This device can be used as a battery tester, or installed in your electric plane and used as a 2-3S Li-Po alarm.  It features two rows of LEDs, one green, the other red, and an extremely loud piezo buzzer.  The row of pins along the edge are designed to plug into the battery tap of the Li-Po battery, and will indicate the battery's voltage and condition.  
 
A fully charged 2S Li-Po will indicate with two green LEDS, a 3S will show three green LEDs. As the voltage drops, the red LEDs will come on, and once the voltage drops below 3 volts per cell, the piezo buzzer will begin to sound.  This unit is enclosed in clear heat-shrink.