There’s more variety available in power tool batteries than might be expected. Of course, various rechargeable Li chemistries dominate, and battery management systems (BMS) are critical, but there are also power tool batteries that can automatically switch their output voltage to suit the needs of specific tools; thermal management can be important for both chargers and battery packs and in some packs, pouch batteries are replacing 18650 and 21700 cylindrical cells.
This FAQ begins with a brief review of BMS considerations, looks at how automatic voltage switching works, presents applications for forced air and phase change cooling technologies, digs into where NiCd and NiMH battery packs are still used, considers the ergonomics and applications of a large 56 V 28.0 Ah Li battery pack, and closes by examining the difference between 18 V and 20 V power tool battery packs.
The BMS in a cordless power tool usually consists of a dedicated IC plus an optional microcontroller (MCU) and various sensors and support devices (Figure 1). The battery pack BMS is important for reliable and safe operation and is designed to work in coordination with the tool and with a custom-designed charger. A basic (low-cost) solution will only include protection and monitoring features while a more capable BMS will also provide fuel gauging, cell balancing, a status display, and other functions including:
- Estimating the battery’s operational state and optionally reporting it to external devices
- Continually optimizing battery performance
- Supporting accurate real-time voltage and current sensing
- Having a low standby power requirement to extend runtime
- Accurately measuring battery state of charge and estimating remaining runtime
Automatic voltage switching
Battery packs are available that can automatically sense the voltage needed by a specific tool and switch their output voltage as required. The BMS in these batteries is more complex and includes active communication between the tool and the battery pack. These packs can deliver 20, 60, and 120 Vdc for tools, and are available in 6, 9, and 12 amp-hour (Ah) capacities.
Tools like 7-1/4″ circular saws, 4-1/2″ to 6″ grinders, reciprocating saws, 1/2″ variable speed stud and joist drills, and portable table saws can require 20 and 60 V battery pack voltages while larger tools like 12″ fixed head compound miter saws and 12″ sliding compound miter saws require the pack to deliver 120 V. With automatic voltage switching, a single type of battery pack can be used for a variety of applications, reducing the cost and complexity of managing batteries on a job site.
Thermal management matters
Two-way communication between the battery pack and charger can be important for several reasons. The most obvious is to speed charging and optimize battery life by controlling the charging current, voltage, and temperature. Temperature control can be enhanced with a fan built into the charger that cools the battery pack being charged, as well as the charger, for longer battery life and faster charging times (Figure 2).
The charger also provides protection for the pack. It communicates with the pack to determine the condition of the battery. For example, if the pack is too hot or too cool for reliable charging, an LED on the charger will flash making the user aware of the problem. Once the pack has cooled to about 57 °C (135 °F) or warms to over 3 °C (37 °F), the charger automatically begins charging, and the corresponding LED will shine normally. The charger also checks for defective packs and will alert the user of any problems and charging will be stopped.
In addition to active thermal management during charging, some packs are available with internal thermal management systems based on a phase-change material to absorb and dissipate heat for cooler operation. Those pouch-based battery packs deliver more power, longer battery life, and faster charging.
Pouches, not cylinders
While cylindrical form factors like 18650 and 21700 are the most common battery cells used in power tool battery packs, premium packs based on pouch cells have appeared. Pouch cells are replacing cylindrical cells for some of the same reasons that the 21700’s replaced 18650’s — their ability to support higher power delivery and faster charging. Both are key considerations for power tool batteries. In some cases, the pouch packs can be recharged in 15 minutes.
Pouch packs are available from several power tool companies and consist of stacks of pouch cells (Figure 3). The initial offerings are focused on 18 and 20 V packs and include a range of capacities like 1.7, 3.5, 5.0, 6.0, and 8.0 Ah. Some of the performance benefits claimed for pouch cells include:
- 50% more power and 50% more work per charge
- Twice the charge/discharge cycles
- Higher energy densities
- 5Ah in about the same size as a 2.5 Ah cylindrical pack
- 0 Ah in about the same size as a 5 Ah cylindrical pack
- 0 Ah in about the same size as a 6.0 Ah cylindrical pack
The performance improvements delivered by pouch packs are enhanced using phase change cooling and advanced BMS technologies. Placed between the pouches, the phase change material stabilizes the battery temperature during discharge enhancing pack lifetimes. These premium packs include a sophisticated BMS that sends data such as voltage, discharge current and temperature to the tool enabling an algorithm in the tool to optimize its use of the energy in the pack. The tool also communicates with the pack for added battery cell protection and extended lifetimes.
Nickel down but not out
While not as common as Li-based packs, NiCd and NiMH cells are still found in power tool battery packs, almost exclusively for smaller tools. They are heavier and have lower energy densities compared with Li packs. But they can support impressive power levels. Compared with both Li and NiMH cells, NiCd cells are more tolerant of impacts and can be used over wider temperature ranges. NiCds are more temperature rugged than NiMH cells. NiCd packs are available with capacities from 1.2 to 2.2 Ah while comparable NiMH packs can store between 2.2 and 3.0 Ah.
NiCd packs are the least expensive but also weigh the most compared with NiMH and Li. NiMH is in between, costing more than NiCd but with lower weight, and cost less than Li but with higher weight. Self-discharge rates are higher for NiCd and NiMH compared to Li, and the nickel chemistries require maintenance consisting of regular deep discharges while Li does not. NiMH is less maintenance intensive than NiCds.
Professional power tools for landscape maintenance like string trimmers, hedge trimmers, blowers, and saws can become unwieldy with large battery packs that are needed for all-day use. That challenge has been addressed with a 19.8-pound backpack battery that can deliver 28 Ah at 56 V (Figure 4); it’s rated IP65 and designed for continuous use in heavy rains. Air vents are included for cooling the pack and supporting all-day use. The pack is also designed to give battery-powered portable tools performance that rivals their gasoline-powered counterparts. The BMS communicates with the charger to constantly monitor each cell’s charge and temperature — to deliver the most efficient and quickest charge. Additional features and specifications include:
- Detachable harness & retractable carrying handle
- Robust, high current capacity cable & plug
- USB charging ports
- 550 W charger with user-controlled fast charge mode that provides a full charge in 210 minutes
On a final note, power tool battery packs are offered rated for 18 V and 20 V. What’s the difference? Marketing. The Li cells used in the packs have a nominal voltage of 3.6 V and a maximum voltage of about 4 V. The packs use 5 cells, so the difference between 18 V and 20 V packs depends on whether the company is using nominal voltage or maximum voltage for its rating.
Portable electric power tools are important in a range of industries from construction to agriculture and landscape maintenance. Lithium-based battery packs are the most common and pouch cells are replacing 1860 and 21700 cylindrical cells in high-performance pack designs. For applications that are more cost-sensitive or require higher levels of environmental ruggedness, NiCd and NiMH cells are still used occasionally. As in many industries, a marketing spin is applied to some power tool battery packs, and there’s no practical difference between 18 V and 20 V Li packs.
Batteries for Power Tools: Safety Testing and Certification, UL
Battery pack: cordless power tool, Texas Instruments
Commercial series backpack battery, EGO POWER
Flex 24V Stacked Lithium Battery: New for 2022, ProTool Reviews
PowerStack Battery, Dewalt