Construct a small, portable solar panel that will charge two AA rechargeable batteries in a day or two. Use the batteries to make any battery-powered device solar powered.
Step 1: Introduction
Construct a small, portable solar panel that will charge two AA rechargeable batteries in a day or two. Use the batteries to make any battery-powered device solar powered. Or use the panel to directly power small DC electronics.
The panel consists of eight 1"x3" solar cells wired in series with a blocking diode mounted on a board and protected by clear plastic. In this configuration the panel provides about 250 milliamps at 4 volts, which will charge two batteries in a day or two, depending on the weather and the batteries' capacity. Other solar cell configurations are possible to provide more or less power to, for instance, directly charge a 3.6 volt cell phone battery, or to provide a faster charge to AA batteries.
There are a number of off-the shelf small solar panels available on the web, but building one yourself gives you the flexibility to configure it to provide exactly the voltage and amperage your project needs. And it could be cheaper.
Step 2: Background
My original goal was to use several small 1"x3" solar cells I'd purchased a year ago to charge my cell phone. For my first panel I connected nine cells in series and very simply mounted them on a board with no cover. That generated enough electricity to directly charge my cell phone.
However, it had several shortcomings. First, I found that I needed to charge the phone when the sun wasn't shining. Second, when I wanted to receive calls the phone was often outside charging while I was inside. Third, the cells got dirty and one broke. Finally, because the circuit could run in either direction (no blocking diode), the cell phone battery would discharge to the panel when the was no light.
My solution to these problems was increase flexibility by charging two AA batteries instead of the cell phone and to put the charged batteries into the Minty Boost to charge the phone. To better protect the cells I glued them to the backing board and covered them with a clear plastic sheet. A blocking diode prevents battery discharge.
The inspiration for mounting the cells comes from otherpower.com
I couldn't find any other resources on the internet with details for creating your own solar panels, but there must be something out there.
Step 3: Solar Cell Basics
It helps to understand some fundamentals about solar cells before designing a panel. All common solar cells, like the multicrystal cells used in this instructable, produce 0.5 volts or so. That is, the front and back have a 0.5 volt difference. The size of the cell determines the amperage. A full-sized cell (6"x6") could produce three amps, depending on its design, but smaller cells may produce only 250 milliamps or less.
To increase the voltage of a panel, wire the cells in series. To increase the amperage, wire the cells in parallel.
Step 4: Materials
eight solar cells (I purchased multicrystal cells online. Try, for instance, Silicon Solar, Plastecs, eBay, or do a Google search.)
small gauge wire
ribbon wire (Flat wire commonly used to connect solar cells to one another. I purchased cells with the ribbon already attached to the front of each cell. You can connect with regular wire, but the flat ribbon is less likely to cause the cells to crack when mounted.)
clear plastic (I used plastic that was 0.1 inch thick)
four to six wood screws
battery holder for two AA batteries
wood, about 1/2 inch thick panel
adhesive (I used an adhesive/sealant intended for bathrooms, but silicon should work fine)
Step 5: Tools
Step 6: Building the Backing
Cut a flat piece of wood the correct size to provide backing for the solar cells.
I arranged the solar cells in two columns of four cells (to make the panel more square than rectangular). That covered 6 1/2" x 4", plus I added additional space for wiring and to secure the cover to the backing, which came to a final dimension of 8" x 5 1/2" for the wood backing.
Sand and paint the wood. Set it aside to dry.
Step 7: Begin Wiring the Cells Together
All the cells will be wired in series, that is, the front of each cell will be connected to the back of the next cell in series.
Cut the ribbon into ten approximately 1 1/2" long pieces. Solder eight of them to the fronts of all eight cells, allowing about half of the piece to stick out beyond each cell. (The extra will be soldered to the back of the next cell in the series.) I found it easiest to apply solder to half of each ribbon first, allow them to cool, then put the soldered part of the ribbon onto the top of the cell, and solder it on without adding more solder.
Step 8: Continue Wiring the Cells
Apply solder to the top of the portion of each ribbon that is sticking out beyond the cells. This solder will be used to connect to the back of the cells.
Carefully flip over the cells and arrange them in two columns of four cells. Position them so they are close to one another, but not touching (perhaps a quarter-inch apart--they will expand in the heat). Bend the ribbons so the soldered portions touch the backs of the cells. Solder the ribbons onto the backs.
Solder the two remaining ribbon pieces to the two unsoldered backs. Remember to apply solder to the ribbon first.
You should now have two sets of four cells with ribbons sticking out the front and back of each set.
Step 9: Continue Wiring the Cells
While the two sets of cells are upside down, place them next to each other as you want them positioned on the finished panel. Make sure that the ends of each set have ribbons connected to opposite side of cells. That is, the cell at the top of one column should have its top ribbon attached to the front while the top cell of the other column should have its top ribbon attached to the back.
Now that they are positioned, cut a piece wire the right length to connect the two top ribbons. Solder it to the ribbons.
You may want to carefully flip the cells over and do a continuity test. In full sunlight they should produce about 4 volts and 250 milliamps. The voltage and amperage will be less in indoor light. Make any fixes now and flip the cells upside down again.
Step 10: Attach the Cells to the Backing
Using a blank piece of paper, create a template by tracing the outline of the backing constructed in step 6 onto the piece of paper. The outline will be used to position the cells onto the backing.
Carefully slide the template under the solar cells and position them within the outline as you want them mounted on the backing. Leave enough space on all sides to attach the clear plastic cover and for the two wires that will be soldered to the cells.
Apply a dab of adhesive about the size of a nickel to the back of each cell. You want enough to adhese the cells to the backing, but not enough to squirt out the sides, nor so much that the cells end up sitting up high off the backing--they need to be close against the backing to fit well under the plastic cover.
Hold the backing over the cells and, using the template outline as a guide, gently press the backing onto the cells. The cells are in danger of breaking in the step, so be gentle but firm. Pull up the backing, hopefully with the cells stuck on, and flip it over. You can do some repositioning and make certain that each cell is well pressed into the backing.
Step 11: Attach the Connector Wires
Cut two approximately six inch lengths of wire. Solder one to the ribbon attached to the back of the cell on the bottom so that the wire is pointed to the right, where it will exit the panel. This is the ground connection. To the remaining ribbon, solder the diode. The orientation of the diode is important--make sure it doesn't block the electrical flow. If it does, reverse it. Connect the second wire to the diode and have it go to the right as well.
Step 12: Create the Cover
Cut a piece of clear plastic the size of the backing. Cut four pieces of plastic to create a frame around the cells. These pieces should be thick enough that when they are arranged around the cells that once the plastic cover is placed on top the cover doesn't touch any of the cells. Make sure that there is a small opening in the lower right side of the frame to allow space for the two connector wires to exit the panel.
Attach the frame pieces to the backing with a small amount of adhesive along the entire length of each piece. Position the connector wires to exit the frame. Once the four pieces and the wires are secure, place the plastic cover over the panel and drill holes for the wood screws. Make the holes in the plastic just a little bigger than the screws to keep the screws from stressing the plastic. Screw the panel in place and seal up any joints, especially where the connector wires stick out. Adhesive or silicon around the sides should keep water out fairly well.
Step 13: Attach the Battery Holder
Solder the battery holder wires to the solar panel's connector wires, making sure that the positive and negative connections are correct.
To protect the battery holder from the rain, put it in a plastic container, like a cheap Tupperware container, with a hole punched through the side for the wires.
Step 14: Put It in the Sun
The solar panel is complete--place it in the sun and charge your batteries. I'm estimating that at 250 milliamps, the solar panel will take 10-12 hours to fully charge my 1800 milliamp/hour batteries.
Although the diode will prevent the batteries from discharging into the solar panel, there is no protection against over charging, so don't leave the batteries out too long. Also, the panel may not be able to take a drenching rain. Even though it is sealed up, I take mine inside if it looks like it is going to rain.
Step 15: Possible Modifications
The solar panel has worked great for me for the past month. AA batteries are charged in a day or two and the plastic cover has so far protected the cells. Minty Boost works to charge my phone with the AA batteries. (I had to splice together my cell phone connector to a USB connector.) By unscrewing the cover, I was even able to resolder the diode to the connector wire after it came free somehow. I also fashioned a jumper out of a paper clip to let me charge just one battery. (The jumper fits in the second battery's place in the battery holder.)
With some time, I'd like to improve the solar panel by adding a logic circuit to prevent overcharging my batteries. Also, I'd really like to leave the panel outside permanently and have the batteries inside, which would be convenient and would protect the batteries from temperature extremes. So far, I've been taking the panel and batteries outside when good weather is predicted and bringing them in during bad.
With a little work, this panel design should scale up to charge 12-volt batteries or perhaps an external laptop battery.
Lucio Liang made it!