The smaller solar cell recommended in the MMB project is great because its nice and compact. But its small size limits the amount of power it can generate. It has an output of about 100mA, which would take about 20 full-sun hours to charge the MMBs 2000mAh battery. A sunny winter day produces about 4.5 full-sun hours and a summer day about 8.5. An average day year-round is about 5 full-sun hours, including cloudy weather. Those numbers depend on the local climate and the figures given are for my area, which is near San Francisco right on the coast. Therefore the small solar cell will take on average 4 days to fully charge the MMB battery. That is fine performance if you only intend to use it for topping off and will charge you iPhone from the grid over-night. But if you want to go pure solar like I do, you need a bit more juice from your solar cell
The big solar cell produces 310 mAh, which can fully charge the MMB battery in just 6.5 full-sun hours. Thats pretty good and should satisfy your iPhone's power appetite on a daily basis much of the year except maybe in the winter when you are playing lots of games or something. I was able to verify that charge time the first time I charged my brand new MightyMintyBoost. But to get that full on performance it helps to have the cell held at the optimal angle to the sun and to turn in occasionally so that it tracks the sun during the day. So I designed a support frame for my solar cell that has three different tilt angles to optimize the cell's angle for different times of the year.
Enough power generated to fully charge and top-off an iPhone 3GS on solar power alone.
Voltage regulated so you can use cells with more than 7 volts output.
Three tilt angles optimized for Winter, Equinox (spring and fall), and Summer sun angles.
Refer to the MightyMintyBoost Project for the construction of the MightyMintyBoost battery and charger part, which is the stuff in the Altoids tin. This project covers how to make a frame for and how to wire a larger solar cell.
Step 1: Tools and Materials
An Analemma (see second picture)
Hot melt glue gun
Wood saw (hand or power)
Miter box (if you use a hand saw)
Carpenter's square (if you are cutting by hand)
7/64" drill bit
Large Solar Cell
Voltage Regulator, 5v
3/4 No. 8 wood screws (qty 8)
Hot melt glue
Super Glue (optional)
Brush on electrical tape
Finished lumber 1x2 x 18 long (poplar is very nice, but any wood will do)
Birch Plywood, 1/4 thick, 2 pieces at least 5 x 4 each
The MightMintyBoost charger will accept up to 7 volts input, but this solar cell puts out 9 volts so it needs to be regulated down to keep from frying the charger. Because the cell produces smooth DC voltage, capacitors that are sometimes used with the 7805 voltage regulator are not required. The voltage regulator will only dissipate about 1.5 watts so a heat sink is not required.
I have both a cabinetmakers table saw and a crosscut saw, which makes this project super simple, quick and very accurate. However if you dont have all that stuff dont worry, it can easily be done with hand tools if you are careful and take your time and maybe make a practice cut or two if you havent done much wood work before. There are only a few cuts to make so you can take all the time you like and still finish the project quickly. Making accurate 90 degree cuts, especially when cutting the 1x2 to length, are important to making the project fit properly so do make use of a miter box. The more precise the lengths are the better the final product, but it will work just fine even if things are not perfect so don't sweat it.
I have been itching to get a chance to use the word "analemma" in text ;-). I used it to estimate when to use the winter, summer and equinox tilts. More on that later.
Step 2: Calculate Your Tilt Angles for You Latitude
You need to calculate three angles:
Equinox Angle = 90 - Latitude
Summer Angle = Equinox + 23
Winter Angle = Equinox - 23
That's pretty simple. In the next step, the drawing shows where to use those angles. They allow you to just set the frame down on one of three edges that are preset to the optimum angles for each season. In the picture below my frame is sitting on its summer angle. Each angle is labeled on the drawing.
Step 3: Cut the Frame Parts, Rails
The frame is made up of four parts. There are two end plates and two rails. The drawing attached below shows the exact dimensions I used based on the size of my solar cell. The cells might vary a little bit so you might need to adjust the dimensions to match your cell.
So the first thing to do is to measure your cell and write those dimensions down.
You will notice that I have made a notch for my cell to sit in on the two rails. That's pretty easy to do if you have a table saw, however it would be tough if you are cutting out parts by hand so if that's the case for you I would just skip making those 1/8" notches. However, if you do want to make them, do that milling operation before you cut your 1x2 to length. Note that a 1x2 has actual dimensions of 3/4" x 1 1/2", which is what I show on the drawing.
Cut your 1x2 into 2 pieces the same length as, or just a tiny bit longer than, your solar cell long dimension. Use your miter box for these cuts. Getting them square is important to things fitting properly later. Do not do any sanding on these parts at this point.
Double check your cut rails for length against the solar cell. They need to be the same length as the cell or a bit longer. If they are too short, you need to cut new ones that are long enough.
Step 4: Cut the Two Side Panels
Use the rubber cement to glue the two panels together with the edges well aligned. Then use the rubber cement to glue the drawing side frame view to the two panels to use as cutting template. Make sure the rubber cement has a chance to dry well so that the panels don't slip around when you are cutting. If you have a vice to hold them that will help with the cutting step.
Start cutting. Since the panels are glued together you will cut out both panels at the same time following the cutting lines on the drawing. The finished panels should look like the picture below. Keep your cuts as square as you can so that the panels will be the same size.
Pry apart the cut panels. Rub the rubber cement off. Give them a light sanding both on the edges and faces to make them look nice and to remove splinters.
Step 5: Assembling the Frame.
Once the frame is glued together. Set it on end on a flat stable work surface and carefully drill pilot holes for your No. 8 screws using the 7/64" drill bit. Drill the hole about the same depth as the screw length. Don't skip this step or you will have a heck of a time with splitting and driving the screws. You can eyeball the screw locations or measure them, which ever you are more comfortable with. Make sure you hold your drill nice and square so the holes are not at an angle. Take care not to break the temporary glue joints of the frame as you do this step.
Drill one hole at a time and then drive its screw in to minimize the problems with the frame glue joints breaking.
Drive the screws in until the head is flush with the wood surface.
Finish sand the piece now. If you want to paint or otherwise apply a finish, now is the best time.
Step 6: Prepare the Solar Cell for Mounting.
The solid red lead is positive on the cell suggested. The negative lead is black with a red stripe. If you use another brand of cell, make sure you understand the lead polarity on that cell.
Cut the positive lead about 2-3" from the connection point on the cell. Strip the insulation back on both ends by about 1/2".
Cut just the insulation of the negative lead, but not the wire, and pull it back about 1/2" to expose wire. If you cannot do that you can cut and strip both ends also, but you will need to cut about 1/2" from the positive lead and restrip it so the lengths will match.
Bend the two outer leads away from the center lead of the 7805 Voltage Regulator and super glue or hot melt glue it to the back of the solar cell as shown in the picture below. Make sure the leads can reach it with enough slack to wrap the stripped wire around the leads of the 7805. Make sure it is oriented like shown in the picture.
Solder the positive lead to the two bent outer leads of the 7805, wrap the leads with the wire to make a secure solder joint. Make sure it is oriented like shown in the picture below. If you get it backwards the voltage regulator will not work.
Solder the negative lead, both leads if you had to cut it, to the center lead.
Test the cell output. I should be 5 volts in full sun. If not double check your leads and correct the hookup.
Insulate the exposed leads with Brush-On Electrical Tape.
Shorten the cell cable (cut strip and solder) and insulate that splice now if you want a shorter cable.
Step 7: Mount the Cell in the Frame.
Use hot melt glue to glue the cable to the back of the cell. Use a nice big glob to act as a strain relief on the cable. See the picture. That will prevent the leads of the voltage regulator from being yanked and damaged later.
Step 8: Using the Frame.
In the summer, May through July, you will want to use the summer angle in the middle of the day. In the spring and fall, which would be March, April, August, September and October use the equinox angle. In the winter, November through February use the winter angle. I determined these months and angles using the analemma shown below. They are only approximate and using the analemma you can determine more accurately the best dates to switch from one angle to the next.
The winter angle is good for early morning and late afternoon all year around when the sun is low in the sky.
You can collect a lot more power if you can rotate the frame every few hours during the day so that it follows the sun. That is especially true in the winter.
You can also harvest a quite bit more power in the winter time, you need all you can get when the days are short, if you set the frame on a large piece of cardboard covered with aluminum foil with most of that to the south of the cell. The foil captures sun light hitting in front of the cell and reflects it up to the cell increasing the current output by as much as 50%.