As illustrated in Figure 1A, passive balancing is simple and inexpensive. However, passive balancing is also very slow, generates unwanted heat inside the battery pack, and balances by reducing the remaining capacity in all cells to match the lowest SoC cell in the stack. Passive balancing also lacks the ability to effectively address SoC errors due to another common occurrence: capacity mismatch. All cells lose capacity as they age, and they tend to do so at different rates for reasons similar to those listed above. Since the stack current flows into and out of all series cells equally, the usable capacity of the stack is determined by the lowest capacity cell in the stack . Only active balancing methods such as those shown in Figures 1B and 1C can redistribute charge throughout the stack and compensate for lost capacity due to mismatch from cell to cell.
Figure 1a/b/c Typical cell balancing topologies
Cell to Cell Mismatch Can Dramatically Reduce Run Time
Cell to cell mismatch in either capacity or SoC may severely reduce the usable battery stack capacity unless the cells are balanced. Maximizing stack capacity requires that the cells are balanced both during stack charging as well as stack discharging.
In the example shown in Figure 2, a 10-cell series stack comprised of (nominal) 100A-hr cells with a +/- 10% capacity error from the minimum capacity cell to the maximum is charged and discharged until predetermined SoC limits are reached. If SoC levels are constrained to between 30% and 70% and no balancing is performed, the usable stack capacity is reduced