Active balancing solutions for series-connected batteries
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 current remaining capacity of an individual cell relative to its maximum capacity as the cell charges and discharges. For example, a 10A-hr cell with 5A-hrs of remaining capacity has a 50% state of charge (SoC). All battery cells must be kept within an SoC range to avoid damage or lifetime degradation. The allowable SoC min and max levels vary from application to application. In applications where battery run time is of primary importance, all cells may operate between a min SoC of 20% and a max of 100% (or a fully charged state). Applications that demand the longest battery lifetime may constrain the SoC range from 30% min to 70% max. These are typical SoC limits found in electric vehicles and grid storage systems, which utilize very large and expensive batteries with an extremely high replacement cost. The primary role of the battery management system (BMS) is to carefully monitor all cells in the stack and ensure that none of the cells are charged or discharged beyond the min and max SoC limits of the application.
With a series/parallel array of cells, it is generally safe to assume the cells connected in parallel will auto-balance with respect to each other. That is, over time, the state of charge will automatically equalize between parallel connected cells as long as a conducting path exists between the cell terminals. It is also safe to assume that the state of charge for cells connected in series will tend to diverge over time due to a number of factors. Gradual SoC changes may occur due to temperature gradients throughout the pack or differences in impedance, self-discharge rates or loading cell to cell. Although the battery pack charging and discharging currents tend to dwarf 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 by 25% after a complete charge/discharge cycle relative to the theoretical usable capacity of the cells.
Figure 2 Stack capacity loss example due to cell to cell mismatch
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% 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.
Figure 3 illustrates how the use of “ideal” active balancing enables 100% recovery of the “lost” capacity due to cell to cell mismatch. During steady state use when the stack is discharging from its 70% 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% 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% 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.
High Efficiency Bidirectional Balancing Provides Highest Capacity Recovery
The LTC3300 (see Figure 4) is a new product 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.
Figure 4 LTC3300 high efficiency bidirectional multicell active balancer. For full resolution click here.
SoC balance is achieved by redistributing charge between a selected cell and a sub-stack of up to 12 or more adjacent cells. The balancing decisions and balancing algorithms must be handled by a separate monitoring device and system processor that controls the LTC3300. Charge is redistributed from a selected cell to a group of 12 or more neighboring cells in order to discharge the cell. Similarly, charge is transferred to a selected cell from a group of 12 or more neighbor cells in order to charge the cell. All balancers may operate simultaneously, in either direction, to minimize stack balancing time. All balancing control commands are delivered to each IC via a stackable, high noise margin serial SPI interface with no limit on the height of the stack.
Each balancer in the LTC3300 uses a nonisolated, boundary mode synchronous flyback power stage to achieve high efficiency charging and discharging of each individual cell (see Figure 5). Each of the six balancers requires its own transformer. 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 – including the cell to be balanced. The number of cells on the secondary side is limited only by the breakdown voltage of the external components.
Figure 5 Bidirectional flyback power stage operation. For full resolution click here.
Cell charge and discharge currents are programmed by external sense resistors to values as high as 10+ amps with corresponding scaling of the external switches and transformers. Sequencing and IPEAK/IZERO current detection through the primary and secondary components depends on whether a balancer is enabled to charge a cell or discharge a cell. High efficiency is achieved through synchronous operation and the proper choice of components. Individual balancers are enabled via the BMS system processor, and they will remain enabled until the BMS commands balancing to stop or a fault condition is detected.
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 must also be high in order to extend run times or to achieve fast charging of the pack. Poor balancer efficiency results in unwanted heat inside the battery system, and must be addressed by reducing the number of balancers that can run at a given time or through expensive thermal mitigation methods.
Figure 6 LTC3300 power stage performance
As shown in Figure 6, 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 with equal balancer power dissipation. Furthermore, higher balancer efficiency produces more effective charge redistribution, which in turn produces more effective capacity recovery and faster charging.
Local Cells Do Most of the Balancing Work
Transferring charge throughout the stack is achieved by interleaving the secondary side connections as shown in Figure 7. Interleaving in this manner allows charge from any group of six cells to be transferred to or from a group of adjacent cells. Note that the adjacent cells may be either above or below in the stack. This flexibility is helpful when optimizing a balancing algorithm. A common misconception with any interleaved system is that redistributing charge from the top of a very tall stack to the bottom must be horribly inefficient due to all the conversions necessary to move the charge from top to bottom.
Figure 7 Interleaving connections and charge transfer performance. For full resolution click here.
However, as shown in the Figure 7 example, most of the balancing is accomplished simply by redistributing charge to or from the cells closest to those in need of balancing. A secondary side stack of 10 or more cells allows a weak cell, which would otherwise limit the run time of the whole stack, to recover over 90% of its “lost” capacity simply by running one balancer. Hence, with the LTC3300 interleaved topology, there is no need to move charge all the way from the top of the stack to the bottom – most of the balancing work is done by the local neighbor cells.
Safety Comes First
In addition to providing excellent electrical performance, the LTC3300 bidirectional active balancer provides numerous safety features to prevent mishaps during balancing and to maintain the highest possible reliability. Data integrity checks (CRC checking on all incoming and outgoing data, watchdog timer, data read back) guard against balancer response to unintended or erroneous commands. Programmable volt-second clamps ensure that current sensing faults during balancing do not result in runaway current conditions. Cell by cell over- and undervoltage checking, as well as secondary side overvoltage detection, prevent sudden battery wiring harness faults from causing damage to the balancing circuits during balancing.
These characteristics enable the LTC3300 to provide both high performance and reliable active balancing in series-connected battery systems. As the cells in such systems age or need replacing, it becomes increasingly important to compensate for the resulting mismatch in cell capacities without further compromising run time, charge time, or the lifetime of the battery pack. The LTC3300 was designed specifically to address this challenge, providing designers with a new level of safety and charge efficiency.
About the author: Samuel Nork is Director, Boston Design Center, Linear Technology Corporation
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