This is the Wltoys XK A180 Upgrade Alarm for the Battery,which Can test whether your battery has enough power or not.
We guess that all players don't want the glider to fall down because of a sudden loss of power when it is halfway in the sky? And this small accessory plays a decisive role. You can freely set the value of the alarm voltage. Then when the battery power drops and the voltage is lower than the alarm voltage, the alarm will sound "dididi".
This alarm can be heard within 50 meters. Lightweight. It can be easily bound or pasted onto the glider.