Recent electric vehicles use Li-ion batteries to power the main electric motor. To maintain the safety of the main electric motor battery using Li-ion cells, it is necessary to monitor the voltage of each cell. DENSO has developed a battery Electronic Control Unit (ECU) that contributes greatly to the reduction of the cost and the improvement of the reliability of the system. Each manufacturer has been developing a dedicated IC for monitoring the voltages of each cell of a battery. However, since the number of cells that can be monitored is limited, more than one IC is required to measure the voltages of a large number of cells. The increase in the number of ICs and the amount of insulator leads to the rise in system cost. DENSO has developed a dedicated IC that uses a proprietary high-breakdown voltage process, and which enables monitoring up to 24 cells with a single IC chip. A battery management ECU using this type of IC helps reduce the cost and physical size of the system by decreasing the numbers of ICs and the amount of insulator. We have also succeeded in cutting down on the variations in the current consumption of the main battery by providing each monitoring IC with power from an insulated power supply on the low-voltage side, which is based on the fact that variations in the current consumed by the monitoring ICs is one of the causes for non-uniform cell voltages. Controlling the variations has resulted in decreasing the equalizing discharge current, which reduces the amount of heat generated inside the battery management ECU and enhances the reliability of the unit that is used under severe conditions where high voltage is constantly applied.