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 typically comprised of series / parallel arrays of lithium polymer or LiFePO4 cells due to their high energy density and 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 need cell balancing.
All Series-Connected Cells Need to be Balanced
The cells in a battery stack are “balanced” when every cell in the stack possesses the same state of charge (SoC). SoC refers to the remaining capacity of an individual cell relative to its maximum capacity as the cell charges and discharges. All battery cells must be kept within a SoC range to avoid damage or lifetime degradation.
The relative SoC for cells connected in series will diverge over time due to temperature gradients or differences in impedance, self-discharge rates or loading cell to cell. A passive balancing scheme is a simple and inexpensive way to force all cells to match the lowest SoC cell. However, passive balancing is slow, generates unwanted heat, and does not address SoC differences due to capacity mismatch cell to cell. All cells lose capacity as they age, and they do so at different rates. Since the stack current flows into and out of all cells equally, the usable stack capacity is determined by the lowest capacity cell in the stack. Only active balancing methods can compensate for “lost” stack capacity due to cell mismatch.
Cell to Cell Mismatch Reduces Run Time
Cell to cell mismatch may severely reduce the usable battery stack capacity unless the cells are balanced. As shown in Figure 1, a 10-cell stack comprised of 100A-hr cells with a +/- 10% capacity error is charged and discharged until 30% and 70% SoC limits are reached. If no balancing is performed, the usable stack capacity is reduced by 25%. Passive balancing may eventually equalize SoC during the charging phase but cannot prevent cell 10 from reaching 30% SoC before the others during discharge. Figure 2 illustrates how “ideal” active balancing enables “lost” capacity to be recovered. When the stack is discharging, charge is added the lowest capacity cell(s) such that all cells reach 30% SoC at the same time. Similarly, charge is removed from the lowest capacity cell(s) during charging to allow all cells to reach 70% SoC together. Over the operating life of a battery stack, variations in cell aging will create capacity mismatch. Only active balancing provides “capacity recovery” by redistributing charge as needed to maintain SoC balance throughout the stack.
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Figure 2. Capacity recoveries due to ideal active balancing
High Efficiency Bidirectional Balancing Provides Highest Capacity Recovery
The LTC3300 (see Figure 3) is designed specifically to address the need for high performance active balancing. The LTC3300 is a high efficiency, bidirectional active balance control IC that is a key piece of a high performance BMS system. Each IC can simultaneously balance up to 6 Li-Ion or LiFePO4 cells connected in series.
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Figure 3. LTC3300 high efficiency bidirectional multicell active balancer
SoC balance is achieved by redistributing charge between a selected cell and a sub-stack of 12 or more adjacent cells. Balancing decisions and algorithms are handled by the BMS that controls the LTC3300. Charge is redistributed from a selected cell to a group of 12 or more neighbor cells to discharge the cell. Similarly, charge is transferred to a selected cell from a group of 12 or more cells to charge the cell. All balancers are controlled via stackable SPI interface.
Each balancer in the LTC3300 uses a nonisolated, synchronous flyback power stage to achieve high efficiency charging and discharging. The “primary” side of each transformer is connected across the cell to be balanced, and the “secondary” side is connected across 12 or more adjacent cells. The number of secondary cells is limited only by the breakdown voltage of the external components. Sequencing and IPEAK/IZERO current detection depends on whether a balancer is charging or discharging. If any type of fault is detected, balancing will automatically stop.
Balancer Efficiency Matters!
One of the biggest enemies faced by a battery pack is heat. High ambient temperatures rapidly degrade battery lifetime and performance. Unfortunately, in high current battery systems, the balancing currents are also high. The LTC3300 achieves >90% efficiency in both the charging and discharging directions, which allows the balance current to be more than doubled relative to an 80% efficient solution.
Local Cells Do Most of the Balancing Work
Transferring charge throughout the stack is achieved by interleaving the secondary connections. Interleaving in this manner allows charge from any group of six cells to be transferred to or from a group of adjacent cells either above or below in the stack. This flexibility helps optimize the balancing algorithm. As shown in Figure 4, it is not necessary to move charge via multiple conversions all the way from the top of the stack to the bottom – most of the balancing work is done by the local neighbor cells.
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Figure 4. Interleaving connections and charge transfer performance
Safety is Key
In addition to providing excellent electrical performance, the LTC3300 provides numerous safety features to maintain the highest possible reliability. Data integrity checks, programmable volt-second clamps, cell over- and undervoltage checking, as well as secondary side overvoltage detection prevent data or wiring faults from causing damage. These characteristics enable the LTC3300 to provide high performance and reliable active balancing in series-connected battery systems. With the LTC3300, mismatches in cell capacities are compensated without compromising run time, charge time, or the lifetime of the battery pack.