A custom lithium battery pack usually goes through seven stages: requirements (RFQ), cell selection, electrical and BMS design, mechanical design, prototyping, certification, and mass production. Most delays happen at the start, because the specification is incomplete, or at the end, because certification wasn’t planned early enough. This guide covers what happens at each stage and what you need to prepare.
Key Takeaways
- A complete RFQ covers voltage, capacity, peak current, size limits, temperature range, target market and annual volume.
- Choose cells before you design the pack. Cell choice sets energy density, current capability, cost and lead time.
- Plan certifications such as UN38.3 and IEC 62133-2 during design, not after prototypes.
- Freeze the design (cells, BMS firmware and enclosure) before certification testing. Changes afterwards can mean retesting.
Stage 1: Write a Complete RFQ
How well your RFQ is written decides how accurate the quote is and how fast engineering can start. Include these parameters:
| Parameter | Example | Why it matters |
|---|---|---|
| Nominal / max voltage | 36 V / 42 V | Sets the series count (10S for Li-ion) |
| Capacity or runtime | 10 Ah or “4 hours at 80 W” | Sets the parallel count |
| Continuous and peak current | 15 A / 40 A for 3 s | Decides the cell type and BMS MOSFET sizing |
| Max dimensions and weight | 180 × 70 × 45 mm, ≤ 1.2 kg | Limits the cell format and layout |
| Operating / charging temperature | -20 °C to 50 °C / 0–45 °C | Affects chemistry and whether heating is needed |
| Communication | SMBus, CAN, UART or none | Changes BMS hardware and firmware |
| Target markets | US, EU, Japan | Decides the certifications needed |
| Annual volume | 5,000 pcs/year | Affects tooling and unit price |
If you only know the device’s power draw and required runtime, share those. An engineer can convert them into voltage and capacity. Our guide to watts and watt-hours shows the calculation.
Stage 2: Select the Cells
Cell selection comes first because everything else depends on it. The main choices are:
- Chemistry: NMC/NCA gives higher energy density. LiFePO4 gives longer cycle life and better thermal stability. See LiFePO4 vs NMC.
- Format: cylindrical (18650, 21700), prismatic, or pouch (LiPo) for thin devices.
- Brand and grade: tier-one cells from Panasonic, Samsung SDI, Molicel or EVE cost more but come with consistent datasheets and traceability. See our 21700 cell comparison.
A cell datasheet gives hard limits you must design within. For example, Molicel’s INR-21700-P45B is rated for 45 A continuous discharge but may only be charged between 0 °C and 60 °C.
Stage 3: Electrical and BMS Design
The electrical design sets the series/parallel configuration, the protection thresholds and how the pack talks to the host device. The battery management system (BMS) handles:
- Overcharge, over-discharge, overcurrent and short-circuit protection
- Temperature monitoring with NTC sensors placed at the hottest cells
- Cell balancing (passive on most packs, active on large ones)
- Fuel gauging and communication (SMBus, CAN, UART)
Cells in parallel groups must be matched on capacity and internal resistance before assembly. Mismatched cells age unevenly and trigger early BMS cut-offs. Our article on cell matching explains the sorting criteria.
Stage 4: Mechanical and Thermal Design
The enclosure has to hold the cells in place, protect them from impact and vibration, and get rid of heat. Decisions at this stage include:
- Soft pack (heat-shrink and fish paper) or hard case (ABS/PC, aluminium)
- Nickel strip thickness and spot-weld pattern, sized for peak current
- Cell holders, foam and potting against vibration
- Connector and wire gauge (for example XT60 vs XT90)
- IP rating, if the product is used outdoors
Stage 5: Prototyping and Validation
Prototypes are used to confirm that the design works. Typical validation tests are:
- Capacity and discharge-curve tests at the rated current
- Temperature rise at peak load
- BMS protection trip tests (overcharge, short circuit, overcurrent)
- Drop, vibration and in-device fit checks
- Cycle-life sampling for longer projects
Our battery testing guide covers the methods. Expect one or two prototype rounds before the design is frozen.
Stage 6: Certification
Almost every lithium pack needs a UN38.3 test summary before it can be shipped. Section 38.3 of the UN Manual of Tests and Criteria covers altitude, thermal, vibration, shock, external short circuit, impact, overcharge and forced discharge tests. Market-specific safety standards come on top of that:
- IEC 62133-2: portable sealed secondary lithium cells and batteries, widely used for CB and CE projects
- UL 2054 / UL 2271 / UL 2580: North American standards for household, light-EV and EV batteries
- End-product standards such as IEC 62368-1 (ICT) or IEC 60601-1 (medical)
Our guide to battery certifications by market explains which ones apply. You can also view Jetray’s certifications.
Stage 7: Mass Production
In mass production, the goal is to make every pack the same as the approved sample. A controlled line follows the same steps shown in our factory process:
- Incoming cell inspection, sorting and matching (OCV and IR)
- Spot welding and assembly
- BMS installation and functional testing
- Pack assembly and fixing
- 100% final testing and inspection before packing
Ask your supplier for traceability down to cell batch level and a first-article inspection report for the first production lot.
Frequently Asked Questions
How long does custom battery pack development take?
It depends on complexity and how many certifications are needed. A simple pack with an existing BMS moves faster than one that needs new firmware, new tooling and several certifications. Certification testing is often the longest single step, so start it once the design is frozen.
Can I change the design after certification?
Changing the cell, BMS or enclosure can invalidate test reports. Talk to your test lab before making any change after certification.
What does a custom pack cost?
Cells, BMS, enclosure tooling, certification and volume drive the price. See our breakdown of custom battery pack cost and MOQ.
Start Your Project
Jetray has designed and built custom lithium packs for more than 15 years, for consumer, medical, robotics and mobility customers. Send your specification through our custom battery pack service page, and our engineers will review the RFQ and suggest a cell and BMS configuration.
