If you are an ebike brand owner, product engineer, or OEM sourcing manager developing a new model, you will eventually face the same question from your battery supplier: “Can you send us your battery specification?”
Many buyers answer with a rough voltage number and a target range. That is how projects start late, samples fail fit-checks, and the final bill of materials drifts 15–20% above the quoted price. A professional custom ebike battery pack program starts with a written design specification — a single document that defines electrical performance, cell configuration, BMS behavior, mechanical envelope, connectors, validation tests, and certifications before any tooling is cut.
This article is a hands-on engineering guide to building that specification, written from the perspective of a China custom lithium ion battery pack manufacturer that develops packs for ebike brands, OEM factories, and wholesale distributors. For the commercial side of supplier evaluation and sourcing models, see our companion ebike lithium battery B2B buying guide; this piece focuses purely on the technical design decisions.
Have a frame drawing and motor spec already? Send them to our engineering team for a free design review →
What Is a Custom Ebike Battery Pack Design Specification?
A battery design specification — sometimes called a Battery Requirement Specification (BRS) or Design Input Sheet — is the technical contract between an ebike brand and its battery supplier. It converts marketing requirements (“750W motor, 80 km range, works in Nordic winters”) into measurable engineering parameters that a pack factory can design, quote, test, and certify.
A complete specification typically contains the following blocks:
- System-level inputs — motor voltage and power, controller current limits, frame battery compartment, charger plan.
- Electrical performance — nominal voltage, capacity (Ah), energy (Wh), maximum continuous and peak discharge current, target internal resistance.
- Cell definition — chemistry, form factor (18650 or 21700), grade, acceptable cell brands/models, cycle life requirement.
- BMS definition — current rating, protection thresholds, balancing specification, temperature sensors, communication protocol.
- Mechanical definition — housing type and dimensions, mounting and locking, IP rating, weight limit, connector models and pinouts.
- Environmental definition — operating and storage temperature ranges, vibration and shock expectations, cold-weather requirements.
- Validation and certification — design verification tests plus UN38.3, IEC 62133, CE, UL or market-specific approvals.
- Commercial anchors — annual volume, MOQ per model, sample and mass-production timing, warranty terms, labeling requirements.
The remaining sections walk through each block in the order engineers actually work, with reference tables you can adapt directly into your own document.
Step 1 — Lock the System-Level Requirements First
Battery design never starts at the cell. It starts at the bike. Before contacting any ebike battery manufacturer, collect the following inputs from your motor, controller, and frame teams:
- Motor type and rated/continuous power (250W, 500W, 750W, 1000W+).
- Controller operating voltage window and low-voltage cutoff (LVC) — the battery voltage window must sit inside it.
- Controller maximum current (A) — the BMS must allow more headroom than the controller draws, not less.
- Frame battery compartment 3D dimensions, or the housing series reference (Dorado, Hailong, Poly DP, Kirin, etc.).
- Mounting and locking interface, cable exit direction, and space for the charge port.
- Target market and sales channel — these decide certifications (EU, US, Japan, Korea) and whether UL 2849 is on the roadmap.
- Target range and rider profile — rider weight, typical terrain, and assist level used in range claims.
- Annual volume and model lifetime — volume decides whether custom tooling and a dedicated BMS firmware are economically justified.
A common and expensive mistake is finalizing the frame before the battery pack is engineered. Battery compartment dimensions, cable routing, and charge-port position are cheap to change on a CAD file and expensive to change on an injection-molded frame. If your frame design is not yet frozen, involve the battery supplier at the frame stage — this is standard practice in serious OEM/ODM programs and is part of our custom lithium ion battery pack service.
Step 2 — Select Chemistry and Nominal Voltage
Virtually all modern ebike packs use lithium-ion cylindrical cells in two chemistry families:
- NMC / NCA (nickel-based): high energy density, mature supply chain, nominal 3.6–3.7V per cell. This is the default choice for 24V–72V ebike packs.
- LFP (LiFePO₄): nominal 3.2V per cell, longer cycle life and better thermal stability, but lower energy density and heavier for the same range. LFP is mostly specified for rental fleets, cargo bikes, and safety-first programs rather than lightweight consumer frames.
Pack voltage is defined by the series count (S). The table below lists the standard configurations used by an electric bike battery manufacturer:
| Configuration | Nominal Voltage | Full-Charge Voltage | Typical Discharge Cutoff | Marketed As | Common Application |
|---|---|---|---|---|---|
| 7S NMC | 25.2–25.9V | 29.4V | ~19.6V | 24V | Folding bikes, older city models |
| 10S NMC | 36–37V | 42.0V | ~28.0V | 36V | EU pedelecs, 250–500W city bikes |
| 13S NMC | 46.8–48.1V | 54.6V | ~32.5–35V | 48V | 500–1000W mid/hub motors (mainstream) |
| 14S NMC | 50.4–51.8V | 58.8V | ~35–37.8V | 52V | 750–1500W performance/fat-tire bikes |
| 20S NMC | 72–74V | 84.0V | ~50–54V | 72V | Speed pedelecs, off-road/hunting bikes |
| 16S LFP | 51.2V | 58.4V | ~40V | 48V-class | Fleet/cargo bikes prioritizing cycle life |
Cutoff values shown are pack-level references based on 2.5–2.7V per NMC cell and 2.5V per LFP cell; actual BMS thresholds are set per project (see Step 5).
Two specification notes worth getting right:
- 52V is not 48V. A 14S pack on a 13S controller can over-voltage the system. Always specify both the S-count and nominal voltage on your drawings.
- Charger and BMS scale with S-count. Charge voltage must match the chemistry and series count exactly (42.0V, 54.6V, 58.8V, 84.0V for NMC). Confirming this on the PO eliminates a surprisingly common returns driver.
Browse our standard voltage range on the ebike lithium battery manufacture page.
Step 3 — Calculate Capacity, Energy and S/P Configuration
Capacity (Ah) decides range, but the pack must also physically fit the compartment — these two constraints are solved together by choosing cell format and series/parallel topology.
Key equations:
- Energy (Wh) = nominal voltage (V) × capacity (Ah)
- Parallel count P = pack capacity ÷ single-cell capacity (must be an integer)
- Total cells = S × P
- Estimated range (km, pedal assist) ≈ Wh ÷ 8–12 for typical mixed-terrain riding at moderate assist with a ~75 kg rider
The table below shows real configurations with 3500mAh 18650 cells and 5000mAh 21700 cells:
| Target Pack | Cells | Configuration | Actual Capacity | Energy | Est. Range (assist) |
|---|---|---|---|---|---|
| 24V 10.5Ah | 18650 3500mAh | 7S3P | 10.5Ah | ~265Wh | 25–35 km |
| 36V 14Ah | 18650 3500mAh | 10S4P | 14.0Ah | ~504Wh | 45–60 km |
| 36V 15Ah | 21700 5000mAh | 10S3P | 15.0Ah | ~540Wh | 55–70 km |
| 48V 17.5Ah | 18650 3500mAh | 13S5P | 17.5Ah | ~819Wh | 70–95 km |
| 48V 20Ah | 21700 5000mAh | 13S4P | 20.0Ah | ~936Wh | 80–110 km |
| 48V 21Ah | 21700 5000mAh+ | 13S4P | 21.0Ah | ~1008Wh | 90–120 km |
| 52V 17.5Ah | 18650 3500mAh | 14S5P | 17.5Ah | ~910Wh | 80–110 km |
Practical specification advice:
- Never accept a label-only capacity. Require the exact configuration (e.g., “13S5P, 18650 3500mAh grade-A cells”) plus the cell brand and model. This single line prevents most capacity-fraud issues in wholesale sourcing.
- Design around integer parallel counts. If your target capacity falls between configurations (e.g., 48V 16Ah), the design will round up to 17.5Ah or down to 15Ah — plan range claims accordingly.
- Range claims need conditions. Always state assist level, rider weight, terrain, and temperature. Winter range can be 15–30% lower even with healthy cells.
For consumer-facing guidance on matching capacity to riding style, our article on how to choose the right lithium battery pack for your electric bike covers the rider perspective; your BRS should keep the engineering view.
Step 4 — Choose Between 18650 and 21700 Cells
Cell format is the decision that most directly trades cost against housing space. Both formats are mature, cylindrical, and widely used in ebike packs — but they optimize differently.
| Parameter | 18650 | 21700 |
|---|---|---|
| Dimensions | 18mm × 65mm | 21mm × 70mm |
| Typical capacity (grade A) | 2600–3500mAh | 4000–5000mAh |
| Energy per cell | ~9.4–12.6Wh | ~14.4–18Wh |
| Typical continuous discharge | 2–3C (high-drain grades) | 2–3C (high-drain grades) |
| Cell-level energy density | ~240–260 Wh/kg | ~250–280 Wh/kg |
| Cells needed for same energy | Baseline | ~25–30% fewer |
| Pack assembly labor per Wh | Higher (more welds, more connections) | Lower |
| Relative cost per Wh at volume | Reference | Typically ~3–8% lower |
| Housing compatibility | Widest — small and legacy housings | Best in mid/large intube and downtube housings |
| Best fit | Cost-sensitive replacement packs, compact 24V/36V housings | Long-range 48V/52V integrated packs, new OEM models |
How to read the trade-off: 21700 packs put roughly 30–40% more energy in each cell, so for a given energy target you need fewer cells, fewer parallel strings, fewer weld points, and fewer cell-level failure opportunities — usually at a slightly lower cost per watt-hour at production volume. 18650 remains the better choice when a housing was tooled around 18mm cells, when the capacity is small, or when matching an existing aftermarket SKU for wholesale compatibility.
Cell acceptance criteria to write into the spec
Cell quality — not format — is the number-one cause of warranty returns. Specify:
- Grade-A, new, traceable cells from established manufacturers (e.g., Samsung SDI, LG Energy Solution, or reputable Chinese makers such as EVE, Lishen, BAK), with batch codes and datasheets.
- Matched grouping (binnings): capacity spread within ±1.5% and internal resistance spread within ±3 mΩ inside one pack.
- No recycled, dismantled, or “equivalent quality” cells without written approval.
- Cycle life requirement: typically ≥500 cycles to 80% capacity for NMC at 0.5C, 25°C, full DoD; LFP commonly ≥1500 cycles.
Two reference products illustrate the two formats: our Shanshan 21700 36V 15Ah ebike battery uses 10S3P construction for a compact, high-energy pack, while the Reention 24V 5.2Ah–10.5Ah series shows how 18650-based designs cover small-capacity folding-bike housings.
Step 5 — Specify the BMS: Architecture and Protection Thresholds
The Battery Management System is the second most frequent source of field failures after cell quality. A BMS specification should name not just its current rating but every protection threshold, because “BMS included” tells you nothing about how it behaves on a cold hill.
Typical protection parameters for an NMC ebike pack:
| Protection Parameter | Typical Setting (NMC) | Release / Recovery | Typical Delay |
|---|---|---|---|
| Cell over-voltage (OVP) | 4.25 ± 0.03V | 4.10 ± 0.05V | 0.5–1 s |
| Cell under-voltage (UVP) | 2.50–2.70V | 3.00–3.20V | 0.5–1 s |
| Discharge over-current (OCP) | 1.3–2× rated continuous | Remove load / reconnect | 0.5–1 s |
| Short-circuit protection (SCP) | Hardware-level | Load reconnect | <10–50 ms |
| Charge over-current | 1.2–1.5× max charge current | Reconnect charger | 1 s |
| Balance start condition | Cell ≥ 3.5–3.6V and Δ ≥ 20–30mV | Δ < 10mV | Continuous while charging |
| Passive balance current | 30–80mA | — | — |
| Low-temp charge cutoff | 0°C (±3°C) | ≥ +5°C | 1–3 s |
| High-temp charge cutoff | 45–50°C | ≤ 40°C | 2–5 s |
| High-temp discharge cutoff | 65–75°C | 50–55°C | 1–3 s |
| Temperature sensors | 2–3× NTC 10kΩ (cell group + power path) | — | — |
| Sleep / quiescent current | < 30–50 µA | — | — |
Current rating: size the BMS to the motor
The BMS must carry the controller’s maximum draw with 20–30% headroom. An under-rated BMS causes the classic field complaint “battery dies uphill” when it cuts out under load; an over-rated BMS wastes cost and space.
| Motor Power | Typical Working Current | Peak Draw | Recommended BMS |
|---|---|---|---|
| 250W | 8–12A | 15–20A | 15A cont / 30A peak |
| 500W | 12–18A | 25–35A | 20–25A cont / 40–50A peak |
| 750W | 18–25A | 35–50A | 25–30A cont / 50–60A peak |
| 1000–1500W | 25–40A | 50–80A | 40A cont / 80A peak |
| 1500W+ | 40–60A | 80–120A | 50–60A cont / 100–120A peak |
Communication and smart features
For premium or mid-motor programs, decide whether the BMS needs communication:
- UART / CAN bus to the motor controller or display (standard on Bosch/Shimano-class mid-drive systems); pinouts and protocol must be frozen before sample build.
- SMBus for charger/device communication in some enterprise and IOT-style applications.
- Bluetooth module + mobile app for SOC, cycle count, and per-cell voltage diagnostics.
- LED/LCD fuel gauge on the housing for aftermarket and rental use.
Wording for your specification: list mandatory protections, then list current rating, balance current, sensor count, protocol, and connector pinout as explicit line items. Any reputable supplier can build to this; vague “smart BMS” language is where misunderstandings live.
Step 6 — Mechanical, Thermal, Connector and Charger Design
Housing and mechanical interface
The housing defines which frames the pack fits and how it survives daily use. Specify:
- Housing series or custom enclosure, with 2D/3D drawings for private tooling.
- Mounting position: downtube (external), intube (integrated), rear rack, bottle/seat-tube.
- Locking mechanism and key profile, plus mechanical retention without the lock.
- Water and dust resistance: IPX5–IPX6 is typical for quality ebike housings.
- Vibration and impact expectations for fat-tire/off-road models.
- Weight target, including cells, BMS, and housing (a typical 48V 17.5Ah pack weighs roughly 4.3–5.0 kg).
- Labels and branding: model label, capacity/voltage marking, warning icons, your private-label artwork, and serial number scheme.
High-capacity intube packs are the growth segment for new OEM models; our Reention Dorado 48V 21Ah inner tube battery is representative of the form factor, and the Dorado family also covers 52V 17.5Ah variants.
Connectors, ports and charger
Wrong connectors turn a correct electrical design into a return. Name every connector, its position, and its pinout on the drawing:
| Item | Common Options | Notes |
|---|---|---|
| Discharge connector | XT60 (≤60A), XT90, Anderson PP45/PP75, Higo/Rosenberger waterproof round plugs | Match wire gauge to peak current |
| Charge port | DC 5.5×2.1/2.5mm, RCA, XLR 3-pin (rack packs), XT30 | Position for daily use and rain runoff |
| Signal connector | 5/6-pin JST or Higo waterproof | Confirm pinout with controller team |
| Charger voltage (NMC) | 29.4V / 42.0V / 54.6V / 58.8V / 84.0V | Must match S-count exactly |
| Charge current | 2A standard; 3–5A for large packs | 0.2C standard, up to 0.5C with thermal margin |
| Charge profile | CC–CV, taper cutoff ~0.02–0.05C | Fast charging above 0.5C accelerates fade |
Always confirm polarity and pinout against a wiring diagram — even “standard” connectors are wired differently across bike brands, and this is a frequent aftermarket complaint. Thermal design at pack level means defined cell spacing, copper/nickel busbar sizing for peak current, heat paths away from the BMS MOSFETs, and NTC placement on the hottest cell group rather than only on the board.
Step 7 — Validation and Reliability Test Plan
The specification must state how the design is proven before mass production, not just what it should do. A typical design verification plan for a custom ebike battery pack combines electrical, environmental, abuse, and housing tests:
Electrical and life tests
- [ ] Initial capacity and energy verification at 25°C (per IEC 62133-2 conditions).
- [ ] Internal resistance measurement; compare against design target.
- [ ] Charge retention / self-discharge after standard storage.
- [ ] Cycle life: ≥500 cycles to ≥80% capacity (0.5C, 25°C, defined DoD) for NMC.
- [ ] Continuous and peak discharge at maximum rated current with temperature logging.
- [ ] BMS function verification: every threshold in the Step 5 table triggered and recovered.
Environmental and mechanical tests
- [ ] Vibration and mechanical shock (per UN38.3 T3/T4 and IEC 62133-2).
- [ ] Temperature cycling (e.g., −20°C to +75°C) and low-pressure/altitude simulation.
- [ ] Free-fall / drop on housing (commonly 1.0–1.2m, packed condition).
- [ ] Ingress testing to the claimed IPX rating; salt-spray for coastal markets.
- [ ] Connector mate/unmate life and lock-cycle life.
Safety and abuse tests
- [ ] External short circuit, overcharge, and forced discharge behavior (UN38.3 / IEC 62133-2).
- [ ] Crush and thermal-abuse behavior at cell level, per the standards.
- [ ] ESD and EMC pre-scan where CE/FCC marking is planned.
Require a signed test report per model — not generic certificates for “similar” packs — and per-shipment outgoing QC including capacity verification, BMS function, insulation/hipot, and an aging (charge/discharge) cycle before packing.
Step 8 — Certification Matrix by Target Market
Compliance is part of the specification because test timelines (often 4–8 weeks for a full program) drive the product launch schedule. The main approvals an ebike brand should plan for:
| Certification / Standard | What It Covers | Region | When Required |
|---|---|---|---|
| UN38.3 | Transport safety: altitude, thermal, vibration, shock, short circuit, impact, overcharge, forced discharge | Global | Mandatory for any shipment by air/sea/land |
| MSDS | Material safety documentation for logistics | Global | Required by freight forwarders |
| IEC 62133-2 | Safety of sealed portable lithium cells/batteries | International | Widely referenced by buyers/retailers and national schemes |
| CE (EMC Directive 2014/30/EU; LVD where applicable) | Electromagnetic compatibility / electrical safety | EU | Market access in Europe |
| RoHS 2 / REACH | Hazardous substances | EU | Standard consumer-product requirement |
| EN 15194 | Electrically power-assisted cycles (battery safety aspects) | EU | Ebike type-approval context |
| UL 2271 | Batteries for light electric vehicles | US | Retailer/city requirements; safety programs |
| UL 2849 | Ebike electrical systems safety | US | Effectively required by major US retailers/channels |
| FCC Part 15 | RF emissions (for packs with wireless/bus electronics) | US | When communication modules are present |
| KC / PSE | National safety schemes | South Korea / Japan | Market-specific, plan at design stage |
Three practical notes: ask for original reports naming your exact model and configuration, since generic certificates do not survive a retailer audit; treat UL 2849 as a roadmap item even if not enforced in your first season, because channel policy changes faster than you can re-certify; and remember that changing cells or BMS after certification can invalidate reports, which is why the specification must be frozen before certification starts.
Cold-Climate Specification Add-Ons
If your bikes sell in Canada, Northern Europe, or mountain regions, winter behavior deserves its own section rather than a footnote:
- Standard NMC packs lose usable capacity below 0°C, and riders see 15–30% reduced mid-winter range.
- Charging below 0°C damages lithium cells through lithium plating; a correct BMS blocks it. Confirm the low-temp charge cutoff is enabled — many cheap BMS units omit it.
- Standard discharge reaches roughly −20°C; low-temperature-optimized cells extend operation to −30°C or −40°C with less voltage sag.
- Optional self-heating modules (wrapped film or heater plates controlled by the BMS) allow charging and full output in severe cold, drawing energy from the pack itself.
- Storage guidance (−10°C to +35°C, 40–60% SOC for long storage) should appear in your user documentation.
For extreme-climate programs we produce dedicated low temperature lithium ion battery designs with widened charge/discharge windows; state your target climate in the inquiry so cell grade and BMS thresholds are specified correctly from the start.
From Spec to Mass Production: The OEM/ODM Workflow
A written specification exists to make development predictable. A typical custom ebike battery pack program with an OEM/ODM supplier follows these stages:
- Inquiry & spec review (1–3 days): you send compartment dimensions, motor/controller parameters, target capacity, certification needs, and volume.
- Design proposal & quotation (5–10 days): configuration (S/P), cell model, BMS selection, electrical drawings, connector pinout, housing plan, sample and tooling timeline.
- Sample build (2–3 weeks): hand-built or soft-tool samples with capacity and BMS test data; charger and connector validation.
- Fit and ride verification (1–2 weeks, on your side): frame fit, cable routing, thermal check under load, range confirmation.
- Certification (4–8 weeks, in parallel where possible): UN38.3 first, then market-specific IEC/CE/UL work on frozen configuration.
- Pilot run & mass production: custom-tool MOQ is commonly 200–500 units per model; standard-housing programs can start at much smaller batches; lead time typically 4–8 weeks depending on housing and certification.
- After-sales: 2–3 year warranty on quality packs, failure analysis on returns, spare-pack availability across the model lifetime.
Wholesale and private-label buyers who do not need custom tooling skip stages 4–5 partially and work from standard housing series with their own labels — useful when speed to shelf matters more than a one-off form factor.
Example: Complete Custom Ebike Battery Pack Specification
The following sample sheet ties every section together and can serve as a template. Values are realistic industry settings for a 750W-class intube model, not fabricated precision:
| Spec Item | Requirement |
|---|---|
| Application | 750W hub/mid-drive city & fat-tire ebike, EU + US markets |
| Chemistry | NMC cylindrical, grade A |
| Configuration | 13S5P |
| Cell | 18650, 3500mAh, ≥2C continuous, named brand with datasheet |
| Nominal voltage | 48V |
| Charge voltage / cutoff | 54.6V / 32.5V pack-level |
| Capacity / energy | 17.5Ah / ≥819Wh |
| Max continuous / peak discharge | 25A / 50A |
| BMS | 25A cont, 50A peak; OVP 4.25V; UVP 2.6V; OCP 45A; SCP <50ms; NTC ×2 |
| Balancing | Passive, ≥50mA; start at Δ ≥30mV above 3.5V |
| Communication | UART to display, 5-pin Higo (protocol frozen before sample) |
| Charger | 54.6V, 3A, CC–CV, taper cutoff 0.15A |
| Cycle life | ≥500 cycles to 80% (0.5C, 25°C) |
| Charge temperature | 0°C to +45°C (BMS-blocked below 0°C) |
| Discharge temperature | −20°C to +60°C |
| Housing | Intube, Dorado-compatible, with lock; IPX5 |
| Connectors | Discharge XT60; charge DC 5.5×2.1; signal 5-pin Higo |
| Weight | ≤ 5.0 kg |
| Certifications | UN38.3, MSDS, CE, RoHS, IEC 62133-2; UL 2271/2849 by program |
| MOQ / lead time | 300 units custom; samples 2–3 weeks; mass 4–6 weeks |
| Warranty | 2 years |
Frequently Asked Questions
How much does a custom ebike battery pack cost? At production volume, NMC ebike packs typically land in the range of roughly US$0.12–0.20 per watt-hour depending on cell brand, BMS features, housing, volume, and certification amortization. Always compare quotes on the same cell model and configuration, not on voltage and label capacity alone.
Can you replace 18650 with 21700 in an existing housing? Usually not without redesign: the cells are wider and longer, so the holder, busbar layout, and possibly the enclosure change. Treat it as a new design with fresh fit and certification checks rather than a drop-in substitution.
Who owns the design and tooling in an OEM program? Terms vary by supplier. Reputable manufacturers give the buyer usage rights over private tooling and label artwork, keep drawings and BMS parameters documented, and supply them on request. Confirm this in writing before tooling payment.
Do wholesale replacement batteries need the same certification as OEM packs? Transport certification (UN38.3, MSDS) is always required, and market access rules such as CE and UL apply regardless of sales model. Replacement lines usually work from certified standard housing configurations, which is faster and cheaper than certifying custom designs.
Get a Free Design Review and Custom Quote
A precise specification is the difference between a battery program that launches on time and one that burns a season on re-samples and re-certification. If you have a frame drawing, motor/controller parameters, and a target range — even on a single page — our engineering team will review it and return a proposed configuration, BMS specification, housing solution, sample timeline, and quotation at no cost.
Flion Power is a China-based custom lithium ion battery pack manufacturer supplying ebike brands, OEM/ODM factories, wholesale distributors, and private-label programs worldwide, with 18650/21700 designs from 3.6V to 72V+, low-temperature options, and full export documentation.
Send your ebike battery requirements to info@flionpower.com → and receive your free custom design proposal, or browse our ebike battery product range and custom battery pack capabilities.
