Battery Management System
Battery Management System (BMS) controls the state of battery through voltage, current, temperature, state of charge (SOC), state of health (SOH) and etc. Because the individual battery cells is to be required to operate normally and the lifetime of the battery cell may be decreased by the working conditions, a sufficient BMS should be adapted to the battery applications. In addition, because the battery cells are used in serial-parallel structure (battery pack) in the high power applications such as electric vehicles, the charge equalization (cell balancing) technique is also enclosed in the category of BMS.
Stage of battery handled by BMS
Voltage : the battery pack voltage, the voltage of individual battery cells, minimum and maximum cell voltage (safe range)
Current : Current in and out of the battery
Temperature : temperatures of individual battery cells and battery pack, coolant temperature
SOC : the charge level of the battery cells
SOH : overall conditions of the battery cells
To increase the battery power, individual battery cells are used in serial-parallel structure. In this case, as the number of charging/discharging periods increases, the unbalanced cells may limits the use of the battery power considering the safe range. Therefore, the cell balancing circuit is required to prevent the cells from unbalancing. Normally, cell balancing circuit is divided by the passive cell balancing and the active cell balancing. In the passive cell balancing, the energy of overcharged cells dissipated by resistors. On the other hand, the energy of overcharged cells is consumed by the undercharge cells.