Feature: Power
representing the cell as a simplifi ed circuit model with a capacitor and a resistor in series during charging and discharging. It is important to note that this abstraction is a necessary simplifi cation for discussion purposes in this article; while suitable for illustrating impedance mismatch eff ects, it does not represent the actual physical and electrical characteristics of a real cell. During charging, an unhealthy cell with higher internal
impedance experiences a greater voltage drop under a given charging current. In this case, if all cells exhibit the same voltage value, the unhealthy cell will actually store less energy. As shown in the fi gure, the unhealthy cell has a smaller Vcell_actual
charging. Additionally, due to the higher power loss caused by its impedance, the unhealthy cell typically experiences a higher charging temperature. During discharging, a higher impedance results in a greater
voltage drop and higher power dissipation under a given discharge current. Consequently, the unhealthy cell experiences a more rapid decline in voltage and capacity and generally operates at a higher discharge temperature. Over time, with repeated charge- discharge cycles, the higher temperature and ageing eff ects further accelerate impedance increase in the unhealthy cell, exacerbating the impedance mismatch issue within the battery pack. By analysing both capacity and impedance mismatches, you
will notice that, although they represent diff erent aspects of cell imbalance, their ultimate eff ects are quite similar. Whether it is a weak cell with lower capacity or an unhealthy cell with higher impedance, both impact the usable capacity and operating voltage of the battery pack. In a battery pack with weak or unhealthy cells, the overall capacity utilisation and safe operating time are signifi cantly reduced. Moreover, these mismatched cells are a continuous threat to the safety and normal operation of the well- performing cells within the pack.
The critical importance of passive/active balancing Passive balancing is a dissipative method that typically operates
Figure 1: Impact of capacity-mismatched cells during battery pack charging and discharging
during the charging cycle. Since weak cells have lower capacity, their voltage rises faster under the same charging current. When they approach full charge fi rst, the excess energy must be immediately dissipated. Although this energy dissipation leads to heat generation and thermal management challenges, it extends the charging time for healthy cells, ultimately improving the overall run time of the battery pack. Passive balancing is widely adopted in BMS, with most cell
value during
monitoring ICs already integrating this functionality. Active balancing, on the other hand, transfers energy between cells using transformers, capacitors and inductors. T is method works during both charging and discharging cycles, redistributing charge effi ciently. While both passive and active balancing have their advantages and disadvantages (shown in Table 1), the choice of balancing method in practical BMS design is not simply based on a direct comparison of their pros and cons. Instead, it depends on the capacity and scale of the battery system. Generally, the balancing current is set to about 1-5% of the cell capacity. For example, in a 4Ah lithium cell, if the balancing charge
www.electronicsworld.co.uk November 2025 27
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