FrSky FBVS01 Battery Voltage Sensor
BATTERY VOLTAGE SENSOR:
This uses a simple voltage divider circuit to measure the pack voltage on your aircraft. It can measure up to 5S LiPo as it is. You can also measure higher voltages, but you will need to modify the circuit to do so. This plugs directly into a D8R or D4R-II or newer series* receiver's voltage monitor port.
Does NOT require Sensor Hub.