Active balancing solutions for series-connected batteries: Page 2 of 6

April 30, 2013 //By Samuel Nork
Active balancing solutions for series-connected batteries
Large, high voltage rechargeable battery systems are now common sources of power in applications ranging from electric vehicles to power grid load leveling systems. These large battery stacks are comprised of series / parallel arrays of individual battery cells, and are capable of storing enormous amounts of energy (tens of kilowatt-hours). Lithium polymer or LiFePO4 cells are common technology choices due to their high energy density and high peak power capability. As in single-cell applications, careful control of the charging and monitoring of the cells is essential to ensure safe operation and prevent premature aging or damage to the battery. However, unlike single-cell systems, series-connected battery stacks present an additional requirement: cell balancing.
these cell to cell variations, the accumulated mismatch will grow unabated unless the cells are periodically balanced. Compensating for gradual changes in SoC from cell to cell is the most basic reason for balancing series connected batteries. Typically, a passive or dissipative balancing scheme is adequate to re-balance SoC in a stack of cells with closely matched capacities.

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

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