Recently, Almost HV, PHEV, EV veheicle use a Li-ion battery as a main battery. In case of the main battery with Li-ion battery, it is neccesarry to monitor all cell voltage in order to secure a safety of Li-ion battery. In this paper, DENSO report that new development ECU which is contributed to the reduction of system cost and the inprovement of reliability. Many semiconductor suppier develop various monitoring IC, but a measurable cell voltage number is insufficient. In order to measure many cell voltage in seriese, many monitoring ICs are needed for battery management system. So the system cost increases due to increase of IC number and insulation device. Therefore DENSO develop the monitoring IC with the original high voltage process of DENSO. This monitoring IC measures 24 cells voltage. The battery management ECU that DENSO developed is contributed to the system cost reduction and the size reduction, because the number of monitoring IC and insulation device for the battery management ECU can declease. And DENSO focus that one of contributor for cell voltage variation is the variation of the current comsumption of monitoring IC. So DENSO acomplish that the variation of current comsumption of the main battery is decreased, by the power supply of the monitoring IC through low voltage side with the insulated power supply. As a result, the internal temperature rising can be reduced by reducion of the balancing current, the reliability of the battery management ECU which is used in the full-time high voltage environment is improved.