Most custom battery projects fail for the same reason: the pack is specified by the battery you already know rather than by what the device actually needs. Getting a custom lithium battery pack right means starting from the load, the runtime and the environment, then choosing cells — not the other way round. This guide walks through the decisions in the order a battery engineer makes them, and what to send a battery pack supplier to get an accurate quote first time.
Step 1: define the electrical requirement
Four numbers drive everything else. Operating voltage (and the cut-off voltage at which your device stops working), average current, peak current and its duration, and required runtime between charges or over the product’s life. The peak matters as much as the average: a pack sized purely on mAh will sag under a transmit pulse or a motor start and trip your undervoltage cut-off long before the energy is gone.
Step 2: primary or rechargeable?
If the device is sealed for years and drains slowly — a meter, a sensor, a tag — a primary cell is usually right, and Li-SOCl2 gives the longest life. If the device is charged regularly, Li-ion or Li-polymer applies. Where safety or temperature dominates the specification, solid-state cells are now a production option. Deciding this early prevents redesigning the pack later.
Step 3: cell selection and configuration
Cells are combined in series to raise voltage and in parallel to raise capacity — the familiar xSyP notation. A 2S2P Li-ion pack, for example, gives roughly 7.4V nominal with double the capacity of a single cell. The format matters too: cylindrical cells (18650, 21700) are robust and cost-effective; pouch and prismatic cells fit thin or awkward enclosures; coin cells suit tiny backup roles. Cells within a pack should be matched for capacity and internal resistance, otherwise the weakest cell dictates the life of the whole pack.
Step 4: protection — PCM or BMS
| Function | PCM | BMS |
|---|---|---|
| Over-voltage / over-current / short-circuit / over-temperature protection | Yes | Yes |
| Cell balancing | Basic or none | Yes |
| State-of-charge reporting & communication | No | Yes (RS485, CAN, MODBUS, UART) |
| Typical use | Small packs, 1S–3S | Multi-cell, smart or high-value packs |
Every lithium pack needs protection. A PCM covers the essential fault conditions and suits small packs. A BMS adds balancing, state-of-charge reporting and a communication interface — specify it when the host system needs to read the pack, or when cell count makes balancing necessary.
Step 5: mechanical and environmental
Give the maximum envelope (length × width × height) and any keep-out zones, the connector or termination type — plug, wire lead, or solder tabs — and the full operating and storage temperature range. If the product goes outdoors or into cold chain, say so: it changes the cell choice, not just the label. Add vibration, shock or IP requirements if the pack is automotive or industrial.
Step 6: certification and volume
Confirm which certifications your market needs — UN38.3 is required for transport in all cases, with IEC 62133, UL, CE, RoHS and REACH depending on destination. Note that pack assembly usually requires its own testing separate from the cells; our guide to battery certifications covers what to request. Then state your pilot quantity and annual forecast, so the supplier can quote tooling and pricing honestly rather than optimistically.
The quote checklist
- Nominal voltage and cut-off voltage
- Capacity or required runtime
- Average and peak current, with pulse duration
- Charge method and charging current, if rechargeable
- Maximum dimensions and connector type
- Operating and storage temperature range
- Certifications required for your market
- Pilot quantity and annual volume
Frequently asked questions
What is the difference between a PCM and a BMS?
A PCM provides core protection — over-voltage, over-current, short-circuit and over-temperature. A BMS adds cell balancing, state-of-charge reporting and a communication interface such as RS485, CAN, MODBUS or UART. Small packs typically use a PCM; multi-cell or smart packs need a BMS.
What does 2S2P mean?
Two cells in series, two of those strings in parallel. Series raises voltage, parallel raises capacity. A 2S2P Li-ion pack gives roughly 7.4V nominal with twice the capacity of a single cell.
What information do you need to quote a custom battery pack?
Voltage and cut-off voltage, capacity or runtime, average and peak current, maximum dimensions, connector type, temperature range, required certifications, and your pilot and annual volumes. With those, our engineers can recommend cells, configuration and protection.
Start your custom battery pack project
JBWCELL is a Shenzhen-based custom lithium battery manufacturer and battery pack supplier with full OEM/ODM capability — cell selection, PCM/BMS design, spot welding and assembly, housing and certification support under ISO 9001/14001. See our OEM/ODM services, or contact us with the checklist above for a specification and quote.
