It is used to indicate whether the single-cell lithium battery (4.2) is insufficient when it is not necessary to accurately know the power of the lithium battery.

Low cost, easily accessible components, and ultra-small size.

Connect the module directly to the lithium battery. When the voltage of the lithium battery is low, the LED light will be on.

Low battery threshold voltage is adjustable, according to the current schematic is about 3.3V. 

The specific calculation formula is: target voltage = 2.5 / R3 * (R2 + R3)

Ultra-low power consumption, when the LED light is off, the power consumption is less than 1 mA. 

Therefore, this module can always be connected to the lithium battery without worrying about consuming the lithium battery.