Power tool batteries are typically composed of multiple cylindrical lithium-ion cells connected in series and parallel to form a battery pack. Common voltage ratings include 12V, 16V, 18V, and 21V, with different manufacturers using varying interfaces. However, two standard interfaces widely adopted by manufacturers are DaYi and Makita, allowing battery packs to be interchangeable across different brands.
Battery Measurement
Most power tool battery packs are made from 18650 or 21700 lithium-ion cells, predominantly ternary lithium batteries. The nominal voltage of a single cell is 3.7V, with a fully charged voltage of 4.2V and an undervoltage protection threshold around 3.2V. The internal resistance of individual cells typically ranges from a few milliohms to several dozen milliohms, with lower resistance being preferable.
Measurement Tools for Individual Cells
- Internal Resistance Meter
- Multimeter
- Charge-Discharge Capacity Tester
- USB Electronic Load Tester
Battery Pack Configurations and Testing
1. Series Connection Testing
Connecting multiple cells in series increases voltage while maintaining the same capacity. Proper testing of series connections involves measuring and recording each cell’s voltage and internal resistance before and after full charging, then using an electronic load to assess discharge time and capacity.
2. Parallel Connection Testing
Parallel connections increase capacity while maintaining the same voltage. To avoid self-discharge losses between cells, batteries with similar voltage, capacity, and internal resistance should be selected.
3. Series-Parallel Combination Testing
A combination of series and parallel connections enhances both voltage and capacity, optimizing power output. Testing follows the same principles as individual series and parallel configurations.
Measurement Methods
1. Individual Cell Testing
Accurate testing requires precision instruments. However, for general testing, an internal resistance tester, multimeter, and charge-discharge tester suffice. It is advisable to label each battery and record key parameters for reference.
2. Series Pack Testing
Each cell in a series connection should be measured separately. Record voltages and internal resistances before and after charging, then use an electronic load to measure discharge capacity and duration.
3. Parallel Pack Testing
Select batteries with similar voltage, capacity, and resistance values to prevent unnecessary energy loss due to inter-battery charging and discharging.
Essential Testing Instruments
- Multimeter – Measures voltage and current
- Electronic Load Tester – Simulates loads, displaying real-time voltage, current, power, resistance, and capacity
- Internal Resistance Meter – Measures battery resistance
- Adjustable Power Supply – Charges batteries and helps manually balance series-connected cells
- Active Balancer Module – Ensures balance in multi-series battery packs
- Battery Protection Board with Bluetooth – Monitors discharge and charge states via a smartphone, using alligator clips for easy testing
Conclusion
Accurate battery measurement is essential for maximizing battery efficiency and longevity. By using the right instruments and testing methods, power tool batteries can be effectively monitored and optimized for better performance.
