Feature: Power
internal impedance, state of charge (SoC), state of health (SoH) and operating temperature. In reality, when a batch of brand new cells is first produced
by the manufacturer, their performance and specifications are generally consistent. However, aſter being put into actual use, as the cells age, inevitable differences in performance arise due to factors such as load, environmental temperature and humidity, as well as the number of charge cycles. When performance differences between cells are small, they
typically do not threaten the normal operation of the battery pack and do not require special attention. But once these differences become significant enough to threaten the proper functioning of the battery pack, it is crucial to address them. We will refer to these performance differences between cells as cell mismatch.
The important role of active balancing in battery management systems
By Frank Zhang, Applications Engineer, Analog Devices
S
implicity and efficiency are one of the main goals for most designers. Here we will explore the design prototype of a simple yet efficient active balancing system for battery management. In a battery management system (BMS), multiple individual cells are typically connected in series to
form a high voltage battery pack. Tis high voltage system is found in various applications, including electric vehicles, high voltage energy storage systems and uninterruptible power supplies. In these series-connected cells, the ideal operating condition is that all individual cells have consistent parameters such as cell voltage,
26 November 2025
www.electronicsworld.co.uk
Cell capacity mismatch As shown in Figure 1, if a few cells in a battery pack have significantly lower capacity than the others, they are referred to as weak cells. Tey can be problematic during both charging and discharging. During charging, a weak cell will reach full voltage more quickly and become fully charged ahead of the others. However, because the cells are connected in series as part of a larger battery pack, the charging current does not automatically stop once the weak cell is full. As a result, the entire battery pack’s charging process must be stopped as soon as the weak cell reaches full charge to avoid overcharging, which could endanger not only the weak cell but the entire battery pack. Similarly, during discharging, the weak cell’s voltage will drop
faster and will reach its fully discharged state sooner than the rest. Again, the discharge process of the entire battery pack must stop immediately once the weak cell is fully discharged, or it risks over- discharging, which is also a safety concern. Observant readers may quickly realise that in a battery
pack containing weak cells, the overall capacity utilisation is significantly reduced. Without cell balancing, healthy cells are unable to fully charge or fully discharge during each cycle. Over time, as the cell undergoes repeated charge and discharge cycles, the weak cell – being subjected to more cycles – tends to experience faster capacity degradation, which worsens the mismatch with the other, healthy, cells.
Cell impedance mismatch Apart from cell capacity, another important parameter of great concern is cell impedance. Similar to capacity mismatch, impedance mismatch occurs when a cell within a pack exhibits significantly different impedance compared to the others. Some engineers use the electrochemical impedance spectroscopy method to measure each cell’s impedance and assess its health. A healthy or relatively new cell typically has lower impedance, whereas an ageing or unhealthy cell tends to have higher impedance; see Figure 2. If a particular cell within the pack has significantly higher
impedance than the others, it is referred to as an unhealthy cell for ease of discussion. Figure 2 shows this phenomenon by
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