In 2005, I built a solar messenger bag based around a Timbuk2 laptop messenger. The deign was simple and has been riffed on many times, most notably in Popular Science in 2007.
The build was fine, but I learned a few things though the process and had been thinking about a Mk.2 version from the beginning. The new design had to feature a larger PV cell and would integrate the wiring and battery into the bag itself, rather than have those things draped around the bag’s storage spaces. Mk. 2 was going to be a backpack as opposed to a messenger bag because the equipment I carry as mixed-media journalist can be heavy after a long day and two straps are better than one for load distribution. I also hoped to up the level of technology employed in the circuit design, or at least make it easier to replicate.
I was both the designer and the audience for this product, which can be a dangerously incestuous design space to get into, but I’ve had such positive reaction to the bag from all kinds of people, I think I hit on a shared desire.
- Timbuk2 Swig backpack Size: Small, Mission 6 lightweight nylon fabric
- 10”x7” WheatherPro Flexible Solar Cell
- Carbon fiber plate for stiffening (basswood could work as well)
- Scrap USB cable with female end
- Misc smallish gauge wire (telephone twisted pair works great)
- Shrink tubing Various sizes
- Gorilla Tape I use the 1 inch wide roll
- Electrical tape
- Breakaway prototyping board
- Right angle breakaway headers (not necessary but nice)
- 5v voltage regulator
- 100uF/25v Electrolytic capacitor
- 0.1uF Ceramic tantalum capacitor
- Rectifier Diode
- Industrial strength Velcro (Sticky Back)
- Small project enclosure (hate to admit it, but I did use a mini Altoids tin)
- Very sharp, clean scissors
- razor blades, or a pen knife
- soldering iron (with solder)
- Multi Meter
- various pliers
- straight length of coat hangar wire (for pulling wires through bag’s innards)
- a strong light source (for testing)
1. Perform Surgery on the Bag
a. I recommend measuring the actual cell you received, then making a blank out of cardboard that is the size of the PV cell, not including the plastic edge around the cell.
b. Place the blank toward the bottom of the flap, so that there will still be enough fabric to give strength to the closure clips.
c. Draw around the blank with a pencil or tailors soap. Cut ONLY the outer nylon fabric, NOT the inner rubberized membrane. This is delicate, so measure twice and cut twice. By that I mean make the first cut smaller than you need it, and then widen it where needed. Also, the less you have to stretch and warp the fabric while cutting the straighter your cuts will be.
d. Use a lighter to very lightly heat the edges of your cut- this will seal the edges and stop fraying. I really doesn’t take much, and over heating will cause the edged to melt and warp. I recommend practicing on some of the fabric you just cut away from the bag.
2. Assemble the Solar Cell
(before you begin, put the cell under a light source and us the multimeter figure out which lead is positive and which is negative.)
a. Cut your carbon fiber (I recommend some industrial scissors different than you used for fabric) to the same size as the blank you used to cut the hole in the bag (the same size as the cell excluding the plastic weatherproofing fringe).
b. Use the Gorilla tape to affix the carbon fiber stiffener to the back of the solar cell. Make sure you bed the leads that come off the back of the cell upwards so they don’t get trapped under your stiffener.
c. Solder a 3 foot length of wire to each lead, use some shrink tubing to cover the joints, and then bend the leads back down over the stiffener board and tape them in place (This is a protective measure to keep them from getting bent and damaged, and also makes things look tidy). Use wires that are maybe 3 feet long.
d. Run the wires to the top left (with the cell facing you and the contacts on top) edge of the cell/stiffener assembly and then tape down the wires so they come out of there together. Remember what color you matched to positive and negative.
e. Take the stiff (scratchy) side of the velcro and affix it all the way around the sun-facing side of the solar assembly. Cut it so that it only occupies the plastic edge that extends past the actual cell. You don’t want anything covering any part of the cell itself. Make sure you really press it on there well. The adhesive backing on the velcro is pretty good, but benefits from some massage.
3. Install the Stiffened Solar Assembly.
a. Use the razor blade and poke a small (5mm long) hole in the rubberized backing fabric at the left side of where the flap is sewn into the bag body. Don’t cut though the outer fabric here. This hole will be the wire way.
b. Use the soft side of the Velcro you cut for the edged of the solar cell assembly and adhere them to the inside edges of the large hole you cut in the outer fabric on the front of the bag. (This should be starting to make sense now)
c. When installing the Velcro, take care to mangle the bag lid as little as possible. The less you stretch the hole while assembling this, the better the finished product will look. Once the Velcro is in, take some time and really massage the fabric and Velcro together. We are asking the backing adhesive to do a lot here and this promotes adhesion.
d. feed your pair of wires through the hole in the front of the bag, and out the wire way hole you cut.
e. Maneuver your cell assembly into place and press the Velcro together. This is the toughest part. Try to do this without stretching the hole or taring the Velcro loose. Just go slow and try to position it the way you want it the first time.
f. Pull the wire through the wire way hole, but leave some slack so as not to damage the solar cell assembly.
4. Build the regulator circuit.
a. The two capacitors must be soldered to the voltage regulator (square black thing with three legs). The positive lead of the electrolytic capacitor is soldered directly to the input lead of the voltage regulator.
b. The positive lead of the ceramic capacitor is soldered to the voltage regulator’s output lead.
c. The negative leads of both capacitors are soldered to the ground (GND) lead of the voltage regulator.
d. Solder the diode to the input lead of the voltage regulator. The other end of that diode will eventually be connected to the positive wire coming off the solar cell assembly.
e. Break off 3 right angle breakaway headers (hence “breakaway”) and stick the straight end through the top of your board. Solder the undersides to three wires that connect to the three leads of the voltage regulator. Mark which goes to what.
f. Put the whole assembly into your project box.
5. Run the wires.
a. I chose to locate the female USB jack in the outside pocket on the left hand side of the bag. You could easily place yours inside, or elsewhere.
b. If you copy mine, poke a small hole on the bag wall in the bottom of the inside of the external pocket. Try to do it so you go through the fabric in the pocket and the foam layer inside the bag wall, but not all the way through the inside layer of fabric.
c. Cut the male end off of your USB cable. Leave the female connector intact.
d. Feed the bare wire cut you just made down into the pocket and into the wall of the bag.
e. Make a small cut in the top left side of the bag nearest the wire way hole you made earlier.
f. Feed the USB cable (this is where the coat hanger comes in handy) up through the wall of your bag and out the hole. then pull it through so the female connector is inside the pocket, but has some slack.
6. Connect it all together.
a. Strip the housing away on the end of the USB cable that you cut (you may want to cut it shorter). It will likely reveal 4 wires, some silver foil and a bit of fuzz. You can cut the fuzz and foil away.
b. Confirm which of the four wires is which (positive, negative and ground). It will likely be red=positive, black=negative. There are many diagrams available on the internet to help figure this out. If you choose the wrong wires, the bag wont work.
c. Connect the positive USB wire to the right angle breakaway header that is connected to the voltage out pin on the regulator. Connect the black wire to the header attached to the ground (center pin) on the regulator.
d. Connect the positive wire from your solar panel to the header attached to the input lead on the regulator.
e. Connect the negative wire from your solar panel to the ground (center pin) on the regulator.
f. Shrink tube or electrical tape the whole mess so the wires don’t cross.
a. Find a spot where your controller box can sit and not mess with your use of the bag.
b. Feed extra wire back through the wire way hole you cut in the lid of the bag. 2 extra feet are OK. That slack will make servicing the bag or doing any repairs much easier (that said, I never once had to open my last bag in 6 years of daily operation).
This panel will generate enough power to activate the regulator circuit and push out the 5-ish (USB power is between 5v and 5.5v) volts needed to charge USB stuff, even in partial sun. The best use of that power is to feed it into a battery that charges via a USB connection. A low weight lithium polymer battery, when plugged into this panel, will slowly charge over several days and accumulate the power collected by the cell.
Think of using power out of this cell as if you were trying to collect rainwater to drink. You might not collect enough in a moment to quench your thirst, but over a rainy day you’d have enough a have a glass of water and then some. Collecting sunlight is the same thing, you have to store it up to make it useful.
Please note that some USB batteries have circuts that wont start charging the battery unless it’s getting a full amp. Those will not charge of the bag. Other batteries have LED indicators that light up when charging. In some cases, those lights wont light, or will only softly glow based on how much power if flowing at a given moment. The battery I linked to does charge on this panel, has a light that glows (softly) when charging, and can recharge my iPhone more than once when fully charged.
Building and using this kind of low power system makes you acutely aware of just how much energy you use. For instance, I tried, unsuccessfully, to power all of my iPhone usage off the cell last summer (when the sun is most intense). You’ll also realize how dramatically less intense the sun is in the winter because charging will take longer (depending on your latitude). It makes me more cognoscente of all my power usage.
Finally, I’ve found that the solar bag attracts people almost better than it catches rays. At least once a week, someone will walk right up to me and ask if it really is a solar panel in there. I tell them it is, that I built it and that it isn’t really that complicated.
And for the guys thinking about building one, my girlfriend would be the first to say, ladies love solar power. 😉
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