The high-voltage battery in the BMW ActiveHybrid 5 supplies 320 volts nominal, which is constantly stressed through energy consumption (electric driving), energy recovery (braking), and during charging. The individual cells charge at different rates, depending on the degree of aging and the manufacturing tolerance of the cells. The different charge states are equalized by a passive balancing system. This requires continuous monitoring of cell voltage and temperature by Cell Supervising Sensor Units and processing of the data by a Battery Management Unit, both developed by Preh.

During passive balancing, the charge of all cells is adjusted by specifically discharging them to the level of the cell with the lowest charge. In this way, the charging of the entire battery is done uniformly and optimal battery performance is achieved.