Cell-balancing algorithms that use only voltage divergence as a balancing standard have the disadvantage of overbalancing (or under balancing) because of the impedance-imbalance effect (Figures 3 " 4). The problem is that cell impedance also contributes to voltage divergence (VDiff_Start and VDiff_End) during charging. The simple voltage-based cell balancing does not distinguish between a capacity or impedance imbalance. Therefore, this balancing cannot guarantee that, at full charge, all the cells are at 100% capacity.

Figure 3: Passive bleed balancing based on voltage

Figure 4: Simple voltage-based balancing may not effectively balance capacity

One solution is a fuel gauge, such as the bq2084, with improved voltage-based balancing. Since the impedance differences between cells can mislead the algorithm, it only balances near the end of the charge cycle. This action minimizes the effect of impedance differences because the IRBAT drop becomes smaller when the charging current tapers toward the termination threshold. Additionally, the IC makes the balancing decision based on all cell voltages, so it is a more efficient implementation. Despite the improvements, the need to rely on voltage levels alone limit the balancing operation to high-SOC regions and only while charging.