how to properly make a charging circuit -- 2s 7.4V to 5V 2A

Question

I have 2 18650 li-ion cells, each at 3.7v

and this bay with this white connector

I saw on youtube that depending on the length of the cell, we can know whether they have integrated "invisible" BMS inside, in a flat circular pcb of the same diameter as the cell, provided the length of the cell. Mines are 64.8mm tall (but still, I don't know if they have integrated BMS as I can't open them).

I'd like to be able to plug and hide the bay in a plastic case.

Now my issues are that:

1. I want to know which modules I need to buy in order to make a circuit able to charge the 2cells bay out of a single USB(-C) port.
2. The bay is here to power a raspberry-pi zero through its GPIO, at 5V +-5% and 2A, because the pi has to:

• be overwhelmed itself by services spinning in it
• power an USB device over GPIO
• power a wifi antenna dongle over GPIO
• share data with a phone over one of the two micro-USB ports (idk if I can prevent it from charging the phone though (I'd like to just transfer data))

3. So adding a USB-C port for charging the overall system is what I intend to do, but still I'm curious to know whether I could charge the whole thing through the remaining unpopulated micro-USB port of the pi...
4. I need a power button to cut the contact between the pi zero and the battery tray when I want to shut off my whole device. Led would be appreciated.

To do so I found this guide :

1. https://www.youtube.com/watch?v=opYVS0EXZIg that uses this module on the below picture, it has everyting : charge and discharrge, led, powerbutton, BUT can only handle 3.7 battery and not my 6.6 up to 8.4V battery tray.

2. This https://www.youtube.com/watch?v=7AFbrKi0t_4 also seems to only handle 3.7V
3. I do found this : https://www.youtube.com/watch?v=CzKiflfyeMk but it requires to solder onto the cells, and I want them to be removable unfortunately
4. Finally I found a guy that used a USB-C port -> that goes into a buck converter outputing 8.4V -> that goes into a 2S BMS (however I looked at BMS on the market and it start at "2S 3A" so idk how to choose the right one) -> that manages the 2 cell battery tray -> then he didn't power some basic arduino/pi, but I guess I'd have to add another buck converter from 8.4V down to 5V ? (but how do I do if the input tension vary from 6.6 up to 8.4?)

Question :

So I'm a bit lost, what modules should I assemble in between the tray and the pi please? What wires diameter should I pick to get these 2A ?

Answer 1

If you have no experience in electronics, the easiest solution is to get a powerbank. There are even powerbanks which accept 18650 cells, such as this one (google "8pcs 18650 powerbank"):

Be careful when you put the cells in it: if you plug a cell which has a substantially different charge level it may get damaged, and plugging a cell with reverse polarity is an easy way to start a fire. You can put up to 8 cells in it, but it will work fine with just two. Put some foam / cardboard paper as a placeholder for unused cells.