How does connecting different solar panels in parallel affect total current?

Question

I've done a lot of research but only found info about batteries in parallel, which says higher voltage will charge lower voltage, which is unwanted, and that their currents add up.

So how is it different in the case of solar panels? Basically I hooked up a 12V panel to my battery charger, but wouldn't charge, turns out current is almost zero. (Is that normal?) It was part of a package to charge batteries built into a light- and motion-sensitive LED light. Not sure what batteries are in that one, panel is probably low-amp on purpose so it doesn't overcharge as it's always connected. I have another one of the same panel, but not worried about that as the double of almost zero will still be almost zero. However I have 2 of the small 5V panels with a bit better current, so I figured, can I hook them to the 12V to add amp?

Basically my main question is, would the amps add up? Also, would it still result in 12V? Would there be a side-effect similar to stronger batteries charging weaker? There doesn't seem to be a battery inside the 12V panel itself, just a panel, not sure about the small ones, I guess there must be as they still supply voltage for a while even when covered from light, they are the emergency chargers. So would the 12V blow them up? Or any other side-effects that I need to know about?

And if it's doable, how do I connect them? Originally I thought weaving the 3 + wires into one, same for -, and treat them as the 2 poles. Then I saw a diagram, where the + is connected to the + of another battery, and from there with another wire on to load, same for -. Or are these two the same? Only asking, because did an experiment with an electronics experiment "toy", and unified the 2 cables into one, and I got half the individual currents instead of the double, or the same, so I was baffled, maybe using the multimeter wrong? Could do videos if necessary but I'm still only trying to understand the very basics. Not progressing well while being confused by contradictory experiences.

Back to the panels though, if my idea is no good, is there another way to use my 4 panels to operate the charger? Additional info: large panels supply 10V actual voltage, small ones do 5.5V. Not sure about exact currents. Also charger runs on 12V 600mA when on mains.

Thanks for help.

Answer 1

Take a look at the equivalent circuit of a solar cell.

^{simulate this circuit – Schematic created using CircuitLab}

When illuminated the cell acts as a constant current source IS. The shunt resistor RSH has a very high value and the series resistor RS a low value. When the cell is open circuit, i.e. no load, the constant current IS flows through the diode and the output voltage is the diode voltage. When a short circuit is applied at the output the short circuit current is, for practical purposes, equal to IS with no current in the diode. The whole point about solar cells is that they can be connected in parallel to increase current and in series to increase voltage, which is how solar panels are created from individual solar cells. But -- a cell/panel requires requires blocking diodes to prevent a load like a battery pushing current back in to the cell/panel and panels require by-pass diodes incorporated to cater for conditions where the panels are mismatched or partly shaded. http://www.solar-facts.com/panels/panel-diodes.php

Answer 2

You cannot connect panels of different voltages and/or power ratings in parallel by simply joining positive and negative wires together. In fact, simple electrical parallel connection is only recommended to identical solar panels (manufacturer and ratings) that will receive the same amount of sun light, meaning they should be close and facing the sun in the same angle. If your system respects these criteria, you should still protect each solar panel against reverse current (becoming a load) by placing a diode, preferably Schottky for lowering power losses.

If you are willing to join multiple solar panels of different ratings, you should consider buying a controller that is capable of handling multiple input feeds. And each group of panels of different ratings are connected to one feed.

For additional information regarding this subject, you may check the Solar Electricity Handbook by Michael Boxwell, specially the "Mixing and matching solar panels" section.

Answer 3

Trouble is two panels even the same type and manufacturer will NEVER produce exactly the same output voltage. To draw equal amounts of current from both devices wired in parallel would be like trying to balance a ball bearing on the edge of a razor blade. I would always go for series wiring, ok you get a higher output voltage and the current is going to be based upon the minimum current of the 2 devices.