I have a 12V 10W solar panel that I'd like to use to charge a power bank (USB input). Here are the stated characteristics: V(max power) = 19.5V, I(max power) = 520mA, V(open-circuit) = 22.5V, I(short-circuit) = 550mA. I have some questions about the 'power conversion' that I'm trying to work out.
I connected a LM2596 based buck converter module (12-40V input to USB output) to the panel and connected the power bank to the buck converter via a USB cable. In full sunlight I was able to get a output current of about 400mA with an output voltage of 4.75V from the buck converter. This is barely 2W! The panel voltage falls to about 9V at this point. Please help me understand why this happens.
Is the buck converter trying to draw too much current/power and the panel cannot keep up, which in-turn causes the panel output (both voltage and current) to drop far below expected? Is the panel simply not 'producing' that much power while still being in full sunlight? It just doesn't make sense to me intuitively. I have tried to understand the I-V curves and MPPT point but I guess I'm not able to keep up very well with those either.
Is the solar panel not working like a 'battery'? If not, what do I need to understand about its working?
What would be the best way to achieve my purpose here without causing massive power losses?
Appreciate your help!