How to Choose the Right Lithium-Ion Battery Cells for Power Tools

Lithium-ion batteries have become the preferred power source for many modern cordless power tools. From electric drills and impact wrenches to angle grinders, saws, and other professional equipment, rechargeable lithium-ion battery packs provide a combination of high energy density, lightweight design, and reliable power output.

However, choosing the right battery cell is not simply about selecting the highest capacity available. Different power tools have different power requirements, operating conditions, and runtime expectations.

For battery pack manufacturers, distributors, and power tool developers, understanding battery cell specifications is essential when designing a suitable battery solution.

In this guide, we will explore the key factors to consider when selecting lithium-ion battery cells for power tools.

Lithium-ion batteries offer several advantages that make them suitable for cordless power equipment.

High Energy Density

Lithium-ion cells can store a relatively large amount of energy in a compact and lightweight design. This helps reduce the overall weight of cordless tools while providing sufficient operating time.

High Discharge Capability

Many power tools require substantial current, especially during startup or heavy-duty operation. High-discharge lithium-ion cells can support these demanding power requirements when selected according to the equipment's specifications.

Rechargeable Performance

Lithium-ion batteries support repeated charging and discharging, making them practical for professional users who operate their tools frequently.

Flexible Battery Pack Design

Cylindrical cells, particularly 18650 and 21700 formats, offer flexibility for battery pack manufacturers. Different series and parallel configurations can be used to achieve the required voltage and capacity.

Two important specifications when selecting power tool battery cells are capacity and discharge rate.

Capacity indicates how much electrical charge a battery cell can store.

For example:

• 2500mAh: Suitable for applications with moderate energy requirements, depending on discharge capability.

• 3000mAh: Offers a balance between capacity and size.

• 4000mAh: Provides greater energy storage for longer operating periods.

• 5000mAh: Offers higher capacity for applications prioritizing extended runtime.

A higher-capacity cell can help extend operating time, but capacity alone does not determine whether a battery is suitable for a high-power tool.

Discharge capability determines how much current a cell can deliver under specified conditions.

For example, an electric impact wrench may require a battery pack capable of delivering substantial current during demanding operations.

Using cells with insufficient discharge capability may result in excessive voltage drop, overheating, or reduced performance.

Therefore, battery pack designers should evaluate both continuous discharge current and peak current requirements rather than considering capacity alone.

18650 and 21700 are two common cylindrical lithium-ion battery formats used in rechargeable battery packs.

Both formats have their advantages, and the appropriate choice depends on the design requirements.

18650 cells measure approximately 18mm in diameter and 65mm in length.

Key characteristics include:

• Compact dimensions.

• Established supply chains and wide model availability.

• Flexible battery pack configurations.

• Various capacity and discharge performance options.

18650 cells are commonly considered for cordless drills, portable power tools, and compact battery pack designs.

21700 cells measure approximately 21mm in diameter and 70mm in length.

Compared with 18650 cells, the larger format provides more internal space, allowing manufacturers to develop cells with higher capacity or different discharge characteristics.

Key characteristics include:

• Potentially higher capacity per cell.

• Various high-discharge options.

• Opportunities to optimize battery pack design.

• Suitability for demanding power applications, depending on cell specifications.

21700 cells can be an attractive option for professional power tools that require a combination of energy storage and power output.

Which One Should You Choose?

There is no universal winner between 18650 and 21700 cells.

If compact dimensions and existing battery pack compatibility are priorities, 18650 cells may be appropriate. If the design requires higher energy per cell or a different power-to-size balance, 21700 cells may be worth considering.

The final decision should always be based on the actual cell specifications and battery pack requirements.

Different power tools have different operating requirements.

Here are several common applications and the battery characteristics worth considering.

Cordless drills are widely used in construction, woodworking, and home improvement.

Battery selection should consider:

• Appropriate voltage for the motor.

• Sufficient discharge capability.

• Balanced capacity and weight.

• Reliable performance during repeated operation.

Impact wrenches often experience high power demand during fastening and loosening operations.

High-discharge lithium-ion cells may be more suitable for these applications, provided their continuous and pulse discharge ratings meet the design requirements.

Angle grinders can require relatively high power, especially when cutting or grinding harder materials.

Battery packs should be designed with sufficient current capability, effective thermal management, and suitable protection systems.

Cordless circular saws and reciprocating saws may require substantial power during cutting operations.

Battery pack designers should pay particular attention to discharge performance, thermal conditions, and operating duration.

Selecting a suitable individual battery cell is only one part of the process. The complete battery pack must also be properly designed.

Series and Parallel Configuration

Series connections increase the total battery pack voltage, while parallel connections increase the overall capacity and available current.

For example, a 5S configuration connects five cells in series, while a 5S2P configuration contains ten cells arranged in five series groups and two parallel cells per group.

The configuration should match the power tool's electrical requirements.

Battery Management System (BMS)

A suitable battery management system helps monitor battery conditions and provides protection functions such as overcharge, over-discharge, overcurrent, and temperature protection, depending on its design.

Thermal Management

High-current applications generate heat during operation. Proper cell spacing, electrical connections, and thermal design are important for maintaining reliable battery performance.

Cell Consistency

Battery packs should use properly matched cells with consistent capacity, voltage, and internal resistance. This helps support balanced operation throughout the battery pack.

Safety should always be considered when selecting and assembling lithium-ion battery packs.

Important considerations include:

• Select cells from reliable suppliers with traceable specifications.

• Verify that continuous discharge ratings meet the application requirements.

• Use a suitable BMS and protection design.

• Avoid operating cells beyond their specified voltage, current, and temperature limits.

• Ensure proper insulation and electrical connections.

• Follow the cell manufacturer's charging and storage recommendations.

Battery pack design, assembly, and testing should follow applicable industry standards and safety requirements.

For battery pack manufacturers and power tool brands, choosing a reliable cell supplier is an important part of maintaining product quality.

Before placing an order, consider the following factors:

1. Product Specifications: Check capacity, nominal voltage, maximum continuous discharge current, internal resistance, and dimensions.

2. Quality Control: Ask about cell testing procedures, batch consistency, and quality inspection.

3. Technical Documentation: Request official datasheets and relevant certification documents.

4. Product Availability: Confirm stock availability, production capacity, and delivery arrangements.

5. Technical Support: Work with suppliers who can help identify suitable cells for different battery pack requirements.

A dependable supplier can help manufacturers make more informed decisions when developing battery solutions for different power tool applications.

Choosing the right lithium-ion battery cells for power tools requires more than comparing battery capacity. Discharge capability, cell dimensions, voltage, battery pack configuration, and thermal management all influence the final performance.

Both 18650 and 21700 cells offer useful options for power tool battery applications. The best choice depends on the specific requirements of the equipment and the intended battery pack design.

By understanding these key specifications and working with a reliable battery cell supplier, manufacturers can develop battery packs that provide suitable power output, operating time, and dependable performance.