This is A.K.E PM3C 3cell Lithium Polymer battery monitor.

You can check the condition of your battery anytime and easlier, or you can install it on your EP plane or helicopter for on-board monitor.

*When the light in Green Colour: The battery is over 3.3V and in good conditions. 
*When the ligh change to Red Colour: The battery is under 3.3V and you must shop flying. 
***This is for 3cells 11.1V battery only.

Specifications: 
Weight：1.9g 
Dimension：28.5mm x 15mm x 4.6mm 
It applies for：3 cells with balance connecter