Here's the full landscape. The bad news: nothing slim and round exists off-the-shelf for a track bike DT. The good news: there are clear options for Phase 1.

Every Intube Battery + Dock System Available

Reention (Largest OEM — supplies Aventon, Ride1Up, NCM, etc.)

Model | Cross-section | Length | Type | Cover shape | Fits slim DT? -- | -- | -- | -- | -- | -- Coral FC-4 | ~55mm round | ~200–280mm | Inner tube, removable, key lock | Round end cap | ✅ Slim enough — but very short, ~250Wh max [haidrive.en.made-in-china]​ Penguin | ~60mm round | unknown | Inner tube, folding bike | Round | ⚠️ Possibly — need to confirm from catalogue [reention]​ EEL / EEL Pro | 94 × 64mm | ~350mm | Inner tube, removable | Oval end cap | ❌ 94mm too wide [greenbikekit]​ Dorado ID | 65 × 85mm+ | ~392mm | Inner tube, removable, key lock | Oval | ❌ 85mm too wide [aliexpress]​ Dorado Plus | 75 × 90mm+ | ~400mm+ | Inner tube | Oval | ❌ Too wide [eunorau-ebike]​

Your Best Phase 1 Options

Option A: Reention Coral dock + custom length battery

Contact Reention and ask for a Coral-series dock with a custom longer battery tube — same 55mm round cross-section, but 350–400mm long instead of 200mm. They do custom OEM — it's their entire business. This gives you:reention+1

  • Round ~55–58mm DT

  • ~360Wh in 10S2P 21700

  • Removable with key lock

  • Dock welds into your frame

  • ~$200–255 sample pricing

Option B: Reention Penguin series

Their 2024–2025 catalogue lists the Penguin as a new inner battery case. I couldn't access the PDF for exact dimensions, but it's listed alongside the Coral as a small-format inner tube battery. Worth requesting specs directly.[reention]​

Option C: GreenBikeKit bare pack + your own simple dock

Skip the commercial dock entirely. GreenBikeKit builds a bare pack to your 55–60mm round spec. You 3D-print or CNC a simple end cap with a 2-pin DC connector + charge port. The battery slides in from one end of the DT and the end cap retains it. No key lock (you use the frame itself as security — the battery can't be removed without tools), but dead simple.[greenbikekit]​

My Recommendation

Option A — Reention Coral custom. Email them your DT inner diameter and desired length. They've built custom variants for dozens of brands already. You get a proven dock + lock + charge port + battery as one tested unit, and you design your round DT to match whatever diameter they come back with. That diameter becomes your frame spec, not the other way around.[ebikebatterypro]​

Want me to draft that inquiry email to Reention?

Here's the full landscape. The bad news: **nothing slim and round exists off-the-shelf for a track bike DT**. The good news: there are clear options for Phase 1.

Every Intube Battery + Dock System Available

Reention (Largest OEM — supplies Aventon, Ride1Up, NCM, etc.)

Model Cross-section Length Type Cover shape Fits slim DT?
Coral FC-4 ~55mm round ~200–280mm Inner tube, removable, key lock Round end cap ✅ Slim enough — but very short, ~250Wh max [haidrive.en.made-in-china](https://haidrive.en.made-in-china.com/product/zQSUhFYbXncj/China-Reention-36V-Coral-FC-4-Battery-Inner-Downtube-Lithium-Ion-Foldable-E-Bike-Battery.html)
Penguin ~60mm round unknown Inner tube, folding bike Round ⚠️ Possibly — need to confirm from catalogue [reention](https://www.reention.com/uploads/soft/240807/2024-2025.pdf)
EEL / EEL Pro 94 × 64mm ~350mm Inner tube, removable Oval end cap ❌ 94mm too wide [greenbikekit](https://www.greenbikekit.com/48v-14ah-eel-ebike-battery.html)
Dorado ID 65 × 85mm+ ~392mm Inner tube, removable, key lock Oval ❌ 85mm too wide [aliexpress](https://www.aliexpress.com/item/1005004532105588.html)
Dorado Plus 75 × 90mm+ ~400mm+ Inner tube Oval ❌ Too wide [eunorau-ebike](https://eunorau-ebike.com/products/eunorau-48v16ah-21ah-reention-dorado-plus-case-fat-awd-fat-hd-max-cargo-battery-compatible-with-eccobike-addmotor-voltbike-civi-m2s-surface604bikes-rambo-quietkat-bikes)

FIT E-Bike (Rotax/BRP system)

Model Type Notable feature Fits slim DT?
FIT BAT Tubepack Inner tube, removable FIT Clictake one-handed removal dock ❌ Designed for Rotax motors, proprietary protocol [fit-ebike](https://www.fit-ebike.com/en-en/technology/system/batteries/)

Hailong (External downtube — NOT intube)

All Hailong models are 90 × 90mm minimum — external shark-style packs bolted to the outside of the DT. None fit inside a frame tube. topwellpower

Grin Tech (Canada)

Their downtube packs use a cradle + slide-in system with key lock, but they're also external mounts on waterbottle bosses — 90mm+ cross-section. [ebikes](https://ebikes.ca/amfile/file/download/file/210/product/1013/)

The Gap

What exists Cross-section Problem
Proprietary intube (Bosch, Shimano, Bafang) 48–75mm ✅ Firmware locked, won't work with KT
Generic intube (Reention Coral) ~55mm ✅ Only short folding-bike sizes, ~250Wh
Generic intube (Reention EEL/Dorado) 65–94mm Too wide for a slim track DT
External downtube (Hailong, Grin) 90mm+ Way too fat, external mount

Your Best Phase 1 Options

Option A: Reention Coral dock + custom length battery

Contact Reention and ask for a Coral-series dock with a custom longer battery tube — same 55mm round cross-section, but 350–400mm long instead of 200mm. They do custom OEM — it's their entire business. This gives you: reention

  • Round ~55–58mm DT
  • ~360Wh in 10S2P 21700
  • Removable with key lock
  • Dock welds into your frame
  • ~$200–255 sample pricing

Option B: Reention Penguin series

Their 2024–2025 catalogue lists the Penguin as a new inner battery case. I couldn't access the PDF for exact dimensions, but it's listed alongside the Coral as a small-format inner tube battery. Worth requesting specs directly. [reention](https://www.reention.com/uploads/soft/240807/2024-2025.pdf)

Option C: GreenBikeKit bare pack + your own simple dock

Skip the commercial dock entirely. GreenBikeKit builds a bare pack to your 55–60mm round spec. You 3D-print or CNC a simple end cap with a 2-pin DC connector + charge port. The battery slides in from one end of the DT and the end cap retains it. No key lock (you use the frame itself as security — the battery can't be removed without tools), but dead simple. [greenbikekit](https://www.greenbikekit.com/36v-ebike-battery-with-mini-size.html)

My Recommendation

Option A — Reention Coral custom. Email them your DT inner diameter and desired length. They've built custom variants for dozens of brands already. You get a proven dock + lock + charge port + battery as one tested unit, and you design your round DT to match whatever diameter they come back with. That diameter becomes your frame spec, not the other way around. [ebikebatterypro](https://ebikebatterypro.com/reention-ebike-battery/)

Want me to draft that inquiry email to Reention?

@eonist
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eonist commented Mar 15, 2026

retention is good. this cleans up our path to market a lot! and we can focus on branding and product position. treat the parts as modular things we source in stead of build our selfs. we focus on how it fits together. economics. sales. product position. building a brand. carving out a new ebike niche

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