Checking a new old Radio Shack Solar Panel - Problem

[pratto]

I bought some hobby Radio Shack solar panels (old stock, but still in the package), and started playing around to see how much power I could get. The package says 9v 1W 112mA.

But when I test the output, it is WAY less than 1W. The best I can get is about 45 mW. But then, I am new to the solar panel game, and I'm not sure I am properly testing it.

Under my desk lamp, with no circuit, it shows 9.0v across the panel leads. But as soon as I put a load on it, the voltage drops and the current is in the mA's.

For example, 1.7 KOhm load results in a voltage reading of 8.6v, with 5.2mA = 45mW
Or, 25 Ohm load results in a voltage reading of 0.27v, with 10 mA = 2.7 mW

How should I be testing this panel ?

[SunEagle]

I bought some hobby Radio Shack solar panels (old stock, but still in the package), and started playing around to see how much power I could get. The package says 9v 1W 112mA.

But when I test the output, it is WAY less than 1W. The best I can get is about 45 mW. But then, I am new to the solar panel game, and I'm not sure I am properly testing it.

Under my desk lamp, with no circuit, it shows 9.0v across the panel leads. But as soon as I put a load on it, the voltage drops and the current is in the mA's.

For example, 1.7 KOhm load results in a voltage reading of 8.6v, with 5.2mA = 45mW
Or, 25 Ohm load results in a voltage reading of 0.27v, with 10 mA = 2.7 mW

How should I be testing this panel ?

Solar panels should be testing outside under the Noon Day sun without any clouds. Any other light source will not produce the required energy a pv panel requires to put out anything close to "nameplate" wattage.

When you do your outside test you should be measuring close to the open voltage rating (Voc) of that panel. If you have the correct meter you can also measure the short circuit amps Isc but the best is to use a solar battery charge controller on a battery that is less than 75% SOC. Then you will see the panel trying to provide it's maximum amp output (Imp) trying to charge that battery.

Oh by the way. Welcome to Solar Panel Talk.

[pratto]

Solar panels should be testing outside under the Noon Day sun without any clouds. Any other light source will not produce the required energy a pv panel requires to put out anything close to "nameplate" wattage.

When you do your outside test you should be measuring close to the open voltage rating (Voc) of that panel. If you have the correct meter you can also measure the short circuit amps Isc but the best is to use a solar battery charge controller on a battery that is less than 75% SOC. Then you will see the panel trying to provide it's maximum amp output (Imp) trying to charge that battery.

Oh by the way. Welcome to Solar Panel Talk.

OK. I will check it tomorrow. I take it that SOC means state of charge ? I intend to use it on a 6v sealed lead acid battery. Is it ok to discharge it to 4.5v first ? I read somewhere that going less than 5v would harm the battery.

thanks for the reply and the welcome.

[SunEagle]

OK. I will check it tomorrow. I take it that SOC means state of charge ? I intend to use it on a 6v sealed lead acid battery. Is it ok to discharge it to 4.5v first ? I read somewhere that going less than 5v would harm the battery.

thanks for the reply and the welcome.

Yes SOC is state of charge. I would not discharge the battery down to 4.5v. That is too low. Just go down to just under 6 volt.

Next I am not sure how that panel will work connected directly to the battery. Although it is only 112mA I am not sure if the 9 volt is too much. Read the battery specs concerning charging. I don't want you to hurt it.

[inetdog]

I bought some hobby Radio Shack solar panels (old stock, but still in the package), and started playing around to see how much power I could get. The package says 9v 1W 112mA.

But when I test the output, it is WAY less than 1W. The best I can get is about 45 mW. But then, I am new to the solar panel game, and I'm not sure I am properly testing it.

Under my desk lamp, with no circuit, it shows 9.0v across the panel leads. But as soon as I put a load on it, the voltage drops and the current is in the mA's.

For example, 1.7 KOhm load results in a voltage reading of 8.6v, with 5.2mA = 45mW
Or, 25 Ohm load results in a voltage reading of 0.27v, with 10 mA = 2.7 mW

How should I be testing this panel ?

The output of a solar panel is limited in current by the amount of light hitting it. And when you take into account internal series and parallel resistances, the equivalent circuit produces a curve of output voltage versus output current. At either end of the curve (zero volts, max current versus zero current, max voltage) the output power is zero.
Your two test points are each too close to the zero power points on the curve. You need to get closer to the middle.

The highest output power will generally be found when you load the panel with a resistance that produces an output voltage of roughly 80% of the open circuit voltage.

If you take the approximate maximum power voltage (Vmp) of 7.2V, a power of one watt would correspond to a current of 139 milliamps. That is far above the short circuit current of 10mA that you measured.
Either the lamp is giving a factor of 15 less light than direct sunlight or the light is not evenly distributed over the surface of the panel (the least illuminated section will limit the overall current) or the panel is not delivering its originally specified performance.

Your experiment outside with the panel aimed directly at the sun should give you a much higher short circuit current. If it does not, you know that the panel is defective.

If it does give you higher current, try a resistor that will drop 7V at 130mA, about 50 ohms.

[pratto]

The output of a solar panel is limited in current by the amount of light hitting it. And when you take into account internal series and parallel resistances, the equivalent circuit produces a curve of output voltage versus output current. At either end of the curve (zero volts, max current versus zero current, max voltage) the output power is zero.
Your two test points are each too close to the zero power points on the curve. You need to get closer to the middle.

The highest output power will generally be found when you load the panel with a resistance that produces an output voltage of roughly 80% of the open circuit voltage.

If you take the approximate maximum power voltage (Vmp) of 7.2V, a power of one watt would correspond to a current of 139 milliamps. That is far above the short circuit current of 10mA that you measured.
Either the lamp is giving a factor of 15 less light than direct sunlight or the light is not evenly distributed over the surface of the panel (the least illuminated section will limit the overall current) or the panel is not delivering its originally specified performance.

Your experiment outside with the panel aimed directly at the sun should give you a much higher short circuit current. If it does not, you know that the panel is defective.

If it does give you higher current, try a resistor that will drop 7V at 130mA, about 50 ohms.

I will do that, and thank you for your clear and helpful explanation. We have had cloudy days here lately, but maybe tomorrow. In any case, when we finally get a sunny day, I will make the tests and post the results.

[pratto]

I will do that, and thank you for your clear and helpful explanation. We have had cloudy days here lately, but maybe tomorrow. In any case, when we finally get a sunny day, I will make the tests and post the results.

OK. Finally a mostly sunny day.

In direct sunlight, with a 50 ohm load, the Solar Panel rated at 9v measures 10.5v, with a current of 123 mA. I make that to be 1.3W. So, unless I did something wrong, it is just as you folks suggested, full sunlight makes a big difference.

Thanks for your help.