When you connect your 7.4v or 11.1v battery to your ESC, the internal BEC of your ESC only generates 6v power to your receiver which use to support most of your electronics on the car.
Unless you use RX8 Gen2 or the VTX8 which allow you to adjust the internal BEC up to 7.4v and 7v, you will need an external BEC to support HV servos.
