It will warn you when it is time to land your helicopter
It will Buzz with very loud BB noise when any of the cells of your battery is under 3.3V, the volume is loud enough to alert you to land your helicopter immediately to avoid accident. You can customize the voltage to alert you with very loud noise and a big red LED light.

• Microprocessor controlled voltage detector
• Reverse polarity protection

• Dimensions: 80mm x 25mm x 12mm
  
• Net Weight: 12g
  
• Battery Range: 2 to 6 Cell LiPo, LiFe
  
• Voltage Display Resolution: 0.001V
  
• Voltage Detection Precision: 0.005V
  
• Cell Display Range: 1.5 to 4.99V
  
• Total Voltage Display: 3 to 4V
  
• Consumption: 8mA

• 1 x Li-Polymer Battery Low Voltage Tester / Buzzer / Alarmer 2-6S (7.4V - 22.2V)

** color of this device might be different (green or red electronic board) sometime but it is the same.

** there was a customer reported that he tested this device with a battery closed to 3.0-3.3V and this device does not work because the battery had no more power to drive this device to work. I have no idea which battery did he use but please keep in mind that a Li-Po Battery with lower than 3.7V is already risky to run out (Battery could die easily). For safety purposes, a Li-Po battery should keep in Voltage between 3.7 - 4.2V. Lower or More in voltage could damage the battery's life.