Battery Knowledge
18650 & 21700 Battery Cells: Selection Guide for OEM Packs is a technical reference for engineers, buyers and product teams. It explains the terms that affect real product design, including cell format, chemistry, pack structure, protection and testing.
The purpose is to help B2B customers communicate more clearly with a battery manufacturer before starting a custom battery pack project.
Table of Contents
1. Basic Concept
A lithium battery cell is the basic energy unit. A battery pack combines one or more cells with protection, wiring, insulation, label and connector. The finished pack must match the device load and charging environment.
2. Cell Chemistry and Format
Common formats include cylindrical Li-ion cells such as 18650 and 21700, lithium polymer pouch cells, and LiFePO4 cells. Each format has different advantages for capacity, size, cycle life, discharge current and mechanical design.
| Format | Typical use |
|---|---|
| 18650 / 21700 cells | Portable equipment, instruments, lighting and multi-series cylindrical packs. |
| Lithium polymer cells | Thin devices or custom-shaped battery spaces. |
| LiFePO4 cells | Lead-acid replacement, RV, backup and long-cycle energy applications. |
3. Battery Pack Structure
Series cells increase voltage, while parallel cells increase capacity and current capability. The pack also needs nickel strips, insulation, a PCM or BMS, wires, connector and mechanical protection.
4. Safety and Testing
Safety design should include over-charge, over-discharge, over-current and short-circuit protection. For demanding applications, temperature monitoring, cell balancing and aging tests may also be needed.
5. How to Apply This Knowledge
Use this knowledge to prepare the project requirements: voltage, capacity, current, size, connector, charging method and working environment. ASOL can then recommend a suitable battery pack structure.
Frequently Asked Questions
What is the difference between a cell and a battery pack?
A cell is a single energy unit. A pack combines cells with protection circuits, wiring, insulation and connectors for use in a finished device.
Why are 18650 and 21700 cells common in custom packs?
They offer stable supply, flexible series/parallel assembly and good mechanical strength for many portable and industrial products.
When should lithium polymer cells be used?
Lithium polymer cells are useful when the device needs a thin battery or a custom shape that cylindrical cells cannot fit.
Send Your Battery Requirements
For project evaluation, share your target voltage, capacity, size limit, load current, connector requirement, application details and expected quantity. ASOL can review the cell structure, BMS/PCM, wiring and sample plan before mass production.
