Options range from chain or gear drive to direct mechanical drive. The design featured in the project on page 29 uses a bicycle trainer that lifts the rear wheel off the ground, and replaces the trainer’s resistance unit with an electrical generator. Mechanical power generated at the roller shaft is coupled directly to the generator shaft, and output voltage depends upon how fast you pedal. To produce electrical power with a bicycle, you need to generate low-voltage direct current (DC), not alternating current (AC). This is because AC appliances require a steady voltage (120 or 240 volts) and frequency (60 or 50 hertz) for proper operation. That would require careful control of the generator’s speed, expressed in rotations per minute (rpm).
When mechanical energy input, such as the spinning of a bicycle wheel or wind turbine, is used to create electricity, the speed (and therefore the frequency in the case of AC) will vary according to the force applied. In order to achieve greater versatility with power management, using DC power provides the necessary flexibility. Generating 12-volt DC power allows you to charge batteries, and battery power can be converted to AC power using an electronic inverter, or it can directly supply DC appliances, such as those made for camping and recreational vehicles.
Build a Bicycle-Powered Battery Charger
This project shows you how to build a 12-volt DC battery charger with a DC-to-AC inverter, so you can use the power generated to supply conventional (AC) electrical devices. I’ve built seven generators with this basic design, but no two are identical. Bicycle trainer designs are constantly changing, and this necessitates modification of the design. Since the trainer you use is very likely to differ from mine, there’s no point in offering detailed templates, and the parts may vary. My goal is to provide enough guidance to get you on the right track using your own parts and materials.
The Parts
The most costly part required for this project is the DC generator, if you buy it new. However, you may be able to find a suitable used unit for a fraction of the cost of a new one. Check online for used and surplus electronic equipment suppliers.
The type of trainer I use has a friction roller that’s driven by the bike’s rear wheel. Prices for trainers vary widely, but magnetic and airresistance types typically are the least expensive. You will remove the resistance unit and connect the generator to the shaft of the roller. Therefore, look for a simple trainer with a flat-bottomed roller mount and a roller assembly that can be removed.
The Wiring
The electrical wiring must provide safe and efficient means to transfer power from the generator to the battery. This requires the proper gauge of wire, solid electrical connections, a sturdy connector between the generator and battery and between the battery and inverter, and fuses to protect against over-current. You will also need a diode, which is an electronic “check valve” that allows current to flow in only one direction. Without the diode, electricity would flow from the battery and spin the generator as a motor, and you would have an electric bike.
This charger design uses clips on the wires that connect to the battery, allowing for easy disassembly and transportation. If you have a more permanent location for the bike and charger, consider using wire terminals that offer a more solid connection to the battery. Most of these parts can be found at auto or electrical supply stores.
The Generator
The generator essentially is a motor that can be operated in reverse to produce electricity rather than consume it: You supply the mechanical energy to spin the shaft, and the generator turns it into electricity. Choose a DC permanent-magnet motor (brush or brushless) rated between one-sixth and one-third horsepower (125 to 250 watts), capable of generating up to 20 amps of current and about 14 volts when spinning somewhere in the vicinity of 2,000 rpm (see Figuring RPM on the next page). The output voltage will vary with the generator speed, which depends on how fast the bike wheel is spinning.
Warning!
The electricity produced by this generator can be potentially lethal. Also, batteries contain harmful acid and can deliver incredible amounts of energy if the terminals are shorted (connected together). If you are uncertain about any electrical or mechanical aspects of this project, consult an expert or qualified professional who can help.
