Vehicle operation when the 48 V battery is in abnormal status is analyzed. In order to keep the vehicle to be driven as a traditional car, the integrated belt-driven starter/generator (IBSG) is no longer working in torque control mode, but in voltage control mode in this condition. For studying the influences of IBSG speed change and 12 V load change on the voltage control mode, validation is performed both in bench test and in vehicle test with 1.0 L engine. Results show that IBSG can enter the voltage control mode smoothly. The voltage control mode ensures that the vehicle keeps running when the 48 V battery is not on the DC bus. As a kind of auxiliary driving mode, this control mode enhances the security and robustness of the vehicle.