Large, series connected strings of batteries are commonly used in electric vehicles, backup power systems and a wide variety energy storage applications. Maximizing the lifetime and ensuring safe usage of such battery stacks requires accurate measurement and balancing of each cells’ state of charge (SoC). Passive, or dissipative, balancing can correct for SoC mismatch due to temperature gradients or cell to cell impedance differences throughout the pack. However, passive balancing cannot compensate for capacity differences that result from cell aging. Cell aging occurs in all cells and occurs at different rates due to the same factors that cause SoC mismatch. Without capacity compensation, the runtime of the battery is limited by the lowest capacity cell in the stack. Active balancers such as the LTC3300 have the ability to correct for SoC imbalance as well as compensate for cell to cell capacity differences. By efficiently redistributing charge from mismatched cells, the LTC3300 maximizes the useable capacity of the battery stack. The LTC3300 provides high current, high efficiency bi-directional cell balancing for series connected batteries. Each IC can simultaneously charge or discharge up to 6 series cells. There is no limit to the height of the stack. Balancer to balancer communication is achieved through a high noise margin SPI bus, and numerous safety features ensure reliable, efficient, high current active balancing.