Figure 1. Common cell balancing topologies.
As with single-cell applications, careful charge control and cell monitoring are essential to ensure safe operation and prevent premature aging or battery damage.
However, unlike single-cell systems, series-connected battery stacks add a new requirement: cell balancing.
All cells connected in series must be balanced.
The cells in a battery stack are “balanced” when each cell has the same state of charge (SoC). SoC represents the remaining capacity of a given cell relative to its maximum capacity as it is charged and discharged. All cells in the battery must be kept within a specific SoC range to prevent damage or degradation over their operational life. Applications requiring the longest possible battery life may use a range of 30% to 70% SoC for all cells. These are typical SoC limits in electric vehicles and grid-tied storage systems, which use very large and expensive batteries.
The relative SoC for cells connected in series will spread out over time due to temperature gradients or impedance differences, self-discharge rates, or cell-to-cell charging. A passive balancing technique, such as the one shown in Figure 1A, is a simple and inexpensive way to force all cells to match the cell with the lowest SoC. However, passive balancing is slow, generates unwanted heat, and does not address the SoC differences caused by unequal cell capacity. All cells lose capacity as they age, and they do so at different rates. Because the stack current flows equally to and from the cells, the usable stack capacity is determined by the lowest-capacity cell in the stack. Only active balancing methods, such as those shown in Figures 1B and 1C, can compensate for the "lost" stack capacity due to cell mismatches.
Cell mismatch reduces operating time.
Cell mismatch can severely reduce the usable capacity of a battery stack unless the cells are balanced. In the example shown in Figure 2, a 10-cell stack made up of 100 Ah cells with a capacity error of +/- 10% is charged and discharged until the 30% and 70% SoC limits are reached. Without balancing, the usable stack capacity is reduced by 25%.
Passive balancing can eventually equalize the SoC during the charging phase, but it cannot prevent cell 10 from reaching a 30% SoC before the others during discharge. Figure 3 illustrates how “ideal” active balancing allows for the recovery of “lost” capacity. When the stack is discharging, charge is added to the cell(s) with the lowest capacity so that all cells reach a 30% SoC at the same time. Similarly, charge is removed from the cell(s) with the lowest capacity during charging to allow all cells to collectively reach a 70% State of Charge (SoC).
Over the operational life of a battery stack, variations in cell aging will create mismatches in its capacity. Only active balancing provides "capacity recovery" by redistributing the charge as needed to maintain SoC balance across the entire stack.
High-efficiency bidirectional balancing offers maximum capacity recovery.
The LTC3300 (see Figure 4) is a new product specifically designed to meet the need for high-performance active balancing. The LTC3300 is a high-efficiency bidirectional active balancing control IC that forms a fundamental part of a high-performance battery management system. Each IC can simultaneously balance up to six Li-Ion or LiFePO4 cells connected in series.
SoC balancing is achieved by redistributing the charge between a selected cell and a substack of 12 or more adjacent cells. Balancing decisions and algorithms are handled by the battery management system that controls the LTC3300. Charge is redistributed from a selected cell to a group of 12 or more neighboring cells to discharge the cell. Similarly, charge is transferred to a selected cell from a group of 12 or more cells to charge it. All balancers are controlled via a stackable SPI interface and can operate simultaneously to minimize balancing time.
Each LTC3300 balancer uses a non-isolated, synchronous flyback power stage to perform high-efficiency charging and discharging (see Figure 5). Each balancer requires its own transformer.
The "primary" side of each transformer is connected to the cell to be balanced, and the "secondary" side is connected to 12 or more adjacent cells. The number of secondary cells is limited only by the breakdown voltage of the external components. The charging and discharging currents (up to 10 A+) are programmed using external sensing resistors. The sequence generation and current detection of IPICO/ICERO depend on whether the balancer is charging or discharging.
High efficiency is achieved through synchronous operation and appropriate external components. If any fault is detected, the balancing process will stop automatically.
The efficiency of the balancer is important!
One of the biggest enemies a battery pack faces is heat. High ambient temperatures rapidly degrade battery life and performance. Unfortunately, in high-current battery systems, balancing currents are also high.
Poor balancer efficiency generates unwanted heat inside the battery system. As shown in Figure 6, the LTC3300 achieves >90% efficiency in both the charge and discharge directions, allowing the balancing current to be more than double that of a solution with 80% efficiency.
Furthermore, greater balancer efficiency results in a more effective load redistribution, which leads to longer operating times.
Local cells handle most of the balancing.
Load transfer across the stack is achieved by interleaving secondary connections, as illustrated in Figure 7. This interleaving allows load to be transferred from any group of six cells to or from an adjacent group of cells, which can be located either above or below them in the stack. This flexibility helps optimize the balancing algorithm.
As shown in the example in Figure 7, it is not necessary to move load through multiple conversions from the top to the bottom of the stack; balancing is mostly done by neighboring cells.
Safety First.
In addition to providing excellent electrical performance, the LTC3300 offers numerous safety features to maintain the highest possible reliability. These include data integrity checks, programmable volt-second adjustment, cell overvoltage and undervoltage monitoring, and secondary-side overvoltage detection to prevent damage caused by data or wiring errors. These features enable the LTC3300 to deliver reliable, active, and high-performance balancing in series-connected battery systems.
With the LTC3300, mismatches between cell capacities are compensated for without affecting operating time, charging time, or battery pack lifespan.
Author
Samuel Nork, Director of the Design Center at Boston Linear Technology Corporation
