SwapCell

Area: Mobility and Logistics · TRL: 3 of 9 (proof of concept on paper) · Prototype budget: about $700 USD · Difficulty: 4 of 5

Open-standard swappable battery pack and wall dock for e-bikes, scooters and cargo trikes, with a defined connector, CAN-based BMS protocol and state-of-health logging.

Exploded render · Detail render · Interactive 3D model · Concept blueprint (PDF) · Review note

Concept rationale

The cells inside almost every e-bike, scooter and cargo-trike battery are the same commodity 18650 or 21700 cells; what locks a pack to one brand is its housing, connector and data protocol. SwapCell therefore puts the effort into a published interface (envelope, pinout, coded interlock and CAN message set) rather than into a better cell. Once the interface is open, any fleet, workshop or small builder can make a pack or a receiver that works with everyone else's, and the pack's state-of-health log travels with it instead of living in a brand's cloud.

The reference build uses parts a garage workshop can source and assemble: commodity cells, an open-source BMS, a folded aluminium tray, a printed lid and a certified off-the-shelf charger, so no custom mains electronics are needed. Keeping it open and buildable is what lets repair shops read and fix packs, and lets other open designs in this portfolio design around one battery instead of each inventing their own.

Burning platform

Light electric vehicles are already the most electrified part of road transport: the IEA reports that about 8 % of the world's two- and three-wheelers were electric in 2023 and that electric models took 13 % of sales that year (IEA, Global EV Outlook 2024). In India, two-wheelers account for 70 to 80 % of all private vehicles (NITI Aayog, draft Battery Swapping Policy, 2022), so how their batteries are charged, swapped and retired matters at national scale.

Closed, mismatched batteries are also a safety problem. In New York City, lithium-ion batteries caused 268 fires in 2023, killing 18 people and injuring 150 (City of New York, 2024). A documented interface with a protected, dead-until-seated output and readable health data is one way to make safe, shared packs the easy choice.

Where it could be used

By industry

Industry	Use
Last-mile delivery	Riders swap a flat pack for a charged one at a depot dock in seconds instead of waiting hours to charge
Bike and scooter sharing	One pack and dock family across mixed vehicle types, with state-of-health data to retire packs on evidence
Municipal, campus and industrial fleets	Shared packs for maintenance e-bikes, cargo trikes and site vehicles
Independent repair and refurbishment	Read a pack's log, find a weak cell group, repair it and return it to service
Agriculture and rural transport	Cargo trikes and e-bikes charged from a shared village dock or a small solar system
Off-grid energy	Retired or spare packs reused as storage in open power products such as PowerBox

By country or region

Country or region	Why it matters there
India	About 880,000 electric two-wheelers and over 580,000 electric three-wheelers were sold in 2023 (IEA), and the government has proposed a battery swapping policy for these segments (NITI Aayog)
China	Nearly 6 million electric two-wheelers were sold in 2023, the largest market in the world (IEA)
Kenya	Newly registered motorcycles, many used as boda-boda taxis, were estimated at 1.5 million in 2018 and could pass 5 million by 2030 (UNEP)
Viet Nam and Southeast Asia	Electric two-wheeler sales in Viet Nam were about 250,000 in 2023, and the electric share across ASEAN was only about 3 % (IEA), so shared packs could lower the entry cost
United States (New York City)	268 lithium-ion battery fires and 18 deaths in 2023, and a city pilot of public battery charging for delivery workers (City of New York)

What sparked the idea

The starting point was India's draft Battery Swapping Policy, published by NITI Aayog in April 2022 (PDF). It asks for swappable batteries for two- and three-wheelers to be BMS-enabled, calls for standards for cables, connectors and repeated coupling tests, and favors open communication protocols so that batteries, vehicles and stations from different makers can work together. Those are the pieces of a common pack that no single brand has an incentive to publish. SwapCell takes that list literally and writes the interface down in the open: envelope, connector pinout with a coded interlock, CAN message set and health log, sized for e-bikes and cargo trikes.

Problem

Every light electric vehicle brand uses its own battery, so fleets cannot share packs or chargers and batteries are scrapped early.

Concept

Open-standard swappable battery pack and wall dock for e-bikes, scooters and cargo trikes, with a defined connector, CAN-based BMS protocol and state-of-health logging.

The core deliverable is the open SwapCell interface v0.3: envelope, blind-mate pinout with a coded interlock, CAN message set with a full bit layout, wake for hosts without CAN, a charge-while-discharging mode and a vehicle latch rating. Other portfolio designs build to it.

Full design precis: docs/02-concept.md · Sizing note: docs/04-calcs/01-sizing.md · General arrangement: cad/drawings/SWC-DWG-002.pdf

Key components

• 26 x 21700 cells in 13S2P: 46.8 V, 10 Ah, about 468 Wh, about 2.85 kg
• Open-source BMS with CAN
• Folded aluminium tray with a printed flame-retardant lid
• Blind-mate power and signal connector with a coded interlock
• Certified 5 A dock charger
• Latch pawl rated for vehicle class V1
• ESP32 dock controller

The priced bill of materials is in bom/bom.csv: about $559 for one pack and one dock, within the $700 budget.

Safety

Contains a 468 Wh lithium-ion battery pack at up to 54.6 V DC. Use a BMS with cell-level protection, fuse the pack, and charge on a non-combustible surface. Build and test packs only inside a fireproof enclosure, never unattended. This is a paper design at TRL 3; nothing here is certified.

Repository layout

Folder	Contents
docs/	Problem, concept, requirements, calculations and design decisions
cad/src/	build123d Python source, the source of truth for all geometry
cad/step/, cad/stl/	Exported models for FreeCAD, other CAD tools and printing
cad/drawings/	2D sketches and dimensioned drawings
bom/	Bill of materials
electronics/	KiCad schematics and PCB layouts
firmware/	Microcontroller code
media/	Renders, perspectives and photos
build-log/	Dated prototyping notes

Documentation

Controlled documents follow the portfolio documentation standard. Each carries a document ID (SWC-PRC-001 for the precis), a version and a revision history. Branded PDFs are built with python .kit/render.py and attached to GitHub Releases when a document is tagged, for example SWC-PRC-001/v1.0.

Credits

Designed by Amish Chadha. See CONTRIBUTORS.md for roles. To cite this design, use CITATION.cff (GitHub shows it as "Cite this repository").

AI assistance (Claude) was used to accelerate concept renders, prototype documentation and first-pass sizing calculations. Design direction and all decisions are Amish Chadha's, recorded in this repository's decision records (docs/decisions/).

Licenses

• Hardware (CAD, drawings, BOM, electronics): CERN-OHL-S v2
• Software (firmware, scripts, notebooks): MIT

A project of the Design Molecule lab.