Charging 18650's using 2S BMS

I got this 2S BMS charger and Im trying to charge 2x18650's which started out at 4.00V (and 8V together) today at 11am. After a sunny day at around 5pm I took them back inside and measured their voltages and found that one battery is at 3.91 and the other at 3.8 and together they give me 7.7-7.8V.

What could be happening?

I gather you are trying to charge the cells from a solar panel. In which case is the charger designed to be powered from solar panels?

I actually don't know that.

What chemistry are the cells? At 4V they are likely already fully charged, so probably they've just been relaxing back towards nominal 3.7V

What is the solar cell open-circuit voltage?

They're scavenged so I don't know specs but they had been for days and were that day at 4v, I connected them to the charger board and 4 hours later they were at 3.7V so I think the connection to the module had something to do with it.

So the BMS might be for a different chemistry?

And the cells might be shot to ribbons?

Ok I did some more testing:

  1. The panel has 2 sets of terminals marked here in green and in red. I had them connected to the green terminals all this time.

  2. The panel itself without anything connected yields 5.24V at the green terminals. The same panel yields 17V at the red terminals.

  3. I tried charging the cells with a wall charger and it worked fine, they reached 4.2V and 4.15V.

  4. When I connect the panel to the charger (without the cells in the charger), at the green terminals(5.24V), I get 5.24V at the charger terminals.

  5. When I connect the panel to the charger (without the cells in the charger), at the red terminals(17V), I get 9V at the charger terminals.

Link to the BMS? What is the rated input voltage?

7.4-8.4v charging or supply voltage

Marciokoko: 7.4-8.4v charging or supply voltage

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Marciokoko: 4. When I connect the panel to the charger (without the cells in the charger), at the green terminals(5.24V), I get 5.24V at the charger terminals.

  1. When I connect the panel to the charger (without the cells in the charger), at the red terminals(17V), I get 9V at the charger terminals.

Do you think there could be some correlation there?

Yes I do. I just found out the bms has that charging voltage specification. But I want to ask and make sure because I don't want to blow up a battery or set my house on fire

Charging cells from solar panels is problematic as the panel voltages will flap around based on the illumination, and if the BMS needs a minimum voltage at its input to work properly, its anyones guess what will happen. If the cells get charged properly when the BMS is powered by a fixed voltage supply greater than the sum of the fully charged cells, but not when charged from the solar panel, then its a sign that the BMS isnt suitable for solar panel charging. Measure the voltage from the Solar panel when its connected to the BMS with the cells in the charger, and see if its collapsing to the sum of the cell voltages. If so, its likley that the BMS simply isnt working properly.