Build a Wind-Powered Battery Charger

How to build a simple windmill that can be used to recharge standard batteries.


Kelly inserts an old and dying battery into Breezy's battery box to be recharged . A close-up (Far above) shows Breezy's recharging mechanism with its rubber-band drive attached to both the four-inch cap and the shaft.


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Imagine using the wind to breathe new life into dying batteries. Most battery chargers require electrical power derived from the burning of fossil fuels. But what if you could capture the power of a breeze and turn it into power that could be stored until later, when you need it? Talk about self-sufficiency and recycling rolled into one! If you like soft energy paths, and want to break the "throw-away" battery cycle in your home, here's a project for you ....

Some Background

The concept of harnessing the wind to do work isn't new. Our ancestors frequently utilized wind power to pump water, saw wood, and grind grains. In fact, the design and implementation of useful windmills dates back hundreds of years. While the burning of fossil and nuclear fuels dominates world-power production, environmental concerns prompt us to now look seriously at "green" energy sources. Wind is clean, free, and abundantly available. So why not simply substitute wind power? Energy experts are quick to point out the fact that our modern per-household power consumption is usually much greater than the wind alone can provide. Does that mean that wind power won't work? Quite the contrary. Rather, our challenge is to "catch, store, and utilize" energy from as many different sources as possible. A little here and a little there saves money, worry, and the Earth.

Building Breezy

By following these simple instructions, you should easily be able to construct this windmill in one afternoon, and be recharging AAA through D-size batteries by evening. So gather up your supplies (see Parts List in the Image Gallery) and follow the instructions below.

Breezy's body is constructed of 1" diameter, schedule 21 PVC pipe. Plastic pipe was selected for its ruggedness, availability, and especially, its cost. You may choose to substitute schedule 40 PVC pipe for additional durability, but it is not necessary.

PVC pipe, normally used for plumbing, is easy to cut and put together. Begin by cutting the 1" diameter PVC pipe into the various sizes noted in the parts list. You can mark the appropriate lengths right onto the pipe with a pencil. (Avoid making unnecessary marks that you'll have to clean off later.) Cutting the pipe is simple, and can be done with a hacksaw or handsaw; make the cuts as straight as possible. You may want to use some fingernail-polish remover or acetone (in a well-ventilated area) to clean off the ink labeling from the pipe. The materials needed for Breezy are commonly available, but feel free to substitute materials on-hand where necessary and applicable. For example, you may choose to use different diameters of PVC pipe. If so, you'll have to change the size of fittings.

Also, keep in mind that if you decrease the pipe's diameter, you must scale down the overall size of the entire unit. Next, sandpaper both ends of each previously cut piece of PVC pipe to ensure a clean fit. Then, following the diagram, temporarily assemble the precut pieces of pipe with the appropriate fittings. Start with one side and assemble the sail members. Don't push the pipe pieces completely into the fittings; instead, gently slide each piece part way into the elbow, tee, or end cap with a back and forth motion. Next, assemble the members and fittings of the other sail. Notice from the diagram that this sail frame is oriented in the opposite direction. This is critical in the function of the unit because Breezy's sails work by "catching" the breeze and then letting it "spill" off the back edge of the sail. With both sail frames assembled, it's time to put together the center vertical member and the
top and bottom cross members. (Refer to Breezy Mechanical Detail in the Image Gallery). Drill a hole in the "tee" separating the top cross members. This hole should be X" in diameter, and can be easily drilled with 3/4" spade bit. (Separate the tee from the assembly before drilling it.) Also, use a vice when drilling such a large hole; the drill bit might grab the tee from your hand. This 3/4" hole will provide rotor stability by allowing the mast (steel rod) to protrude through it. Center this hole perfectly so the windmill won't be off balance.

Making and Assembling the Sails

Breezy's sails can be made from any lightweight, but durable material. (I sacrificed a sheet for mine.) If you purchase new material, launder it first to avoid shrinkage later. Then cut it into two identical pieces, 40" wide x 38" long. Turn one of the newly cut pieces wrong side out, and fold it in half so it is now 20" wide by 38" long. Stitch the two edges (opposite the fold) together to form what looks like a pillowcase with open ends. This seam should be run f" in from the cut edges. When cutting the seam-thread, leave it longer than normal (approximately six inches). Next, turn the material right side out and stitch a 1/2" hem (as you would with pants) on both ends. These hems will increase the life of your sails.

Repeat this procedure with the other piece of material. You can now glue the various pieces together. First, recheck your prior assembly for proper fit and symmetry. If satisfied with the temporary structure, start disconnecting the tee (on both sides) that connects the top cross member to the top sail members. This is necessary for sliding the sails onto the vertical sail members; if you don't, you may find yourself with no means to attach the seamed sail. Once the sails are in place, begin disassembling, gluing, and reassembling the structure one piece at a time. Use PVC Cement, available at your local hardware store. It may drip when applying, so protect your work surface with newspaper. I swab the cement inside the rim of the fitting, and then insert the piece of pipe with a quick twisting motion. Pre-check your alignment before each insertion. When you finish gluing the assembly, set it aside.

Recharging Mechanism Assembly  

This is the part of Breezy that allows for the conversion the wind's mechanical power into stored, electrical battery power. Breezy is self-starting, always turns the same way, and doesn't care from which direction the wind is blowing. Again, temporarily piece together the mechanism to check for proper fit. Take the earlier cut pieces of pipe and assemble as indicated in the mechanical detail. Keep in mind that the elbow (motor shroud/enclosure) must face toward the 4" ABS cap (rotor pulley), and be even with it.

This is so the rubber band, acting as a belt, can have one end around the motor shaft and the other around the 4" cap. This simple pulley arrangement has a step-up effect, which greatly multiplies the rotor speed (40:1), and applies it to the motor shaft. Next, remove both end caps, and with a 3/4" spade bit, drill a hole in the top center of each one. Using a 1 1/4" spade bit, drill a hole through the center of the 4" ABS cap, and then check all three holes for proper clearance.

The two end caps will easily slide over the main mast (steel rod). The 4" cap should just fit around the 1" PVC pipe. You can now begin gluing the pieces together, starting with the pipe piece fitting into the bottom of the four-way fitting. Next slide the 4" cap, open-side down, over this glued piece of pipe. Apply a ring of glue around the drilled hole in the ABS cap to secure it to the bottom of the four way. The piece to follow should be a coupler; glue and attach to the bottom (under the cap) of the same piece of pipe. By pushing the coupler up against the 4" cap, you'll secure the various pieces into a solid rotor pulley mechanism. Make sure the 4" cap rotates with the windmill assembly, and doesn't slip. Finish by gluing another 2" piece of pipe into the bottom of the coupler, and following it with an end cap having a pre-drilled 3/4" hole.

Only a few remaining pieces need PVC cement. In the mechanical detail, study the portion of the mechanism just below the pulley assembly. First, glue the tee into place on the 48" piece of 1" diameter PVC pipe. This piece of pipe is called the support mast, and provides the vertical support for the entire assembly. It is important that the motor enclosure be even with the rotor pulley (4" ABS cap), so the rubber drive band won't be misaligned. The only portion not requiring cement is the motor enclosure (elbow). Last, attach the battery box opening up to a 6"-long piece of 3/4" PVC pipe. (This is the only part of the assembly that requires 3/4" PVC.) I found that a plastic outlet box (located in the electrical section of your hardware store) and cover will work nicely to waterproof the recharging batteries.

If you use this type, check the size of the opening in the box and adjust your 6" piece of PVC to the appropriate diameter to perfectly fit the opening. As far as the matching cover, if you can find only one with either a light switch or two plug-in plate, don't worry. You can use diagonal cutters to enlarge the opening to a 2.5" by 2.5" square (approximate measurements) to allow easy access to your batteries. Attach the cover to the box so it opens upward, providing more rain protection.

The wind-power converter or "generator" is simply a DC motor which is held in the top of the 5"-long PVC pipe by several wrappings of friction tape. Wind several inches of tape around the bottom of the motor case to allow the motor to extend from the pipe end. Test fit it in the pipe, and continue the process until you're sure that the motor will stay in this position. Detail the motor enclosure elbow (see Detail) with a 3/4"-long horizontal cut on both sides of the horizontal segment. This will allow the drive belt (rubber band) to extend from the motor enclosure without touching the sides, which would make Breezy less efficient.

Cut these out with a hacksaw or on a mill so they are slightly larger than the width of the rubber band. Next comes the cutting and drilling of the bearing. The most efficient material I have found for this application is UHMW, (Ultra-High Molecular Weight) plastic. This relatively new plastic is self-lubricating, wear-resistant, and doesn't need to be oiled or replaced like a ball-bearing would. The bearing should be cut from a 2" diameter UHMW rod, and should be 1" thick. Also, a 3/4" hole must be drilled through the center, so it can move freely around the steel rod, or mast. This completes the basic mechanical construction part of Breezy.

Electrical Circuit  

The simplest form of charging system for Breezy could be formed by directly connecting the motor/generator to the battery through a diode. With this system, the diode allows power generated by the motor/generator to be applied to the battery, but blocks any power trying to return from the battery. Without the diode, the battery would rapidly discharge itself back into the motor during times of low or no wind. However,

I don't recommend this charging system. With the 40:1 step-up mechanism attached to the motor/generator, voltages as high as 10 volts can be created in high-wind conditions. These high voltages would overcharge the 1.5 volt batteries and could possibly damage them. Instead you will need to add a couple of other components to limit the amount of power made available to the battery. Refer to the Breezy Mechanical Detail in the Image Gallery, and notice the motor/generator is connected through the 1 N4007 diode and the 51 ohm, one watt resistor, to the battery holder. These parts are available from your local Radio Shack store. In addition to the diode and resistor, a special type of transistor (Field Effect Transistor) is utilized to limit the amount of power in the circuit. When the windspeed increases above the optimum level needed for charging, the motor/generator output voltage increases above three volts

The Field Effect Transistor senses this voltage increase and begins to dissipate the extra electrical energy as heat. Additionally, the 51-ohm resistor limits the amount of charging current applied to your batteries to keep them charging properly. If the wind gusts strongly, the transistor will continue to clamp the charging voltage harder and harder until it eventually "slips" the rubber drive band temporarily. This action will not harm the unit, but rather acts like a governor on the system. The LED (Light Emitting Diode) is an option that you can add to Breezy that gives you a visual "OK" indication. Building the circuit is easy. Using a small soldering iron, simply connect the various parts. Keep your work neat and remember that you will need to install the circuit in the battery box later. Remember to use red wire on the positive side, black on the negative. If not, you might uncharge a battery and damage it. Using a small voltmeter, check the polarity of your motor/generator. With the wind blowing, the rotor turning, and the motor/generator outputting a voltage, determine which lead or terminal is positive, and which is negative.

If you don't connect the positive side to the diode, the unit won't charge; double-check this on your Breezy. Congratulations, you're now done with the electrical part as well.

Final Assembly

At last, Breezy is ready to be put to the test. Secure the sails before taking your windmill to your chosen site. (Move Breezy from time to time to test various sites around your place.) Use 20-pound fishing line to hold the sails in the proper position. This strength of fishing line provides a high-wind safety mechanism. If the wind is too strong, the line will break and stop the windmill from spinning. To begin the final assembly process, drill four holes (W" in diameter) through the top and bottom end caps. Locate the holes, one per top and bottom, for both sails. These holes will hold the sail in the correct position. You may need someone to hold the sail up while you fasten it.

With a needle, thread some 20-pound test fishing line from above, through to the bottom of the top end cap. Continue with the line down through the inside of the sail (along the seam), through the top hole and out the lower hole on the bottom end cap. The fishing line should be slightly loose to allow some give in the sail, and must be knotted on the top and bottom end caps. I knotted the line around a small washer to help hold it in place.

Next, with the same needle run a small bit of fishing line through the sail material and over the top elbow in order to hold the sail up on this end. Repeat the procedure to properly position the opposite sail.

Using Breezy

Take Breezy to the operating site and begin by driving the X" rod into the ground about 18". Be careful not to "maul" the top of the rod too badly; the main rotor assembly must slide over the top.

With the rod in place, use a carpenter's level to make sure the rod is perfectly vertical; Breezy must be vertical to operate at maximum efficiency. Using a ladder, slide the support mast assembly over the top of the rod, and push it into the ground approximately 4". Drop the UHMW bearing into place atop the support mast. Then, slip the rubber drive band around the rotor pulley and ease the main rotor assembly over the top of the rod. With the rotor assembly resting atop the support mast and UHMW bearing, work the rubber drive band over the motor/generator shaft. Position the motor enclosure elbow so that the rubber drive band operates smoothly. Guy (or steady) Breezy using "clothesline" wire and tent stakes at a ground radius of approximately eight feet. The guy lines should hold the support mast firmly, and keep Breezy from moving from side to side as it rotates. I like to use three guy lines, but you may be more comfortable with four. Well, that's it just select a battery to charge. With a light breeze, my Breezy takes about eight hours to fully charge a D cell that is completely run down. Smaller C, AA, and AAA batteries will take even less time. Experiment with your unit to determine what works best in your area. Good luck with your Breezy and congratulations-you've broken the "throw-away" battery syndrome and made the world a better place.

Editor's Note: Kelly Isaac is a high school teacher and neo-naturalist specializing in applied biology, chemistry, and physics.