Teaching Kids to Build and Use a Solar Battery Charger George Hagerman Copyright © 1990 by George Hagerman Over the lifetime of these students, the Earth's population will double. At current rates of consumption, fossil oil and natural gas will disappear even sooner. These trends suggest that anyone who is too young to remember the "energy crisis" of the 1970's will experience one far more severe, unless serious changes are made in the ways we produce and consume energy. Here is one way to open their minds to renewable alternatives. This article is the second of two about a "short-course" approach to teaching junior high school (and older) students the basics of photovoltaic electricity and battery storage. The first article (in HP #15) described various lectures and experiments, conducted as group activities in the classroom and outside. This article describes the solar battery chargers built by the students of El Ingenero'89, custom designed for their favorite portable gadgets. Building the charger can be a course by itself. It also may be combined with the lectures and experiments described in the first article. Table 1 shows how both activities can be paralleled. Charger construction can also be undertaken as an individual student's project for either a regular school class or a science fair. It is described here, however, assuming that it will take place in a class, supervised by a teacher. Ordering the Necessary Materials The basic charger design is sketched in Figure 1. The key components are a solar panel, a blocking diode, a battery holder, and rechargeable batteries. Component access information is given at the end of this article. INSERT FIGURE 1 The parts to be ordered for each student depend on the battery configuration of the student's favorite gadget. Specifically, one must know the size (AA, C, D, etc) and number of batteries required, which can be determined simply by looking at the battery compartment. The students should be asked to provide this information on some sort of questionnaire or registration form circulated 4 to 6 weeks before the class meets. This way, the component parts can be ordered and will be on hand when work starts on the charger. The students should also be asked to bring their gadgets to the first class session. INSERT TABLE 1 Session A - Measure Current Drain of Portable Gadget So why build a battery charger? One demonstration is worth a thousand words. Hold up a portable tape recorder and pull the batteries out. Place them in series with a large-display multi-meter (Photo 3). Put on a tape that the kids know. My favorite is Paula Abdul's Forever Your Girl (copyright 1988, Virgin Records America, Inc.). It really makes the class come alive. Show how current drain increase when you crank up the volume. They'll love it! INSERT PHOTO 1 Photo 1. Demonstrating the current drain of a portable cassette recorder. Photo by George Hagerman Now pull the cells out of circuit. No energy, no tunes. What's the price of hearing Paula play? Put a chart on the board, which the students should copy, that shows the capacities and prices of Ni-Cad and alkaline cells (Table 2). Explain the relationship between battery capacity, current drain, and time to discharge. Give some simple examples, and then compare the cost of disposable vs. rechargeable batteries for the demonstration tape recorder. Talk the class through the calculations, explaining that they'll be doing a similar exercise for their own gadgets. To begin with, we've seen (and heard) that at maximum undistorted volume, the tape recorder draws 150 mA. With alkaline cells, I'll get 34 hours of total listening time for only $5.20 (such a deal!). On the other hand I'd have to pay a whopping $13.60 for four Ni-Cad C cells, and they'd be fully discharged in just 9 hours. Not much of a bargain until you consider that these guys are rechargeable. If they last 500 cycles (and an expected life of 1000 cycles is not unreasonable; see HP #4), then I'll get 4500 hours of total listening time from a single set of batteries! To get the same total listening time with alkaline cells would require the purchase of 133 sets of four, at a total cost of $691.60 plus sales tax. That's no small piece of change. An effective demonstration at this point would be to pile 532 discarded C-cells on a table in front of the class, so that the students can see the solid waste disposal problem that's involved. INSERT TABLE2 NOTE: Although dwarfed in number by their disposable cousins, Ni-Cad cells represent a solid waste problem because cadmium is a highly toxic heavy metal. As described in the last issue of HP, Ovonic Battery Company has developed a rechargeable cell that contains no such toxins. Their nickel-hydride cell has nearly twice the capacity of the same size Ni-Cad, yet is expected to cost only half again as much. This will further improve the economic comparison made above. Production C cells are available now, and AA cells will be soon. Each student should now measure the current drain of their portable gadget. The connections are best made by the teacher or by assistants who know their stuff. BE SURE to run a continuity check on the battery compartment terminals of each device, before hooking up (Figure 2). I blew a lot of meter fuses by shorting batteries across the wrong terminals until I figured this out. INSERT FIGURE 2 Notes on Table 2: 1) Capacities based on data in Enercellª Battery Guidebook, copyright 1985 by Radio Shack. Prices based on the 1990 Radio Schack Catalog. 2) The more economical C-sized Ni-Cad is Radio Shack's standard (Std.) version. The more economical D-sized Ni-Cad is Radio Shack's high-capacity (HC) version. 3) For alkaline cells, "Radio/Cassette Service" assumes between 37.5 mA. to 187.5 mA. drain, two to four hours daily. "Flashlight Service" assumes a 375 mA. drain (for AAA, AA, and C cells) or 667 mA. drain (for D cells) for 4 minutes per hour for 8 hours daily. As a homework assignment, the students should use the information in Table 2, together with the measurements they've made, to compare the cost of using disposable vs. rechargeable batteries in their gadgets. Session B - Glue Hinge to Solar Panel Find a chalkboard that won't be erased overnight, and write down the entire sequence of steps for building the charger. The students should copy the instructions in their notebooks, but it helps to have them on the board, so the kids can look up at a glance to be sure they're on the right track. BEFORE ANYONE GLUES ANYTHING, they should outline the "footprint" that the short plate of the hinge makes on the solar panel, and that the long plate of the hinge makes on the battery holder. Set up a "sanding station", with many small pieces of coarse (40-weight) sandpaper. The students should thoroughly roughen both hinge plates, as well as the "footprints" marked on the battery holder and solar panel (Photo 2). INSERT PHOTO2 Photo 2. Solar panels for battery chargers. Left-to-right: SunWatt F-3A, with cover removed to show blocking diode; Solar World SPE50-6; Solar World SPC180-6E, front and back. The back view shows where the blocking diode goes, and where the "footprint" of the hinge plate has been roughened. Photo by George Hagerman. After everything has been sanded, the student should move on to the "gluing station", where several sets of two- part, quick-setting epoxy have been laid out. The glue that has worked best for me is "Duro"ª brand "Depend II"ª Industrial Strength Adhesive (part number MTB-1, distributed by Loctite Corporation, Cleveland, Ohio). It requires no mixing (Part A goes on one surface, Part B on the other), and the parts only have to be pressed together for a minute or so (Photo 3). It achieves full strength within 45 minutes, which is an advantage if you have only one day to build the charger. Key ingredients at the "gluing station" are paper towels and isopropyl rubbing alcohol to wipe up spills and clean hands. The students should be encouraged to USE BOTH HANDS when working the plunger to dispense the glue. That way the amount of glue that comes out the tip can be carefully controlled. INSERT PHOTO 3 Photo 3. Two students from El Ingenero'89 hold hinge plates against the back sides of their SunWatt solar panels, while the glue sets. They put the panels down after about a minute, leaving them undisturbed for an hour. They were ready to glue on the battery holders after lunch. Photo by Aubrey Evelyn. Session C - Glue Hinge to Battery Holder It is important that the end of the battery holder which has the wire leads coming out of it be placed next to the hinge pin (Photo 4). This minimizes the length of the wiring connections. Occasionally, a student's gadget will take only one battery. In order for the charger to sit level in such cases, a block of wood or spare battery holder should also be glued to the hinge. INSERT PHOTO 4 Photo 4. Lorena Nieto carefully glues the battery holder to the long hinge plate of her charger. Note the proper positioning of wire leads to minimize the length of wiring connections between the solar panel and the batteries. Photo by Aubrey Evelyn. Once the battery holder is glued in place, the hinge should be fully folded and the panel placed face down on a table. This way, the battery holder is in a horizontal position, and won't "creep" off the hinge while the glue dries. Because all the instructions were given during Session C, and there is no sanding to be done, there should be time to give a brief lecture after all the students have finished their gluing. This should cover such topics as the care and feeding of Ni-Cads and the adjustment of panel tilt to receive maximum solar energy from month to month. The use of the charger is explained. Fully charged batteries are placed in the gadget, while discharged ones are placed in the charger. The use of the gadget should be such that its batteries are discharged no faster than the other batteries are charged by the sun. This means that the students can use their gadgets more hours per day in the summer than in the winter. As a homework problem, the students should calculate how many hours of gadget use they should limit themselves to on an average December day (with their charger panels tilted at latitude + 15 degrees) and how many hours on an average June day (with a tilt angle of latitude - 15 degrees). The solar energy data for this exercise should be available from your local solar equipment supplier, or the Solar Energy Research Institute in Golden, Colorado, 303-231-1000). In talking about Ni-Cads, be sure to mention that they will self-discharge in 60 days (at room temperature). This is particularly important for gadgets that are used infrequently, such as flashlights and camera flash units. To be safe, the Ni-Cads in such gadgets should be swapped with those in the charger once a month, if unused, and after every period of heavy use. Also, it should be pointed out that the rate of battery charging behind a sunny window is much less than its outside value. Typically a single pane of south-facing window glass transmits only 80% of the direct beam energy falling on it during the middle of a clear winter day. Due to reflection, it will transmit even less at other times of day or when the sun is high in the sky during the summer. Transmission of diffuse energy (such as on overcast days) is even more reduced. Put a window screen in front of the glass, and the charger will receive only 50-60% of what gets through the plain glass. Therefore, leave your charger outside if you want to charge up a set of dead batteries as quickly as possible. Bring it indoors once they're fully charged (and again, the students can calculate how long this should take), and the smaller amount of energy coming through the window can be used to offset any self-discharge, keeping the batteries fresh and ready to go. Session D - Make Wiring Connections and Test Charger Among other things, this session involves soldering and the application of heat shrink tubing. The actual "hot work" should be done ONLY by the teacher and/or a thoroughly checked-out assistant. The directions that follow are for these "hot workers". The students should watch while their particular chargers are being worked on. This situation is far from ideal, but made necessary by the mini-panel designs now available on the market. For the SunWatt panel, which comes with a blocking diode and speaker-wire lead, the speaker wire should be cut to a 2-inch length, the two wires peeled apart about an inch, and a half-inch of insulation stripped away from the individual wires. Solar World panels come with screw terminals, so no wire preparation is necessary for them. The battery holder leads should be cut so that when connected to the panel, there is about one inch of slack. For Solar World panel SPC180-6E, which comes without a blocking diode, an additional inch should be cut off the battery holder's positive lead, to make room for the diode. Once cut, a half-inch of insulation should be stripped away from the battery holder leads, and these should be tinned. The connections are made somewhat differently for each panel type. With the SunWatt panel, two one-inch pieces of 3/32-inch (unshrunk diameter) heat-shrink tubing should be cut, and slipped over the battery holder leads. Slide these as close to the battery holder as possible, so that your soldering work doesn't inadvertently shrink the tubing! Twist the leads together and slip the heat-shrink tubing over the twisted connections. Hold the panel vertically and use a cigarette lighter to shrink the tubing. Do not hold the flame too close to the tubing or it will melt. Also be sure to pull the wires away from the panel surface first, so that you don't scorch it. Solar World panel SPE50-6 comes with a diode, and screw terminals on the front face of the panel. Once the battery holder leads have been cut and tinned, the students can make their own connections, using a standard screwdriver. Solar World panel SPC180-6E requires a 41001 blocking diode. The leads from this diode should be clipped one-inch from the barrel, and the anode lead should be bent into a fishhook shape (see Figure 1). A one-inch length of 3/32- inch heat-shrink tubing should be slipped over the positive wire from the battery holder. Remember to slide it away from your soldering work! The red wire should then be twisted around the diode's cathode lead, and the diode screwed in place as shown in Photo 2. This will hold it securely while you solder the diode-wire connection. Then slip the heat-shrink tubing up over the diode barrel (it'll be snug!), and shrink it down as described above. The negative wire from the battery holder can be screwed in place by the student. The use of a butane (lighter fluid) soldering iron is STRONGLY RECOMMENDED. This avoids a multitude of safety hazards and heats quickly. Radio Shack sells one for $29.95 (catalogue number 64-2161), and you can often find them on sale at electronic parts outlets. The next step is to test the charger, which the students can do on their own. First, they should put their new rechargeable batteries into their gadgets, to convince themselves that the batteries are "out of juice". Then, they should pop the batteries into their chargers and go outside. They should orient the panel so that the sun's rays are striking it squarely, and wait ten minutes on a clear day, longer on a partly cloudy or bright overcast day. Finally, they should put these batteries back into their gadgets. Voila! Sunshine at work! Having verified that the connections are correct, the students should tack down the wires on the back surface of the panel. Last summer, we used dabs of "Krazy Glue"ª for this job. Any sort of instant glue will work, but I found this particular brand to be well packaged for student use. Session E - Wrap-Up The homework problems assigned in Sessions A and C should be returned to the students and reviewed in class. A table should be put on the chalkboard, showing the cost comparison between disposables and rechargeables for each student. How much money will the entire class save? The disposable ethic is pervasive in surburban America. Using time and energy to drive to the store for a set of batteries once or twice a month is viewed as "less trouble" than fussing with a charger. It is easier to buy alkaline cells and toss them when they're dead. Most kids have never seen a landfill or incinerator, so it's no big deal. I have this lingering fear that the chargers may end up collecting dust instead of rays. One way to help prevent such a fate is to impress the students with the environmental consequences of not using their chargers. Here's an idea. Go through the homework assignment from Session A and have each student read out loud the number of disposable batteries that would have to be used and discarded for each set of rechargeables. Then pull out a box containing that many used batteries (of the correct size), walk over to the student's desk, and dump them. Repeat this for everyone. Finally, tell the students that they must pick up "their" batteries and put them back into the boxes labelled with their names, before anyone can leave the room. You'll hear some complaints (pity the owner of a six-D-cell boom box!), but maybe the kids will now think twice before putting their chargers on the shelf. This also is a way to put literally thousands of used batteries to good use, rather than in the air or groundwater. I've started setting up collection bins at various places around town, and it'll be interesting to see how long it takes to collect enough for a class of twenty. Access Table 3 and the accompanying vendor list provide all the information you need to order materials for a class, with one very important exception, and that's the hinge. While I was fortunate enough to obtain some prototype material that was already in stock, it was of limited quantity. Although the hinges would cost very little individually ($1.00 or less), an order must be placed that would utilize one whole coil of steel, which would be enough for about 12,000 hinges! The solar energy education business is not yet sure enough for me to justify an investment in that kind of inventory. Possibly a couple of education groups could get together for a collective order, which I'd be happy to help coordinate. Any suggestions out there? You can contact me at SEASUN Power Systems, 124 East Roasemont Ave., Alexandria, VA 22301 ¥ 703-549-8067. The hinge is not necessary for the charger to work, but it really helps improve its performance, as well as teaching some basic solar astronomy concepts. If you don't want to use the hinge, then Sessions B and C can be combined, and the students would simply glue the battery holder directly to the solar panel. A word about matching cells, panels, and loads. AA cells are The most common size that a teacher will encounter are AA cells. AA cells are ideally charged at 50 mA. This is too low for many gadgets. For example, if a panel delivers 50 mA under one full sun, three sun-hours would be required for a student to listen to only one hour of cassette tape. In overcast weather, that could take over a week of charging! That's why I've specified rapid-charge AA cells, and panels that deliver 150-200 mA under one full sun. This also happens to be a good charging rate for standard C cells, another common battery size. INSERT TABLE 3 Notes on Table 3: 1) Charging current is average expected under one full sun (1kW/m2) incident radiation. 2) Solar World panel SPC180-6E requires a 1N4001 blocking diode available for Radio Shack at two for 49¢. 3) Three-cell holders (AAA, AA from Digi-Key) have wires running beneath them, and require a thick application of glue. 4) Purchase tow sets of batteries for each student, which are to be rotated between the charger and the gadget. 5) Price breaks vary from item to item. The prices given are for ten chargers. No experience has been had with charging 9-volt rectangular transistor batteries. These are frequently found in radios and remote-controlled vehicles. A six-cell Ni-Cad is underpowered for the job, but Plainview Batteries has an eight- cell version with enough voltage to replace its disposable cousin. A suitable panel would have an open-circuit voltage of Å15VDC and Å10 mA.short-circuit current. Acknowledgements Richard Perez planted the seeds of the battery charger project and encouraged me to write up the results. He also answered a zillion technical questions when I had only a couple of weeks to work out the design. Richard Komp of SunWatt Corporation, and Brad Thompson of Solar World, agreed to modify their standard panel designs, and offer these at very reasonable prices. Thanks also to Geri Walker, of Wagner Products Corporation, who ended my quest for the perfect hinge, and kindly supplied me with prototype material. As mentioned in my last article, the students of El Ingenero'89 were terrific. They did all their gluing (Sessions B and C) in just a few hours and were then given the Session D lecture. This was a lot of heavy-duty material at the end of a full day, and I was really impressed with how attentive these kids were. It was this kind of response that inspired me to document what we did last summer. Much credit, then, goes to the students of El Ingenero'89: Olga Aquino, Stefan Barney, Carla Bernal, Vanessa Bernal, Hermes Borges, Malibea Burguillo, Peggy Cabrera, David Ferragut, Erik Ferragut, Edgar Garay, Jr., Richard Giles, Xenox Garavito, Rodrigo Gutierrez, Monique Mauge, Gerardo Molina, Lorena Nieto, Lupe Nieto, Ruben Pena, Yirla Portobanco, Emeline Tirado, and Louisette Vega. Thanks again to Lucy Negron-Evelyn (and Ivan Vera), who brought us all together.