Turn on a device if voltage drops below a configurable level?
The zener sets the reference voltage. The pot sets the trigger voltage. In this circuit, if the pot (v-) voltage drops lower than the reference, then the LED turns off. If you want it to turn on, then reverse the V+ and V- terminals to the comparator.

Consider that if this is done with a Li-po, there is a possibility that current will be drawn when the battery is already low, potentially lowering the battery voltage to dangerous levels. 


Source: https://www.electroschematics.com/4004/low-battery-indicator/