Small device to ensure your RC model has enough power when playing Low voltage alarm buzzer for 7.2v-14.4v lipo battery Quick test for your lipo before flight Used as inflight alarm if you are not sure the maximum flight time Small device to check the battery voltage to avoid permanent damage How To Use: When testing full power(>3.6V) 2S(7.2V) lipo battery - 2 green led keeping light When testing full power(>3.6V) 3S(10.8V) lipo battery - 3 green led keeping light When testing full power(>3.6V) 4S(14.4V) lipo battery - 4 green led keeping light When voltage <3.3v， it will buzz with red led light. Really useful in flight if you can't see the light.