Connecting small PV panels in parallel

[angust_uk]

I'm working on a small solar-powered device and am using 5W panels (~6V open circuit). Because I can't find a single one of the correct size I'm using 2, and would like to connect them in parallel (in series would result in a voltage too high for my charger, and larger losses due to the linear charger being used).

However if one is in full sun and one in shade, is there a risk of damage to the shaded one? Obviously I could include diodes, but I'd rather not add any losses if at all possible.

[moffy]

Parallel connection shouldn't be a problem as long as the panels are similar, you don't want a higher voltage panel causing the breakdown of a lower voltage panel. The panel in shade would only contribute a small amount of current compared to the unshaded panel. see:- https://www.alternative-energy-tutorials.com/solar-power/parallel-connected-solar-panels.html

[Peabody]

You could test this by setting up your panels in parallel but with a low-value resistor in each line, leading to a common load.  You could measure the voltage across one resistor as that panel goes into shade, and see if it ever reverses polarity, which would indicate current is flowing back into the shaded panel.  I think moffy is right that it would just contribute less and less current to the load.  But I wonder what would happen if the maximum open circuit voltage of the shaded panel is lower than the voltage being produced by the full-sun panel.  It seems that might be a problem, but I've never tested it.

I wonder if ideal diodes could be used in low-current solar systems in cases where a diode is actually needed, such as to prevent  the battery feeding back into the panel at night.

[gf]

Depending on the darkness of the shadow, the shaded panel can either still contribute to the harvested power, or a reverse current can flow through the shaded panel and heat it up. Reverse current originating from only one sunny panel is usually tolerated by the shaded panel, but reduces the power that can be harvested from the sunny panel. Reverse current originating from say 10 sunny panels can however damage the shaded panel. The tolerable limit is somewhere in between. I suggest measuring the currents under representative lighting conditions. Btw, does the charger prevent reverse current from the battery into the panels?

[moffy]

It seems, gf is correct in that the reverse voltage/current can degrade certain types of panels over time, but with a 6V system that might not be much of an issue, but as the others have suggested test and measure.

[angust_uk]

Thanks for the excellent advice everyone. To respond to gf's question, the charger does indeed protect against reverse current from battery to panel.

I set up a small experiment with the parallel panels, constant load and a crude voltage logger on the output of the charger to see what happens over time. It's only been ~14 hours but everything looks good so far, even with one panel in direct sun.

Thanks again!