Active balancing solutions for series-connected batteries: Page 4 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.
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

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