downtube electronics

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

Author

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

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/)

Manufacturing speed — spot welding is faster on a production line than placing cells in holders [evlithium](https://www.evlithium.com/Blog/spot-welding-vs-soldering-lithium-batteries.html)
Vibration resistance — spot welds can't rattle loose, springs theoretically can (but proper BeCu holders with 8–14N insertion force handle this fine) [alibaba](https://www.alibaba.com/product-insights/how-to-choose-the-best-21700-battery-holder-a-complete-buying-guide.html)
It prevents user servicing — which is the actual goal [bike-eu](https://www.bike-eu.com/51466/eu-commission-dismisses-e-bike-battery-single-cell-replacement-in-amendment)

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

Author

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.

Author

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.

Author

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:

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/)

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](https://www.bafang-e.com/en/products/batteries/)
Darfon — Shimano-locked as we discussed [darfonenergy](https://www.darfonenergy.com/en/product/18071309102090)
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](https://varstrom.com/products/36v-ebike-battery)

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)

Author

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)

Author

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:

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)

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](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?

Author

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.

Author

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

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

The aluminum DT is your outer shell — it's stronger, more impact-resistant, and more thermally conductive than any plastic battery case [letrigo](https://letrigo.com/blogs/knowledge/ebike-lithium-battery-cooling-solutions)
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](https://www.gebbattery.com/news/waterproof-and-shockproof-design-in-modern-e-b-85280390.html)
The aluminum frame actually doubles as a heat sink, passively cooling the pack through the tube wall during hot summer rides [letrigo](https://letrigo.com/blogs/knowledge/ebike-lithium-battery-cooling-solutions)

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](https://www.reddit.com/r/ebikes/comments/16fhfs0/what_setup_do_i_need_for_an_aftermarket_second/)
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.

Author

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

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)

Author

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

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)

Author

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.)

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?

Author

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

Author

eonist commented Mar 15, 2026

Exactly. This is the shift from being an engineering project to being a product company. You don't build the motor (Bafang), you don't build the battery (Reention), you don't build the lights (LightSKIN), you don't build the controller (KT), you don't build the torque sensor (Bikone). You design how it all comes together — and you own the experience.

What Omera Actually Is Now

The Business Model

This is how every premium brand works:

Bang & Olufsen doesn't make their own DACs or speaker drivers — they source them. They design the enclosure, the interaction, the feeling.
Brompton doesn't make their own brakes, shifters, or tires. They design the fold, the ride, the lifestyle.
Cowboy/VanMoof sourced Bafang motors and generic cells. They sold the app, the look, the brand.[hypeandhyper]

The difference between a €400 AliExpress e-bike and a €2,500 Omera isn't the bill of materials. It's the curation, the integration, the identity, and the story.

Your Immediate Path

Contact Reention — get Coral dock specs + custom battery quote
Set DT diameter based on whatever Reention comes back with
Model the frame in SCAD — now with a simple round DT, which is actually easier
Source everything else — H1E, SFE-SC, G020, KT, Bikone, standard track components
Build one prototype — welded frame, all sourced parts bolted on
Brand it — name, logo, colorway, photography, website, social
Sell the first 10 — pre-orders, Instagram, Oslo cycling community

The engineering is now a bill of materials and an assembly spec, not a multi-year R&D project. The custom battery scaffold, the user-swappable cells, the advanced BMS — that's all Phase 2, when you have revenue, a brand, and customers asking for it.

You just went from building a bike to building a brand. That's the right move.

Exactly. This is the shift from being an **engineering project** to being a **product company**. You don't build the motor (Bafang), you don't build the battery (Reention), you don't build the lights (LightSKIN), you don't build the controller (KT), you don't build the torque sensor (Bikone). You **design how it all comes together** — and you own the experience.

What Omera Actually Is Now

You source	You design
Bafang G020 250W hub motor	Frame geometry + silhouette
Reention Coral intube battery + dock	DT diameter to match battery
KT controller	Electronics module layout in DT
Bikone BB torque sensor	Head tube cable routing
LightSKIN H1E handlebar	Plastic antenna top cap
LightSKIN SFE-SC seatpost	Paint, finish, graphics
Standard headset, fork, cranks, wheels	Brand identity + positioning

The Business Model

This is how every premium brand works:

Bang & Olufsen doesn't make their own DACs or speaker drivers — they source them. They design the enclosure, the interaction, the feeling.
Brompton doesn't make their own brakes, shifters, or tires. They design the fold, the ride, the lifestyle.
Cowboy/VanMoof sourced Bafang motors and generic cells. They sold the app, the look, the brand. [hypeandhyper](https://hypeandhyper.com/e-bike-in-scandinavian-style/)

The difference between a €400 AliExpress e-bike and a €2,500 Omera isn't the bill of materials. It's the curation, the integration, the identity, and the story.

Your Immediate Path

Contact Reention — get Coral dock specs + custom battery quote
Set DT diameter based on whatever Reention comes back with
Model the frame in SCAD — now with a simple round DT, which is actually easier
Source everything else — H1E, SFE-SC, G020, KT, Bikone, standard track components
Build one prototype — welded frame, all sourced parts bolted on
Brand it — name, logo, colorway, photography, website, social
Sell the first 10 — pre-orders, Instagram, Oslo cycling community

The engineering is now a bill of materials and an assembly spec, not a multi-year R&D project. The custom battery scaffold, the user-swappable cells, the advanced BMS — that's all Phase 2, when you have revenue, a brand, and customers asking for it.

You just went from building a bike to building a brand. That's the right move.

Author

eonist commented Mar 15, 2026

I now have all the data needed. Let me compile the complete filled-in table.

Here's your spec table, fully filled in with confirmed data from Reention's product pages and the SEGL Energy Penguin IPE listing:

Reention Inner Battery Cases — Complete Size List (Filled)

Model | Cross-section (W × H) | Length | Cells | Voltage | Capacity | Shape -- | -- | -- | -- | -- | -- | -- Coral FC-4 | ~55 × 55mm (round-ish) | ~200–280mm | 20× 21700 | 36V | ~10Ah / 360Wh | Round end cap [reention] Ruby (IRU-A) | 68 × 96mm | 436mm | 50–52× 18650 | 36V / 48V | 14–17.5Ah | Oval, top removal [reention] Flamingo | 75.3 × 77mm | 359.6mm | 40× 21700 (36V) / 39× 21700 (48V) | 36V / 48V | 20Ah/720Wh (36V) · 15Ah/720Wh (48V) | Oval, aluminum removable cover, hidden down-take lock[reention] Qiulong | 75.3 × 69.2mm | 433.3mm | 30× 21700 (36V) / 26× 21700 (48V) | 36V / 48V | 15Ah/540Wh (36V) · 10Ah/480Wh (48V) | Oval, detachable outer cover, second lock drawcord [reention] Penguin | 70.0 × 77.0mm | 415.4mm | 40× 18650 (36V) / 39× 18650 (48V) | 36V / 48V | 14Ah/504Wh (36V) · 10.5Ah/504Wh (48V) | Side removal, elegant/slim design[reention]

What Was Filled In

Flamingo — Direct from reention.com/en/gndch/Flamingo/151.html:[reention]

359.6 × 75.3 × 77mm, 21700 cells, 36V/48V, 10S4P / 13S3P
Matched tubes: ELK, FLAMINGO, GIRAFFE; IPX5; ABUS lock compatible
Note: This is a notably large inner-tube case — not slim in absolute terms, but the downtube uses a slim-profile hydroformed design with 5mm wall thickness

Qiulong — Direct from reention.com/en/gndch/Qiulong/248.html:[reention]

433.3 × 75.3 × 69.2mm, 21700 cells, 36V/48V, 10S3P
Extrusion-process downtube, second lock + drawcord retention, IPX6, up/down removal options

Penguin — Extracted directly from the Reention product page meta at reention.com/en/gndch/Penguin/177.html:[reention]

415.4 × 70.0 × 77.0mm, 18650 cells (not 21700), 36V/48V, 10S4P / 13S3P
Side-removal (unique in the lineup), IPX5, 6Pin discharge, split mounting base
Cross-confirmed via SEGL Energy's Penguin IPE listing at 415 × 62 × 77mm — the slight width difference (70 vs 62mm) likely reflects inner vs. outer housing measurement[seglenergy.com]

Key Observations for Your DT Design

These three models share very similar cross-sections — roughly 70–75mm wide × 69–77mm tall — which is why Reention pairs them with the same family of hydroformed oval downtubes. All of them are well outside your ~58–62mm inner diameter target, which confirms that the Coral FC-4 remains the only viable off-the-shelf starting point for a round downtube at that diameter.

The Penguin's side-removal mechanism is architecturally interesting for your use case: it doesn't require the battery to slide axially out through the bottom bracket end, which could allow a shorter DT stub design. Worth noting in your inquiry to Reention if you explore the Coral platform.

I now have all the data needed. Let me compile the complete filled-in table.

Here's your spec table, fully filled in with confirmed data from Reention's product pages and the SEGL Energy Penguin IPE listing:

Reention Inner Battery Cases — Complete Size List (Filled)

Model	Cross-section (W × H)	Length	Cells	Voltage	Capacity	Shape
Coral FC-4	~55 × 55mm (round-ish)	~200–280mm	20× 21700	36V	~10Ah / 360Wh	Round end cap [reention](https://www.reention.com/en/gndch/xiniuxilie/86.html)
Ruby (IRU-A)	68 × 96mm	436mm	50–52× 18650	36V / 48V	14–17.5Ah	Oval, top removal [reention](https://www.reention.com/en/gndch/xiniuxilie/86.html)
Flamingo	75.3 × 77mm	359.6mm	40× 21700 (36V) / 39× 21700 (48V)	36V / 48V	20Ah/720Wh (36V) · 15Ah/720Wh (48V)	Oval, aluminum removable cover, hidden down-take lock [reention](https://www.reention.com/en/gndch/Flamingo/151.html)
Qiulong	75.3 × 69.2mm	433.3mm	30× 21700 (36V) / 26× 21700 (48V)	36V / 48V	15Ah/540Wh (36V) · 10Ah/480Wh (48V)	Oval, detachable outer cover, second lock drawcord [reention](https://www.reention.com/en/gndch/Qiulong/248.html)
Penguin	70.0 × 77.0mm	415.4mm	40× 18650 (36V) / 39× 18650 (48V)	36V / 48V	14Ah/504Wh (36V) · 10.5Ah/504Wh (48V)	Side removal, elegant/slim design [reention](https://www.reention.com/uploads/soft/250415/1-250415093301.pdf)

What Was Filled In

Flamingo — Direct from [reention](https://www.reention.com/en/gndch/Flamingo/151.html).com/en/gndch/Flamingo/151.html: reention

359.6 × 75.3 × 77mm, 21700 cells, 36V/48V, 10S4P / 13S3P
Matched tubes: ELK, FLAMINGO, GIRAFFE; IPX5; ABUS lock compatible
Note: This is a notably large inner-tube case — not slim in absolute terms, but the downtube uses a slim-profile hydroformed design with 5mm wall thickness

Qiulong — Direct from [reention](https://www.reention.com/en/gndch/Qiulong/248.html).com/en/gndch/Qiulong/248.html: reention

433.3 × 75.3 × 69.2mm, 21700 cells, 36V/48V, 10S3P
Extrusion-process downtube, second lock + drawcord retention, IPX6, up/down removal options

Penguin — Extracted directly from the Reention product page meta at [reention](https://www.reention.com/uploads/soft/250415/1-250415093301.pdf).com/en/gndch/Penguin/177.html: reention

415.4 × 70.0 × 77.0mm, 18650 cells (not 21700), 36V/48V, 10S4P / 13S3P
Side-removal (unique in the lineup), IPX5, 6Pin discharge, split mounting base
Cross-confirmed via SEGL Energy's Penguin IPE listing at 415 × 62 × 77mm — the slight width difference (70 vs 62mm) likely reflects inner vs. outer housing measurement [seglenergy.com](https://www.seglenergy.com.tw/en/product_1591483.html)

Key Observations for Your DT Design

These three models share very similar cross-sections — roughly 70–75mm wide × 69–77mm tall — which is why Reention pairs them with the same family of hydroformed oval downtubes. All of them are well outside your ~58–62mm inner diameter target, which confirms that the Coral FC-4 remains the only viable off-the-shelf starting point for a round downtube at that diameter.

The Penguin's side-removal mechanism is architecturally interesting for your use case: it doesn't require the battery to slide axially out through the bottom bracket end, which could allow a shorter DT stub design. Worth noting in your inquiry to Reention if you explore the Coral platform.

Author

eonist commented Mar 15, 2026

That's fine — and the Coral is on your side here. The FC-4 has a round cross-section, not a rounded rectangle. It was designed to sit inside round folding bike tubes. So your DT will be a clean circle or, if you prefer a slightly wider but shorter profile, a gentle oval.[haidrive.en.made-in-china]

Shape Options Based on the Coral Dock

The round tube is the most track-authentic. Think Cinelli Vigorelli, Dolan Pre Cursa — they all use round or near-round DT profiles. It's the signature of a track frame. A rounded rectangle screams "commuter e-bike" — exactly the generic sedan energy you're avoiding.

One Thing to Confirm With Reention

When you email them, specifically ask:

"Is the Coral FC-4 dock system round/cylindrical? We need it to fit inside a round or oval downtube — not a rectangular frame profile."

Some of their larger models (EEL, Dorado) use flat-sided docks designed for rectangular tube cutouts. You want confirmation that the Coral dock is radially symmetric — meaning it works inside a round tube without needing flat walls. If it is, your round DT just slides right over it.

That's fine — and the Coral is on your side here. The FC-4 has a **round cross-section**, not a rounded rectangle. It was designed to sit inside round folding bike tubes. So your DT will be a clean circle or, if you prefer a slightly wider but shorter profile, a gentle oval. [[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)](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)

Shape Options Based on the Coral Dock

DT shape	Look	Works with round Coral?	Notes
Round (65mm)	Classic track bike	✅ Native fit — dock sits centered	Simplest to source and weld
Oval (68×62mm)	Slightly more aero / modern	✅ Dock still fits — extra clearance on the wide axis	Needs custom tube or hydroformed
Rounded rectangle	Generic e-bike look	❌ You don't want this	—

The round tube is the most track-authentic. Think Cinelli Vigorelli, Dolan Pre Cursa — they all use round or near-round DT profiles. It's the signature of a track frame. A rounded rectangle screams "commuter e-bike" — exactly the generic sedan energy you're avoiding.

One Thing to Confirm With Reention

When you email them, specifically ask:

"Is the Coral FC-4 dock system round/cylindrical? We need it to fit inside a round or oval downtube — not a rectangular frame profile."

Some of their larger models (EEL, Dorado) use flat-sided docks designed for rectangular tube cutouts. You want confirmation that the Coral dock is radially symmetric — meaning it works inside a round tube without needing flat walls. If it is, your round DT just slides right over it.

Author

eonist commented Mar 15, 2026

Not quite — the 55mm round and 280mm length are the outer dimensions of the battery module itself, not your downtube's minimum inner diameter. Here's how to think about it:

What 55mm Actually Means

The Coral FC-4 battery capsule is ~55mm in diameter, but it sits inside a dock/cradle that mounts to the tube wall. That cradle adds wall thickness, wiring clearance, and tolerance — typically 3–5mm per side in Reention's other systems. So your downtube inner diameter realistically needs to be ~60–65mm minimum to house the Coral system, which aligns perfectly with your 58–62mm inner diameter target (you'd be right at the tight end).

The 280mm Length Question

280mm is the short end of the Coral's range (~200–280mm confirmed) — it was designed for compact folding bike frames. That's quite short for a full-size urban bike DT. For context: [reention](https://www.reention.com/en/gndch/xiniuxilie/86.html)

A typical urban DT runs 350–500mm in usable length
Ruby is 436mm, Penguin is 415mm, Qiulong is 433mm — those are more representative of full-size bike battery lengths
At 280mm max, the Coral in stock form gives you roughly 10Ah / 360Wh, which is modest but usable for urban riding

What to Negotiate With Reention

This is exactly why the custom conversation matters. You'd be asking them to take the Coral platform (round cross-section, ~55mm capsule, dock+lock+charge port system) and extend it to 350–400mm length — which likely means going from 20× 21700 cells to ~25–28× cells, pushing you toward 12–14Ah / ~450–500Wh at 36V. That's a meaningfully better battery in the same tube geometry, and since they do custom MOQ from 1 sample, it's a real conversation to have.

So yes — Coral is your platform, but the length is the key variable to unlock with them.

Author

eonist commented Mar 15, 2026

Excellent research. With the full picture now clear, the conclusion is definitive:

The Coral FC-4 Is Your Only Option

Everything except the Coral is clustered around 70–77mm — designed for chunky hydroformed city/trekking bike DTs. The Coral sits in its own class at 55mm, purpose-built for small folding bike tubes.

What You're Working With

The Coral FC-4 gives you:

~55mm round battery → ~62mm DT inner diameter with dock → ~66mm OD round DT
200–280mm battery length → ~250–360Wh depending on what Reention can stretch it to
Round end cap — clean, no rectangular cutouts
Key lock + charge port + DC output — all integrated
Dumb DC output — works directly with your KT controller

The Two Questions for Reention

Can they extend the Coral battery length to 350–400mm? That gets you from ~250Wh to a proper ~360Wh (10S2P 21700 at full length). Same cross-section, just more cells in series along the tube.
Exact dock outer diameter? You need this number to set your DT inner diameter. The battery is ~55mm, the dock adds a few mm — you need the final number to lock your frame geometry.

Everything else — the Penguin's side-removal, the Flamingo's hidden lock — is interesting for future reference but too fat for your track bike DNA. The Coral is the one. Get those two numbers and your frame design can start in earnest.

Author

eonist commented Mar 15, 2026

Good constraint — and it actually works in your favour. Here's the breakdown:

Reention's Shape Options

Coral FC-4 — round/circular cross-section (~55mm). ✅ Your best fit [reention](https://www.reention.com/en/gndch/xiniuxilie/86.html)
Ruby (IRU-A) — oval (68 × 96mm). ✅ Oval is fine if your DT can be that wide
Flamingo — oval (75.3 × 77mm), nearly circular actually. ✅ But oversized for your target
Qiulong — oval (75.3 × 69.2mm). ✅ Same — oversized
Penguin — the 70 × 77mm profile with side-removal suggests a more rectangular housing. ⚠️ Worth confirming with Reention

What This Means for Your DT

A true oval downtube (like a track-style or aero road tube) is the natural upgrade path if you ever want more capacity than the Coral can give in a round tube. The Ruby at 68 × 96mm is already a well-established platform with lots of bike builders using it — and an oval DT at ~72–74mm wide × 100mm tall is a completely normal urban/cargo bike profile aesthetically.

But if you're committed to a round tube specifically, the Coral is your only Reention option, and the custom-length conversation is the one to have. A round DT also has a cleaner, more minimalist look for a premium urban bike — which sounds like your direction.

Author

eonist commented Mar 15, 2026

That makes sense — extending the Coral lengthwise while keeping the 55×55mm cross-section is a straightforward engineering change. The reasons:

Why Length Extension Is Plausible

The Coral's 10S2P 21700 layout means 10 cells in series along the tube length, 2 side by side. Each 21700 cell is ~70mm long. Adding cells in series just extends the tube linearly — no change to the cross-section at all.
The dock/cradle rails run the length of the tube — making them longer is trivial. Same extrusion, just cut longer.
The BMS, charge port, key lock, and connector all sit at one end — they don't care how long the battery tube behind them is.
Reention's entire business is building custom variants of their standard platforms for OEM bike brands. Changing length is the most common customization request.[ebikebatterypro]

What a Stretched Coral Gets You

The sweet spot is probably 350mm — that fits a full 10S2P pack of 20× 21700 cells at ~360Wh, same as the Bafang BT F040 but in a round format that works with your KT controller.

Did the other agent have specific dimensions or confirmation from Reention, or was it a feasibility assessment? Either way, this is looking like a solid path — one email to Reention and you have your Phase 1 battery locked.

That makes sense — extending the Coral **lengthwise** while keeping the 55×55mm cross-section is a straightforward engineering change. The reasons:

Why Length Extension Is Plausible

The Coral's 10S2P 21700 layout means 10 cells in series along the tube length, 2 side by side. Each 21700 cell is ~70mm long. Adding cells in series just extends the tube linearly — no change to the cross-section at all.
The dock/cradle rails run the length of the tube — making them longer is trivial. Same extrusion, just cut longer.
The BMS, charge port, key lock, and connector all sit at one end — they don't care how long the battery tube behind them is.
Reention's entire business is building custom variants of their standard platforms for OEM bike brands. Changing length is the most common customization request. [ebikebatterypro](https://ebikebatterypro.com/reention-ebike-battery/)

What a Stretched Coral Gets You

Coral variant	Length	Config	Capacity
Standard FC-4	~280mm	10S2P (short)	~250Wh
Stretched to ~350mm	~350mm	10S2P (full)	~360Wh
Stretched to ~400mm	~400mm	10S2P + extra parallel?	~360–500Wh

The sweet spot is probably 350mm — that fits a full 10S2P pack of 20× 21700 cells at ~360Wh, same as the Bafang BT F040 but in a round format that works with your KT controller.

Did the other agent have specific dimensions or confirmation from Reention, or was it a feasibility assessment? Either way, this is looking like a solid path — one email to Reention and you have your Phase 1 battery locked.

Author

eonist commented Mar 15, 2026

Reference | DT OD | Inner diameter (est. 2mm wall) | Battery fit? -- | -- | -- | -- Classic steel track bike | 28.6mm | ~25mm | ❌ Specialized Langster | ~44–50mm | ~40–46mm | ❌ Cannondale Track 3.0 | 50.8mm (2") | ~47mm | ❌ barely Your target | ~63–65mm OD | ~58–60mm | ✅ Coral fits

Your e-bike DT would be about 12–15mm larger in diameter than the iconic Cannondale Track tube — visually noticeable up close, but very much in the same visual family. Frame it as "oversized track-inspired" and it reads as intentional and premium rather than chunky. The extra girth is your battery — hidden in plain sight.

Author

eonist commented Mar 15, 2026

Ah — you're asking whether Reention could build a Coral-style battery at 60, 65, or 70mm cross-section instead of 55mm. Yes, almost certainly.

Why Bigger Is Easier

The Coral at 55mm is the hardest size to build — they're cramming 2× 21700 cells (42mm) into a 55mm shell with barely any room. If you ask for a larger diameter:

Battery OD | What changes | Config options -- | -- | --

A bigger cross-section at the same 350mm length is less constrained engineering for Reention, not more. They already solved 55mm — anything above that gives them more margin.

What This Means

You don't have to design your DT around the existing 55mm Coral and then add dock + clearance on top. You can flip it:

You decide the DT diameter that looks right on the frame — say 65mm round
Reention builds the battery + dock to fit inside that 65mm tube
The battery cross-section grows to maybe ~58–60mm, dock fills to ~63mm, 1mm clearance

You're the customer specifying the envelope. They fill it. That's how OEM works.

So yes — pick the tube diameter that gives you the right silhouette, and Reention can build a Coral-class battery to match, at 350mm length and 360Wh. You're not constrained to 55mm.

eonist/downtube electronics.md

USB Charging & Battery Protection: Already in the Design

USB-C Charging ✅

Battery Protection (BMS) ✅

Battery-to-Frame Connector ✅

Downtube Oval Size: ~50×65 Is Flexible

eonist commented Mar 15, 2026

The Real Numbers

Power Loss in Practice

Why the Industry Argument Is Overblown

Your "Replace All At Once" Policy

The Real Numbers

Power Loss in Practice

Why the Industry Argument Is Overblown

Your "Replace All At Once" Policy

Uh oh!

eonist commented Mar 15, 2026

In-Tube Battery Dimensions

The Darfon E2C0D Is Interesting

What This Means for Omera

In-Tube Battery Dimensions

The Darfon E2C0D Is Interesting

What This Means for Omera

Uh oh!

eonist commented Mar 15, 2026

In-Tube Battery Dimensions

The Darfon E2C0D Is Interesting

What This Means for Omera

In-Tube Battery Dimensions

The Darfon E2C0D Is Interesting

What This Means for Omera

Uh oh!

eonist commented Mar 15, 2026

Why Nothing Exists in This Space

What This Means

Why Nothing Exists in This Space

What This Means

Uh oh!

eonist commented Mar 15, 2026

Bafang In-Tube Battery Lineup — Completed Datasheet

Bafang In-Tube Battery Lineup — Completed Datasheet

Notes on the filled data

Your Frame Fit Analysis — Updated Numbers

Uh oh!

eonist commented Mar 15, 2026

What's Available

GreenBikeKit Custom Bare Pack

AliExpress Empty Battery Case (DIY)

Woosh / Internal Mini Packs

The Phase 1 Recommendation

What's Available

GreenBikeKit Custom Bare Pack

AliExpress Empty Battery Case (DIY)

Woosh / Internal Mini Packs

The Phase 1 Recommendation

Uh oh!

eonist commented Mar 15, 2026

The Generic Intube Battery Landscape

What "Intube" Actually Means on Alibaba

GreenBikeKit Specifically

The HIMO C30 OEM Pack — Closest Ready-Made Option

The Fundamental Problem

Practical Verdict for Your Build

Uh oh!

eonist commented Mar 15, 2026

What Goes Between Pack and Tube

Why This Works

Securing the Pack

What Goes Between Pack and Tube

Why This Works

Securing the Pack

Uh oh!

eonist commented Mar 15, 2026

The Round-Tube Strategy: Sound Engineering Logic

What the Aventon Soltera OEM Pack Actually Is

Standard Cross-Sections in the Generic Market

The Real Unlock: Serviceability + Certification

The Round-Tube Strategy: Sound Engineering Logic

What the Aventon Soltera OEM Pack Actually Is

Standard Cross-Sections in the Generic Market