I want to make my rpi 3 portable with adding a rechargeable battery pack to it. I have 6 Li-ION batteries that are packed together in 2 series rows of 3 batteries in parallel(each battery is a 3.7 2200mah Li-ION battery and their whole pack voltage is around 7.4v). There are only 2 red and black wires come out of the pack and the manufacturer told me that it has charge circuit in itself and the charging and discharging can happen from that 2 wires. I connected these wire to a XL4005 circuit for decreasing the voltage to 5v for connecting to rpi3. My question is that how should I charge these batteries when they are connected to XL4005? Where should I connect the wires of USB port for charging? Is there any other circuits needed for charging or a simple USB port is enough? Thank you.