2S 18650 solar charge

[panban]

Good day to all members.

I want to power an Rpi4 of the grid that will run with a 2 series 18650 through a power management board.

My setup is a 10w polycrystaline solar panel 18v (open circuit 24v) that connects to a CN3722 mppt board(not sure if its 2s or 3s) and then to a 2s HX-2S-JH20 BMS. After that to 2 series 18650 Panasonic batteries. Now as i had connected the setup i can see the charge of 8.2-8.6 v to the batteries. Don't know if 8.6 is safe for the batteries but at the same time the BMS start to smoke. Input to the BMS is the same after the mppt board 8.2-8.6v while the panel output is on the 22-24v range. Any ideas on how to approach a similar setup?
I will post photos later

 

[Substrate]

STOP. For your project and skill level, you need PROTECTED cells, especially if they are in series, bms or not.

Panasonic does not directly sell protected cells, but the protection circuit like those from Seiko are attached rewrapped by reputable companies that sell protected cells.

Ie, they may indeed contain a venerable cell like a Panasonic NCR18650g, but have the all-important Seiko cell protector attached inside a new wrapper. Companies like KeepPower do this kind of quality assembly and rewrapping.

Voltage? 8.6v is TOO HIGH for your non-lfp application. There is no need to take them above 4.2v individually, and given manufacturing variances, I'd set the system no higher than 8.2 or 8.3 total.

You can get these protected cells from reputable flashlight dealers - like those offering Fenix, Olight, and others manufacturers of quality lights. What you want to avoid are cheap dangerous cells - anything offering outrageous capacity like Zapfire 7000's. Anything above 3500mah in an 18560 size is marketing bs and a red-flag that your safety and quality is not a concern.

Using unprotected bare cells as you seem to be indicating, and with your level of experience, is dangerous even if you think it's just a simple RPI project.

Put those bare Panasonic's aside and start over with protected NEW cells from a reputable dealer.

Also, pick up a DECENT 2-cell charger or analyzer to make sure you start out on the right foot, rather than let your bms balancing bleeders try do all the work initially. Xtar, NiteCore, Opus and others come to mind. Avoid the cheap "Wowza 2000" chargers.

 

[panban]

Thanks Substrate for your prompt reply, i think i found the NiteCore NL1835, KeepPower and the Orbtronic that have the integrated Seiko ic. Didn't knew these requirements existed.

My question would also be, what type of controller-charger do the charging after the panel and before the cells. I assume a BMS with these batteries wont be needed.
A step down DC-DC voltage from 20v to a TP5100 or a faux? bq24650, cn3722 mppt controllers?

If its safer but more bulkier i would try to consider charging a 5Ah 12v UPS battery.

Thanks again.

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