Bike Generator

June 20, 1999
.....copied from a post I did last year.

I have not had a chance to thoroughly search the web and usenet posts for articles or stories about using a bicycle for generating power. The story below is my own personal experience. I have deliberately left out a lot about the theory of electricity to keep the story short. Comments and constructive criticism welcomed. Rick

The Exercise Bicycle Generator

It all started back in the early 60's when I was in the 5th grade and I got my first three speed bike. I lived in Washington, DC, and roamed the streets with my friends back when it was safe. I remember seeing my friends with their generator driven lights on their bikes and I just had to have one too. I saved my money by returning grocery carts full of abandoned pop bottles and soon had enough to get my generator. I went down to Pep Boys and bought my shiny new generator and headlight. Soon after, I wanted to hook a battery to the generator so the light would shine even when I stopped. I bought a 6 volt lantern battery and soon had it hooked across the generator. I was disappointed when I came back a few hours later to find a generator glowing red hot and smoking, and a battery to hot to touch. This discouraged any further attempts at hooking up and using a battery and generator together. The bike was then stolen and further dashed my hopes of generating electricity.

I recent years, the talk of disaster survival, emergency communications, etc., I looked again towards the possibility of a bicycle driven type of electrical generating device. The questions I had were, how much power can a person easily produce? How much energy could one store? Would it be cost effective? Very little has been written that I have seen and little has been done to come up with viable figures for producing electricity. We have all seen old war movies showing a GI hand cranking a generator for radio communications and possibly seen army surplus hand crank generators for sale in some surplus magazines.

Just how much usable energy did they produce and how much energy can an average person produce on a sustained or intermittent basis? First, let's discuss what unit of energy we will use. Energy, whether produced or consumed, is rated in watts. No matter what the voltage or current, the unit measure is still in watts. If you know the voltage and the current, whether produced or consumed, it is calculated as the voltage times the current (sometimes referred to as voltamps). A generator that produces 5 amps at 117 volts is just about 600 watts output. An automobile headlamp that draws 4 amps is said to consume 48 watts (most in heat, some in light).

I found an old exercise bike at a yard sale for $10.00. I scrounged an alternator and regulator from an old junked automobile. With a few nuts and bolts, a 68 inch belt from a retired riding mower, and some angle iron from a junked bed frame, I went to work with a hack saw and drill. I mounted the alternator just in front of the solid rubber front tire of the exercise bike. The belt stayed comfortably aligned in the center of the tire tread and around the alternator pulley with a little adjustment. Wiring the alternator to the regulator was quite simple if you have a schematic from a "Motors Manual". I also had a riding mower battery hooked up to it to provide an exciter voltage and storage medium. The ratio of the peddles to the alternator was one turn of the peddle to 24 turns of the alternator. Now for the results:

My first test almost sent me into cardiac arrest! I should have charged up the battery first, in order to lessen the resistance (drag) I encountered. It was extremely tough peddling the bike with that much of a load, so I put a very small capacity ni-cad battery for an excitation voltage, and a 1156 automotive light bulb as a load. One thing you must remember! An automobile alternator cannot start to produce electricity all by itself. An excitation voltage is needed in the rotating field to begin producing electricity. If you don't have a battery hooked up to provide this startup voltage and to smooth out the electricity, and if you peddle erratically, it may stop producing electricity. A permanent magnet generator does not require a field voltage, but doesn't have the voltage and current carrying capability of a conventional automobile alternator.

To understand how a normal automobile alternator works, when you turn the ignition key on in a vehicle, a large amount of current is applied to the field. This will allow the alternator to convert a lot of power at a low RPM. As the speed and voltage increase to acceptable levels, the field voltage is reduced by the regulator to drop back the output power thus performing regulation. This conventional means of regulation is not acceptable for a bike powered application because you have to peddle hard to overcome the initial low RPM field current applied to it. You'd have to peddle like you are on a "Interstate highway" to do this!

With just a 1156 light bulb that draws about 24 watts, I was able to keep it lit for about a half minute before I became exhausted. A small "dashboard" bulb could be lit with very little effort.

What Does That mean as a consumer? Obviously, 10 watts isn't enough to be a serious energy source by itself, but storing 10 watts over a period of time in a battery such as a deep cycle battery, one could store enough energy to be put to good use.
The next term to be understood is watt-hour. This is "watts per hour". The rate of production or consumption. You see this figure on your electric bill. A typical home can draw as much as 15,000 kWh (kilowatt hours) a month in the summer and if your house is total electric, as much as 25,000 kWh a month. If you peddle the generator/bike that produces 10 watts for one hour into a storage battery, you would store about 10 watts, or 10 watt/hours. Now, you have a more usable amount of energy to work with. You can now draw this approximate 10 watts from the battery for about one hour. If the battery is of a large enough capacity, you can draw much more power for a shorter time. 10 watts per hour is the same as 20 watts in a half hour, 40 watts in 15 minutes, 80 watts in 7.5 minutes, and almost 200 watts in 2 minutes! Now you can imagine how much energy you can use if you peddled for 6 hours. Maybe sharing the peddling with a few others. A typical deep cycle marine trolling battery can output as much as 20 amps continuously for about 4 hours. About 115 amp/hours. The actual usable capacity is sometimes stated on the battery itself.

Of course, the above numbers are all theoretical. Other factors to build into the equation is efficiency, power loss, and self discharge. This can lead to as much as 10 to 20 percent loss and needs to be calculated or estimated into the final equation for usable output ratings. And don't forget, energy cannot be created or destroyed. It is converted to and from different forms whether it is heat, light, mechanical, chemical, or electrical. Heat is usually the unwanted or wasted power.

To sum it up, yes, you can generate electricity from an exercise bike, about 5 watts continuous, but the amount is somewhat low according to my preliminary tests. The real amount depends also on the health, physical shape, endurance, energy (capacity) and size of the person peddling. Obviously, an automobile alternator is not the best means of creating power by human means. There is no way one can peddle up to one horse power (65 amps at 14 volts) of energy that the automobile alternator is capable of producing. The mass of the unit also means a mechanical efficiency power loss too. It will however be almost indestructible and probably rugged enough to outlast the bike and the peddler. It is tolerant of extremely high temperatures and constant high power demands. A more suitable alternator or generator would be from a salvaged riding mower unit. These units are used as a combination starter motor and generator. They deliver a small amount of power, in the amount of about 1 amp at a constant speed.

On the other hand, a 24" X 48" high efficiency poly crystalline solar panel (costing about $300.00 used) can produce up to 15 volts at 4 amps (60 watts) in direct sunlight. Without lifting a finger, you can see almost 360 watts stored in a typical summer afternoon! These panels are light weight compact, easy to move and store, and can last over 20 years if treated properly. Kind'a makes you wanna forget the whole idea about bike power generation, doesn't it? Unless you're hold up in a cave somewhere without sunlight, it just doesn't seem worth the trouble.