I read several other postings regarding BMS's but as this is my first attempt to use them and I guess it can be a fire hazard choosing the wrong part, I'm going to ask:

I have a battery-powered LED light which does not charge. I opened it up and exposed all the internal circuits. There are 4 18650 batteries inside, connected in series and parallel to give 7.4V. After probing connections and voltages it seems to me that the tiny board connected to the batteries must be broken. I assume it's a BMS and am trying to figure out what I can replace it with. if you look at the BMS I linked from aliexpress (below), there are exactly the same number of pads and they have the exact same labels: BM, B-, B+, P-, P+. This is a strong indication to me that it is a BMS. The LED is supposedly 10W. The charging circuit takes 9V input. I have no idea what current/voltage the battery charging PCB gives to the 4 18650 batteries during charging, nor do I know how much current should flow to the LED, although I think it might be around 400mA since I got a reading of ~25.4V from the end of the charging PCB that is connected to the LED with the light switched on--I can get the light to work with and alligator clip that bypasses the chip on the "BMS". To reiterate: if I desolder the black wire, currently at "P-" on that little board and move it over to the left to the "B-" pad, the LED's light up. The only thing between the B- and P- pad is a little chip. So I assume that chip is blown. I think if I replace that board which appears to be a BMS, the whole assembly will function normally again.
I also measured around 1A when the light is switched on, on the other side of that charging board, so current input. That side is connected directly to the batteries. It looks like the LED is COB with 9 smaller LED's. I think the bigger charging board (last row in pic) is not defective as if I plug in the 9V wall wart, I can make the LED light up via that board, i.e. circumventing the battery pack. The charging board gets really hot though and I don't think the light is meant to run like that.

Is choosing a BMS for 18650 lithium ion batteries pretty forgiving and I can choose something like one of these https://www.aliexpress.com/item/1S-2S-3S-4S-Li-ion-Lithium-Battery-18650-Charger-Protection-Board-PCB-BMS-Cell-Charging/32921590504.html ? It sort of seems like it could work to me but I really don't know.

Or does picking one out need more attention to parameters and searching for a more appropriate board?

Image shows take-apart of LED light.  2nd row pics show what I believe is the defective BMS board (defective I think).  Final image lower right I assume is the charging circuit.

  • \$\begingroup\$ It looks like your cells are not dead, and the boost DC-DC converter works as well. So the charging part fails. Why do you believe you have a BMS at all? Can you expose a bit more the battery connection, where the middle point comes to? And could you measure node voltages when the 9-V charger supply is connected for charging? I would suggest to draw a block-diagram of all connections between individual parts, label the joint points, and post voltages. \$\endgroup\$ – Ale..chenski May 2 at 22:39
  • \$\begingroup\$ thanks for the reply. stackexchange auto-suggested I edit my original posting rather than put my reply in this comment if I'm adding new info, so I did that. I hope the updated post clarifies why I think it's a BMS, why I think the BMS is blown and why I think the charging part is not the defective part. \$\endgroup\$ – infinitesimal kitten May 3 at 23:24

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