Capacity and efficiency of the High-voltage main battery used for driving the vehicle because of the development of envi-ronmentally-friendly vehicles is becoming more and more improved.,but the vehicle electrical systems are still use the Lead-Acid battery.due to the problem of the poor start of the vehicle,it cannot be free the use of Lead-Acid battery used in HEV/PHEV/EV.So,to solve above problems by using the main battery to be mounted on xEV the ability to charge/diagnosis of the auxiliary battery is developed.