Step 1: Introduction
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
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
To increase the voltage of a panel, wire the cells in series. To increase the amperage, wire the cells in parallel.
Step 4: Materials
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 6: Building the backing
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
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
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
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
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
Step 12: Create the cover
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
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
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
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.