Cell-Balancing in the bq208x Advanced Gas Gauge Solutions

Serial connection of cells in a battery pack can result in reduced-capacity performance if some of the cells have different capacity or charge levels. The problem is that during charging or discharging of the pack, individual cell differences lead to different voltages in each cell. Since the charger monitors only the summary voltage, some cells can be undercharged and others overcharged. Undercharging amounts to a decrease of overall pack capacity, and overcharging can lead to damage of the cells and can create safety issues. To address this known problem of serial battery packs, the bq208x advanced gasgauge IC in a design with the bq29311 analog front end (AFE) offers a unique capability to monitor the voltage of each cell separately and to bypass part of the charging current of each single cell to achieve a uniform state of charge for all cells. This application report explains the principles of cell-balancing operation, addresses implementation issues specific to cell balancing, provides a detailed description of a test procedure, gives the results of our testing demonstrating the efficiency of the procedure, and provides a guide for choosing correct values for user-defined registers controlling the cell-balancing algorithm.