How to Assemble a Cylindrical Battery Pack: Process & Equipment Guide

Cylindrical battery packs used in power tools, e-bikes, drones, portable power stations and energy storage systems may differ greatly in voltage, capacity and pack structure, but most production lines follow a similar assembly sequence.

The main steps include cell capacity grading, insulation paper application, OCV and internal resistance sorting, cell arrangement, polarity inspection, spot welding, BMS installation and testing, finished pack testing, packaging, coding and charge-discharge aging.

These processes can be arranged as standalone workstations, a semi-automatic line or a fully automated production line.

Cylindrical Battery Pack Assembly Process

Cell Capacity Grading → Insulation Paper Pasting → OCV & IR Sorting → Cell Arrangement → Polarity Inspection → Spot Welding → BMS Testing & Installation → Battery Pack Testing → Packaging & Coding → Charge/Discharge Aging

The first few steps focus on cell consistency and correct arrangement. Welding and BMS assembly establish the electrical connection of the pack, while finished-pack testing and aging are used to verify the assembled battery before shipment.

Main Components of a Cylindrical Battery Pack

A cylindrical battery pack normally includes:

• Cylindrical lithium-ion cells
• Battery Management System (BMS)
• Cell holders or brackets
• Insulation paper
• Nickel or nickel-plated steel strips
• Insulation tape
• Wires and wiring harnesses
• Temperature sensors
• Connectors and output terminals
• EVA or cushioning materials
• Heat-shrink film or an external enclosure
• Labels or traceability codes

The series connection determines pack voltage, while the parallel connection increases capacity and current capability.

Common Cylindrical Cell Sizes and Applications

Supported Cell Formats

14500 · 18650 · 21700 · 26650 · 32650 · 32700 · 32140 · 33140

Not every machine uses the same tooling. Fixtures, cell holders and feeding mechanisms may need to be changed when the cell diameter, height or pack structure changes.

Typical Applications

• Power tools
• E-bikes
• E-scooters
• Electric motorcycles
• Drones and UAV battery packs
• Portable power stations
• Energy storage systems
• UPS systems
• AGVs and mobile robots
• Customized industrial battery packs

What Equipment Is Used to Assemble a Cylindrical Battery Pack?

A complete production setup may include the following equipment:

Step 1 — Cell Capacity Grading

Before assembly, the cells are charged and discharged to measure usable capacity and basic performance.

The purpose is to prevent cells with large capacity differences from being grouped into the same battery pack. A weak cell can limit the usable performance of the entire series or parallel group.

A lithium ion cell grading machine runs programmed charge-discharge cycles across multiple independent channels and records the data for each cell.

ACEY grading systems can be configured for common cylindrical formats such as:

18650 · 21700 · 26650 · 32650 · 32700 · 32140 · 33140

Available configurations include:

• 5V test systems with different current ranges
• 128 / 256 / 512 channels
• Standard linear systems
• Energy-feedback systems
• PC-based data recording and management

For high-volume cell processing, multi-channel grading allows hundreds of cells to be tested at the same time before they enter the pack assembly stage.

Recommended Equipment: Battery Cell Grading Machine

Step 2 — Insulation Paper Application

Insulation paper is usually applied around the positive terminal after grading.

This additional insulation helps separate the positive terminal area from the metal cell casing during assembly.

An automatic insulation paper sticking machine cuts and applies the paper with consistent positioning. ACEY machines can be equipped with changeable tooling for different cylindrical cells.

Production speed can reach up to 4,800 cells per hour, depending on the cell and machine configuration.

Common formats include:

18650 · 21700 · 26650 · 32650 · 32700

Larger cylindrical cells such as 32140 and 33140 require suitable tooling and machine configuration.

Recommended Equipment: Battery Insulation Paper Sticking Machine

Step 3 — OCV and Internal Resistance Sorting

Capacity grading alone is not enough for final cell matching.

Before assembly, cells are normally measured again for:

• Open-circuit voltage
• Internal resistance

The measured values are compared with preset ranges, and the cells are automatically separated into different groups.

ACEY cylindrical battery sorting machines are available with 5, 6, 10 or 11 sorting channels. Precision instruments such as HIOKI 3561 or compatible testers can be integrated for OCV and internal resistance measurement.

Barcode scanning can also be added when the production process requires cell-level traceability.

Cells with large differences in voltage or resistance may behave differently under charge and discharge. Sorting helps the manufacturer build packs from cells with closer electrical characteristics.

For production lines that combine insulation paper application and sorting, these processes can also be integrated to reduce repeated cell handling.

Recommended Equipment: Cylindrical Battery Sorting Machine

Step 4 — Cell Arrangement and Polarity Inspection

Sorted cells are placed into the required series-parallel arrangement using cell holders or dedicated fixtures.

Before welding, the positive and negative orientation of every cell should be checked.

For small production runs, this may be done manually. On semi-automatic and automatic lines, a CCD polarity inspection system is more reliable because it checks the complete cell group against the preset pattern.

ACEY CCD inspection systems can be configured for cylindrical cells including:

18650 · 21700 · 26650 · 32650 · 32700 · 32140 · 33140

Any reversed or incorrectly positioned cell can be identified before the pack enters the welding station.

Recommended Equipment: CCD Tester

Step 5 — Battery Pack Spot Welding

Spot welding is one of the most important operations in cylindrical battery pack assembly.

The cells are connected in series and parallel by welding nickel or nickel-plated connecting strips to the cell terminals.

The welding result is affected by more than welding current. The main process variables include:

• Strip material
• Strip thickness
• Cell terminal material
• Welding current
• Welding time
• Electrode pressure
• Electrode condition
• Welding point position

ACEY provides 5000A, 8000A and 10000A resistance welding configurations for cylindrical battery pack assembly.

Machine options include:

• Gantry spot welding machine
• Slide-rail gantry spot welding machine
  
• Single-side automatic spot welding machine
• Double-side automatic spot welding machine
• Double-side automatic spot welding machine with conveyor belt
• Customized welding fixtures

Automatic models can store welding programs for different battery pack designs and move between preset welding positions.

For production use, welding parameters should be validated with the actual cell and connecting strip. A parameter that works well on one battery pack should not automatically be copied to another design.

The inspection criteria may include weld appearance, connection strength and electrical resistance according to the manufacturer’s own process standard.

Recommended Equipment: Battery Pack Spot Welding Machine

Step 6 — BMS Testing and Installation

After the cell groups are electrically connected, the battery management system is installed according to the pack design.

Before installation or final integration, the BMS can be tested to verify its protection functions.

A BMS tester mainly check functions such as:

• Overcharge protection
• Overcharge recovery
• Overdischarge protection
• Overdischarge recovery
• Overcurrent protection
• Standby current
• Protection delay and recovery behavior

The required test range is determined by the BMS series count, voltage and current rating.

After BMS verification, assembly work includes:

• BMS mounting
• Balance-wire connection
• Main power cable connection
• Temperature-sensor installation
• Connector installation
• Insulation
• Wire fixing

This part of the process varies significantly from one battery pack to another. BMS specifications and electrical drawings are therefore important when configuring the assembly workstation or production line.

Recommended Equipment: BMS Tester

Step 7 — Finished Battery Pack Testing

Once the BMS, wiring and electrical connections are complete, the battery pack is tested as a finished assembly.

This step checks components and connections introduced during battery pack assembly rather than the performance of an individual cell.

A battery comprehensive tester can be configured to test:

• Open-circuit voltage
• AC internal resistance
• Charging
• Discharging
• Charge protection
• Discharge protection
• Overcurrent protection
• Short-circuit protection

Upper and lower limits are set in the test program, allowing the equipment to identify qualified and unqualified packs.

Finished pack testing is particularly useful for detecting problems related to:

• Poor welding
• Incorrect wiring
• BMS connection errors
• Connector problems
• Abnormal protection behavior

ACEY provides different voltage and current configurations for small and larger battery packs.

Recommended Equipment: Battery Comprehensive Tester

Step 8 — Packaging

After electrical testing, the pack is prepared for final packaging.

The packaging method may include:

• PVC heat-shrink film
• POF film
• PP (Polypropylene)
• PE (Polyethylene)
  
• Plastic housing
• Metal housing
• Customized enclosure

For heat-shrink packaging, the pack passes through a temperature-controlled shrink tunnel.

ACEY heat shrink machines are available with adjustable conveyor speed and temperature settings for different pack sizes and film materials.

Step 9 — Coding and Traceability

Pack identification is added before shipment or aging.

Printed information includes:

• Battery model
• Rated voltage
• Capacity
• Production date
• Batch number
• Serial number
• Barcode
• QR code

A handheld printer may be enough for small-batch production, while conveyor-based or continuous inkjet printers are more suitable for production lines.

If traceability is required, the printed code can be linked with earlier cell or pack test records.

Recommended Equipment: Battery Pack Coding Machine

Step 10 — Charge/Discharge Aging

Aging is one of the final electrical verification steps.

The battery pack is charged and discharged according to a programmed test sequence. The system can record:

• Voltage
• Charge current
• Discharge current
• Capacity
• Charge/discharge time
• Protection behavior
• Cycle data

ACEY battery pack charge-discharge and aging systems cover a wide range of battery pack applications.

Available configurations include:

• 8V–1500V voltage range
• 10A–1200A current range
• Multiple independent test channels
• Standard or energy-feedback systems
• PC-based data management

The selected tester must cover the pack’s maximum voltage and required charge/discharge current. Channel quantity is usually selected according to the planned production output and aging time.

For higher-power applications, energy-feedback systems can return part of the discharge energy to the electrical network instead of dissipating it only as heat.

Recommended Equipment: Battery Pack Aging Machine

Semi-Automatic or Fully Automatic?

The higher automation option is not always the better choice.

A semi-automatic production setup is more flexible when the manufacturer produces several battery pack models or changes fixtures frequently.

Many cylindrical battery pack lines use a combination of both.

For example, cell sorting, CCD polarity inspection and automatic spot welding can be automated, while BMS installation, wiring or enclosure assembly remains manual.

This approach often gives better flexibility than trying to automate every process from the beginning.

How to Configure Cylindrical Battery Pack Assembly Equipment

The equipment should be selected from the actual battery pack design rather than only from the cell model.

Two 21700 battery packs may require completely different fixtures, welding machines and test systems if their series-parallel configuration, current requirement and mechanical structure are different.

Before equipment selection, it is useful to confirm the following information.

Cell Information

• Cell model
• Cell chemistry
• Cell capacity
• Nominal voltage
• Maximum charge/discharge current

Battery Pack Design

• Series and parallel configuration
• Number of cells
• Pack voltage
• Pack capacity
• Maximum working current
• Pack dimensions or drawing

Welding Information

• Nickel or nickel-plated strip material
• Strip thickness
• Busbar structure
• Welding pattern
• Number of welding points

BMS Information

• Number of series
• Protection current
• Communication function
• BMS drawing or datasheet

Production Requirement

• Packs per hour or per day
• Number of battery pack models
• Planned automation level
• Workshop layout
• Available operators

Testing Requirement

• Cell grading criteria
• OCV and IR sorting limits
• Finished pack test items
• Charge/discharge current
• Aging procedure
• Required test channels
• Data traceability requirements

These details make it much easier to match each machine to the actual process and avoid buying equipment with either insufficient capacity or unnecessary specifications.

ACEY Cylindrical Battery Pack Assembly Solutions

ACEY supplies individual machines, semi-automatic production equipment and fully automatic assembly solutions for cylindrical lithium battery packs.

Equipment can be configured for cells including:

18650 · 21700 · 26650 · 32650 · 32700 · 32140 · 33140

Available processes cover:

1. Cell grading
2. Insulation paper application
3. OCV and internal resistance sorting
4. CCD polarity inspection
5. Resistance spot welding
6. BMS testing
7. Finished pack testing
8. Heat-shrink packaging
9. Coding
10. Charge-discharge aging

For production-line projects, the configuration is developed around the customer’s battery pack, required output and test process rather than from a fixed machine list.

Planning a Cylindrical Battery Pack Project?

Please provide:

• Cell model
• Series/parallel configuration
• Pack voltage and capacity
• Pack drawing or dimensions
• Connecting strip material and thickness
• BMS specification
• Required production output
• Testing requirements
• Preferred automation level

This information allows us to recommend the required machines, tooling, welding configuration and test system for the actual pack.

Discuss Your Battery Pack Assembly Project →

Frequently Asked Questions

*What machines are needed to make a cylindrical battery pack?

A typical production setup may include a cell grading machine, insulation paper sticking machine, OCV/IR sorter, CCD polarity tester, spot welder, BMS tester, battery comprehensive tester, packaging machine, coding machine and battery pack aging tester.

Small-batch production does not necessarily require all processes to be automated.

*What cylindrical cells can be used?

Common formats include 18650, 21700, 26650, 32650, 32700, 32140 and 33140.

Whether one machine can handle several cell sizes depends on its mechanical range and tooling. Fixtures, feeders and holders may need to be changed when switching cell formats.

What is the difference between capacity grading and OCV/IR sorting?

Capacity grading measures cell capacity through charge-discharge testing.

OCV/IR sorting measures open-circuit voltage and internal resistance and groups the cells according to preset ranges.

They are different steps and are often used together before battery pack assembly.

Why check cell polarity before welding?

A reversed cell can create the wrong series-parallel connection once the nickel strip is welded.

CCD inspection checks the complete cell arrangement before welding and identifies cells placed in the wrong direction.

What type of spot welder is used for cylindrical battery packs?

Resistance spot welding is commonly used for nickel and nickel-plated connecting strips.

ACEY provides 5000A, 8000A and 10000A welding power supply, including gantry, single-side automatic and double-side automatic configurations.

The machine and welding parameters should be selected according to the cell terminal, strip material, strip thickness and welding pattern.

Can one production line make different battery packs?

Yes, but not without changeover.

If the cell diameter, pack dimensions or welding pattern changes, the holder, fixture, feeder or welding program also need to be changed.

Semi-automatic lines offer more flexibility for customers producing several battery pack models.

What information does ACEY need before recommending a production line?

The most useful information is:

• Cell model
• Series-parallel configuration
• Pack voltage and capacity
• Pack drawing
• Nickel strip material and thickness
• BMS specification
• Required output
• Test requirements
• Automation level

With these details, the equipment and tooling can be selected around the actual battery pack instead of using a generic line configuration.