3.7V Li-ion & LiPo battery voltaj indicator circuit

How to make a voltage indicator for 3.7V  Li-ion and Li-Po batterys. Low and Full level indicator circuit.

1 x  Green Led
1 x  Red Led
1 x  BJT NPN N5551 Transistor  (BJT NPN BC548 Transistor can also be used)
1 x  1N4004 Diode (1N4001-4007 can also be used)
1 x  220R resistor
1 x  820R resistor
1 x 1K resistor

Suitable for 1S (3.7V) LiPo and Li-ion battery.

For a 1S battery, the fully charged voltage value is 4.2V. This circuit activates the green led between 3.7V and 4.2V. This means the battery can be used. The voltage value that is considered empty is 3.3V. In order for a battery to have a long life, it should not be used at voltages below 3.3V

This indicator gives a warning between 3-7V and 3.3V by enabling both leds to be active together. When only the red led lights up, the critical level has been reached. In other words, a voltage value lower than 3.3V has been reached.