I have connected my solar panel (rated 6 V, 3 W), and it is generating approximately 10 V.
This 10 V is fed to an XL6009 to generate ~15-16 V, which is further fed to a 4S BMS to charge.
I checked that the open circuit voltage is 10 V, but when I connect it to the XL6009, the 10 V input drops to 3 V.
Please suggest and advise.
That's normal behavior for a solar panel, when you try to draw more current than it's capable of delivering.
The maximum current is determined by the size of the panel and the amount of light falling on it. If you draw less than that current, then the voltage is fairly stable. Draw too much, and the voltage collapses.
You need more light or a bigger panel. Or re-design your circuit to run at a lower current.
that pretty normal considering your input source is low, 6V 3W is only 0.5A but is still potential, but solar cell is not constant source. this is why when you load this solar cell for anything >3W it will drop below rated voltage.
buck converter is usually load with 80-95% efficiency, let's say you have the output on buck is 15V 0.5A = 7.5W with efficiency consideration is around ~8W this is overkill for you solar cell.
consider use proper solar cell V & Watt rating.
BMS 4S could draw more than 0.5A with 2Ah battery each cell (single row) for charging. cheap chinese BMS is drawing 1A for charging each series row cell 1A
i hope this help you
Input power is 3W maximum (from the solar planel). Boost converter output will be slightly less than that (it's not 100% efficient), say 2.5W.
With the output fixed at 15V, maximum output current will be \$\frac{2.5W}{15V}=170mA\$.
If your battery charger tries to draw more than 170mA, the boost converter will be drawing more than 3W of power from the solar panel, and the panel's EMF will drop well below its 6V rating.
