The batteries used in electric and hybrid vehicles consists of several cells with voltages between 3.6V battery and 4.2 V in series or parallel combinations of configurations for obtaining the necessary available voltages in the operation of a hybrid electric vehicle. How malfunction of a single cell affects the behavior of the entire battery pack, BMS main function is to protect individual cells against over discharge, overload or overheating. This is done by correct balancing of the cells. In addition BMS estimates the battery charge status.