Intro: How to Measure a Solar Cell's Power Output.
So you have just scavenged a solar cell that was about to become part of a landfill. Good for you, good for the environment, and even better for your next solar project. But what does that solar cell really do? Maybe a bad solar cell was the reason the whole assembly was in the dumpster.
The aim of this instructable is to give you the tools and understanding needed to examine any solar cell that you find; so that you have a better understanding of how it truly behaves, thus allowing you to incorporate it better into your next project.
While this sort of analysis is usually done with a computer-controlled DC load with a forced voltage function, the equipment needed will usually cost about $500. This method may not be as fast as with a DC load, but for the budding hardware hacker, it's a lot more cost effective.
Step 1: Gather Your Resources.
You will need the following:
- A solar cell or solar panel to test.
- A good quality multimeter, that can read voltage and preferably current. Don't worry if your mutlimeter lacks a current setting. We can get by without it.
- A variable resistance box. This is nothing more than an easy way to vary the resistance to known settings while it is still in the circuit. For truly accurate readings, I'd recommend going through and manually measuring all the resistance settings, as I have seem them vary by as much as 5% from the values printed on the dials.
- (not pictured) Short lengths of wire to connect all the components together.
- (not pictured) A spreadsheet program to help keep track of your data and do your calculations. Yes you can get by without it by using paper & pencil, but lets take advantage of the tools we have.
- (optional) A second multimeter that can read current over a wide range. Once again, this is something that we can get by without, but it's nice to have.
Step 2: Hook It All Up.
This is simply a solar cell connected in parallel with a load, and a multimeter set to measure voltage. Before you start, insure the load is set to 'open'. If you have an additional multimeter that can measure current, you can also connect it in series with the load. This makes things a little easier, but it is not necessary.
Step 3: Point It Toward the Sun.
I realize that you might not have a sunny day on which to measure your cell. Unfortunately, I have yet to find a viable substitute for the sun. Yes, you can use a bright light, but that will give you only a fraction of the sun's energy. I have used halogen lamps with varying degrees of success. Yes, they give me the most power of any artificial light source I have tried, but they also heat up the solar cells which degrades performance.
Beyond using a good light source, you need to align the solar cell toward it properly to get optimal power. Think of it as positioning a sail towards the wind. The best way I have found to align a solar cell towards the sun it to mount a small stick perpendicular to the solar cell's surface then adjust the cell to minimize the stick's shadow. When the shadow is minimized (preferably not visible), then the solar cell is faced toward the sun.
Step 4: Measure the No-load Voltage.
This value is simply taking the voltage measurement across the solar cell's output with no load connected to it. After confirming the load resistance is open, record the voltage measurement. This is the maximum voltage the cell will produce, under the current light conditions.
Step 5: Measure the Short-circuit Current (optional).
This measurement will tell you the maximum current your solar cell can provide.
With the load still 'open', switch the multimeter to measure current. Record the result, then set the meter back to voltage measurement. A current measurement like this is the equivalent of a short circuit across the output of the cell, so don't keep it like this any longer than you have to. While I have not found any evidence confirming that this will damage your solar cell or a good meter, it's best not to take chances.
Now you have the maximum voltage your cell can produce and the maximum current it can produce. However, these results are what happens at open and maximum load. The real understanding comes from what happens between those two extremes.
Step 6: Sweep the Load, While Recording Voltage (and Possibly Current).
Now the fun really begins. With your load box set to it's maximum resistance, change the setting from 'open' to 'resistors'. Record the following data points: Resistance setting, voltage, and current (if you are recording it). Once you have recorded these, switch the resistance to the next lowest value and record the results for that setting. Repeat the process, until you have recorded values for all resistance settings.
Once finished, you should have a data set similar to what is seen in the above spreadsheet image. You should also notice a trend of the voltage dropping as the resistance decreases. Now that you have your data, you can begin analyzing it!
Step 7: Calculate the Power!
The electrical power for any setting is simply the product of the voltage and the current. If you were able to measure the current for each step, you are good to go. If not, you can find the current by dividing the voltage by the resistance. Once you have the current, just multiply it by the voltage.
To clear things up, lets look at the above spreadsheet. In row 2 we have 22.20 volts and 1 megaohm (1,000,000 ohms). The current for that entry is 22.2/1,000,000 or 2.22e-6 amps (2.22 microAmps). The power is 2.22e-6 amps X 2.22 volts, which comes to 4.93e-5 watts (or 493 miliwatts). Repeat this process for each resistance setting. Having a spreadsheet, means you can input the formulas, then copy/paste for all entries.
Step 8: Graph Your Results.
Once you have the power for each resistance setting, you can graph it. I have found that the most understandable way to read the power output of a solar cell is to use an X/Y (scatter) plot , with voltage along the horizontal axis and power on the vertical axis.
The graph above is constructed from the sample data. It becomes readily apparent that the maximum power is above the 1.5 watt rating. We can also see that from the graph, that the maximum power correlates above 12 volts. This is about where we want the maximum power to be, when we are charging a 12 volt battery.
Step 9: Always Improve.
Occasionally, I will add the current to the power/voltage graph. While this is not critical in most cases, it does sometime yield some useful insights. For instance, in the above graph we can see there is a cluster of data points at the high voltage end. As this is where the current drops off, that suggests that these represent high resistance values on the load. To improve resolution in the maximum power area, we can calculate what range of additional resistors we'd need. I see one point at (about) 16 V volts and 105 ma. That suggests a resistance of about (16V/.105A) 150 ohms. Our resistance box jumped from 220 ohms, to 150 ohms, to 100 ohms. A load made from a 100 ohm variable resistor in series with a 50 ohm resistor would give much better resolution for the area we are interested in. Also note, that you can see from the power curve, that you would need resistors rated for at leas