Active balancing solutions for series-connected batteries: Page 5 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.
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

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