Battery Warning Alarm Buzzer and Digital Display 1 - 8 cell . 
This is Very loud and is an essential piece of equipment in anybodys kit. 

1S test mode test range 3.7 to 30V, which means you can test the battery packs without balance connectors like RX battery pack etc., LiPb or other power source voltage. (acts as a simple digital voltage meter).
Loud buzzer can be heard from a distance - you can put this device on your plane and use it as on-board alarm, so you can hear the low voltage alarm during flight.
- Use for 1-8S Lipo/LiMn/Li-Fe
- Voltage detection precision ±0.01V
- Unit voltage display range 0.5-4.5V
- Total voltage display range: 0.5 - 36V
- 1S test mode voltage range: 3.7-30V
- Low voltage alarm mode for 2-8S
- Alarm set value range: OFF-2.7-3.8V
- Weight: 8.7g/ 0.31oz