Tuesday 27 October 2015

Why is my solar panel short circuit rating much higher than in reality?


I have two 6 volt 100mA solar panels. I have connected them in parallel and was expecting to see a maximum of 200ma when I short circuit it. As expected, I got 6 volts, but only 65ma. Does this always occur? The sun was really bright at that time.




Answer



100mAh means 100 milliamps for an hour at maximum possible brightness of the sun, not per an instance of a short. They are not batteries which can put substantial current into a short circuit. The 65mA reading you were getting is what the solar cells could put into a short circuit with the light you had.

Solar cells are actually photo-diodes and the current output is limited by how much light is hitting the diode and its efficiency in converting photon energy into excited electrons. They do NOT behave the same as a charged battery when shorted.

That is why solar cells used to charge batteries often have a charger IC designed to properly charge the batteries based on current sunlight conditions and the charging needs of the battery. Look up 'solar cell charger IC' on the web. They are cheap to buy and will prevent overcharging your batteries. They can also boost the charge current when the sunlight is low. If very low sunlight they stop charging.

Huge panels of solar cells that are over 2 meters sq are often fused and can be a shock hazard. The small ones you have pose no threat except possibly overcharging a small battery if a regulator is not used.


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