You have the scheamtic below. As you can see the Bluetooth receiver and the amplifier have separated batteries, the Bluetooth reciever will already have its own small battery. That's because during tests, I realized having two batteries is better and we have less noise. But we do connect 5V from the USB input so we can charge both batteries at same time. Connect the audio output from the 3.5mm jack to the audio amplifier and then the speakers to the outputs. We add 3 LEDs that will later be glued on the exterior of the case. Those LEDs are for the charging process and the Bluetooth indicator so we could know when it's connected.