You have the option to power the board via a USB cable or by attaching an external power source to the IN+ and IN- pads on the left-hand side. The lithium battery is connected to the BAT+ and BAT- pads on the right-hand side. If you are using the board with the protection circuit, you can connect the output to the OUT+ and OUT- pads.
The voltage on a lithium battery ranges from 4.2 V when fully charged to 2.7 V (this varies by battery). You’ll need a circuit that will lower the voltage when the battery voltage is higher than 3.3 V and boost the voltage when the battery voltage is below 3.3 V. A 3.3 V buck-boost converter solves this problem.
The lithium battery is connected to the BAT+ and BAT- pads on the right-hand side. If you are using the board with the protection circuit, you can connect the output to the OUT+ and OUT- pads. Connect the output wires to the BAT+ and BAT- if your board does not have a protection circuit.
When charging a battery using the above board connect the battery to B+ and B- and disconnect OUT+ and OUT- from your circuit. When using the battery disconnect the 5V input and take the output voltage from OUT+ and OUT- to your circuit.
This is a boost converter meaning that it will take lower voltage and convert it into higher voltage. To adjust the voltage we have to do a couple of steps. Connect the converter with the battery or other power source. Set the multimeter to read the voltage and connect the output of the converter to it.
Solder the OUT+ wire to the IN+ pad and the OUT- to the IN- pad on the boost converter. Solder two wires to the output pads on the boost converter. Use your multimeter to find the correct pads if they are not labeled. I used thicker 22 AWG solid wires since I will be connecting to a breadboard.