Passive balancing could theoretically equalize each cell's SoC during the stack charging phase, but could do nothing to prevent cell 10 from reaching its 30 percent SoC level before the others during discharge. Even with passive balancing during stack charging, significant capacity is “lost” (not usable) during stack discharge. Only an active balancing solution can achieve “capacity recovery” by redistributing charge from high SoC cells to low SoC cells during stack discharging.
illustrates how the use of “ideal” active balancing enables 100-percent recovery of the “lost” capacity due to cell-to-cell mismatch. During steady state use when the stack is discharging from its 70-percent SoC “fully” recharged state, stored charge must in effect be taken from cell 1 (the highest capacity cell) and transferred to cell 10 (the lowest capacity cell) – otherwise cell 10 reaches its 30 percent minimum SoC point before the rest of the cells, and the stack discharging must stop to prevent further lifetime degradation. Similarly, charge must be removed from cell 10 and redistributed to cell 1 during the charging phase – otherwise cell 10 reaches its 70-percent upper SoC limit first and the charging cycle must stop.
Figure 3: Capacity recovery due to ideal active balancing
At some point over the operating life of a battery stack, variations in cell aging will inevitably create cell to cell capacity mismatch. Only an active balancing solution can achieve “capacity recovery” by redistributing charge from high SoC cells to low SoC cells as needed. Achieving maximum battery stack capacity over the life of the battery stack requires an active balancing solution to efficiently charge and discharge individual cells to maintain SoC balance throughout the stack.