Make a series of indicators for a full 3.7V battery - red, LOW, green, FULL 

how to make a full 3.7v lithium battery indicator circuit
sampurasun.
This full battery circuit functions to determine whether the battery is still full or low, with indicators like this we are no longer confused.
ingredients
BD139 NPN transistor
1K & 680 ohm resistor
5mm 3V green & red led
18650 3.7V lithium battery
Thank you, hope it's useful