Power tool batteries come in various forms and technologies, and it's essential to understand their distinctions. Among the most common are rechargeable lithium-ion (Li) batteries, which dominate the market. Other significant factors include battery management systems (BMS) and the ability for some batteries to automatically adjust their output voltage based on the specific requirements of different tools. This adaptability can enhance performance and safety, especially when considering aspects like thermal management, which is crucial for both chargers and battery packs. In recent years, pouch batteries have emerged as a viable alternative to traditional cylindrical cells, offering certain advantages in power delivery and charging speed.
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This article will explore the fundamentals of BMS, the functions of automatic voltage switching, various thermal management applications, the current uses of NiCd and NiMH battery packs, and analyze the ergonomic and practical aspects of the high-capacity 56 V 28.0 Ah Li battery pack. It will also clarify the essential differences between 18 V and 20 V power tool battery packs.
The BMS in cordless power tools is typically made up of a dedicated integrated circuit (IC), optionally complemented by a microcontroller (MCU), along with a range of sensors and supporting devices (as illustrated in Figure 1). The BMS is key to ensuring the safe and effective operation of the battery pack, ensuring its smooth cooperation with both the tool and a specially designed charger. Basic BMS solutions may merely provide essential monitoring and protection, while more sophisticated options might offer additional features such as:
- Real-time assessment of the battery's operational status with options for reporting to external devices
- Continuous performance optimization
- Precise voltage and current monitoring
- Low standby power consumption to extend overall runtime
- Accurate measurements of the state of charge and remaining runtime estimates
Automatic Voltage Switching
Some battery packs are equipped with the capability to detect the voltage needed by a specific tool and adjust their output accordingly. This requires a more intricate BMS architecture with active communication between the tool and battery pack. These versatile battery packs can supply various output voltages, such as 20, 60, and 120 Vdc, and are available in different amp-hour (Ah) ratings, including 6, 9, and 12 Ah.
For example, tools like 7-1/4” circular saws, grinders with variations in size from 4-1/2” to 6”, reciprocating saws, variable speed drills, and portable table saws may operate efficiently on 20 V and 60 V batteries. In contrast, larger equipment, such as 12” fixed and sliding compound miter saws, typically necessitate 120 V batteries. This automatic voltage switching eliminates the need for multiple battery types for different tools, simplifying battery management and potentially lowering costs on job sites.
Importance of Thermal Management
Effective thermal management is crucial for battery longevity. Two-way communication between the battery and charger is paramount to servey charging speed and optimizing battery life. By controlling the current, voltage, and temperature during the charging process, some chargers can utilize built-in fans to cool both the charger and the battery pack, promoting longer battery life and shorter charge times (Figure 2). Additionally, the charger acts as a vital protective measure, checking the battery pack's temperature and other conditions to prevent overheating and potential hazards.
Some advanced power packs feature internal thermal management systems, including phase-change materials that handle heat dissipation and ensure stable operating temperatures. These innovations contribute to enhanced power output, increased battery life, and faster recharge cycles.
Switch from Cylindrical to Pouch Cells
Historically, cylindrical cells were the standard in power tool batteries. However, the rise of premium packs utilizing pouch cells marks a significant technological shift. Pouch cells provide better power delivery and faster recharge capabilities than their cylindrical predecessors. They are increasingly being integrated into 18 and 20 V battery packs with varying capacities, enabling users to benefit from benefits such as:
- 50% more power and efficiency per charge
- Double the charge/discharge cycles
- Higher energy density, with compact form factors
The transition to pouch packs fosters improved performance through advanced BMS technology and thermal management systems, contributing to durability and efficiency in demanding applications.
Nickel-Based Batteries Still Have a Place
Though less prevalent than lithium-based alternatives, nickel-cadmium (NiCd) and nickel-metal hydride (NiMH) batteries still play a role in powering smaller tools. Despite being heavier and having lower energy densities than lithium batteries, they can handle high power levels while functioning over broader temperature ranges. NiCd packs boast capacities ranging from 1.2 to 2.2 Ah, while NiMH batteries provide capacities of 2.2 to 3.0 Ah. The choice between these battery types often boils down to considerations of weight, cost, maintenance requirements, and general usage context.
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