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