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

Thread Starter

eznoa

Joined Mar 2, 2021
3
I have 2 18650 li-ion cells, each at 3.7v

(in case you don't see the pictures, they are available here : https://raspberrypi.stackexchange.com/posts/121847 )

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.
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 ?
 

Audioguru again

Joined Oct 21, 2019
2,875
A review shows that your "9800mAh" batteries are only 500mAh and are absolute GARBAGE! A rip-off. A name-Brand 18650 cell is truly 2000mAh to 3600mAh.
Why do people buy No-Name-Brand fake junk on ebay?

A "Power bank" has a 5V USB input, a single Lithium cell charger and protection circuit and a voltage booster/regulator producing a 5V output. It is less expensive to buy a Power Bank locally (guaranteed to work properly) than to buy cheap ebay parts that do not work and put them together.
 

LowQCab

Joined Nov 6, 2012
271
You can also get "Power-Banks" in a wide variety of mAh Capacities,
I've seen some that are almost comically Huge, and some worthless teeny-tiny ones.
And the prices continue to creep-down.
.
.
.
 

Thread Starter

eznoa

Joined Mar 2, 2021
3
Thank you for your wise advices. So you guys suggest me to use a 2P configuration instead of a 2S. I was wondering how to do it myself rather than buying a powerbank in order to keep control over the form factor to embed in my box. Also I'd like to do it the 2S way because I need to remove and borrow 2S trays from a community where it's a standard. Would you guys know how to do it the 2S way please? Regards
 

Audioguru again

Joined Oct 21, 2019
2,875
I bought a 16,000mAh BlueHive Power Bank 2 years ago on a Black Friday sale. It is too small to hold 18650 cells so it must have a Li-Po one cell pouch battery. I am charging it from USB on my pc at this moment.
 
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