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

Question

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 1

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.

Answer 2

The I-V (current-voltage) curve for a solar cell is a little complex. Below is an example of the relationship:

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What that basically says is that the more current you try to draw for a solar panel, the lower its terminal voltage will be. You get the maximum current at short circuit, and the maximum voltage at open circuit.

Solar cells are specified with two values - short circuit current (in your case 100mA), and open circuit voltage (in your case 6V). 100mA does not mean 100mAh, and the rating of the cell is definitely in mA not mAh (mAh would be completely meaningless for solar cells). You may be wondering why it is rated in terms of short-circuit current and open-circuit voltage, well using those two data points you can get a rough idea of the I-V curve for any given irradiance level.

So why do you not get 200mA short circuit current like you expect? Well, the cell generates current as light hits it and transfers energy to electrons which are then free to move as a current. As such it makes sense that higher the intensity of the light applied to the cell, the larger the output current can be. In other words, the short-circuit current is dependent on the light intensity. If you put the cell in a dark room, the short circuit current will be much lower.

You will be seeing only 65mA as you are probably operating at a lower light intensity than when your panel was characterised.

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There is another thing to take away from the relationship. If you try and draw the maximum possible current from the cell, it won't actually give you any power - all of the power is dissipated internally over the cell (which is not good for it!). Conversely if you try to run the output at the highest possible voltage, you won't get any power either as there is no current (again the energy from the light source will simply be dissipated in the cell). To get the most power out, you need to operate somewhere around the corner of the I-V curve where the output power is maximal.

There are some good answers on EE.SE about using solar panels and a process called maximum power point tracking.