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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.

Edit: I added pictures. circuit schematic battery battery2 battery3

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    \$\begingroup\$ Hi! Welcome to EE.SE. Add a link to (preferably a datasheet of) your pack. Use the edit button rather than responding in comments =). \$\endgroup\$ – K H Apr 16 at 5:14
  • \$\begingroup\$ @KH my pack doesn't have a datasheet because it's handmade. I added pictures and circuit schematics. \$\endgroup\$ – Nimda Apr 16 at 11:10
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    \$\begingroup\$ "Where should I connect the wires of USB port for charging?" - You can't charge a 7.4V battery from a 5V supply without some additional electronics. Also, your circuit diagram is wrong. The way it's drawn, there would be about 0V at the input of the XL4005 at all times. \$\endgroup\$ – JimmyB Apr 16 at 12:52
  • \$\begingroup\$ @JimmyB I painted diagram only for showing a berief of the circuit and if there is a mistake ignore it please. How should I charge the batteries ? Why you say 5V supply ? I didn't tell anything about supply. \$\endgroup\$ – Nimda Apr 16 at 14:08
  • \$\begingroup\$ "Why you say 5V supply ?" - Because you said you want to use a "USB port for charging". However, if you do charge with the appropriate voltage and current you should be able to connect the charging circuit directly to the batteries, i.e. to the inputs of the XL4005. \$\endgroup\$ – JimmyB Apr 16 at 15:21

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