Student-Built Bicycle Generators

Today I stopped at the high school to meet up a with a former student who wanted to borrow our bicycle generator. Her college in Canada (I forget which one now) is having a green fair, and she and he compatriots are hoping to rehabilitate our delapitated machine.

Two summers ago we built 5 bicycle generators with a group of summer school students using Subaru alternators and deep cycle batteries... and some wires and stuff (heh heh). All 5 worked by the end of the 2-week session (which seems amazing to me now), and since it was the first time anyone in the group (myself and my co-teacher included) had ever done something like this we made a lot of mistakes that would certainly affect the way we would build them in the future.

In any case, this is what we did:
We took Subaru alternators (cause we could get them for free, and Subaru is like the official car of Vermont, so they're pretty abundant), and we made sure there was a good electrical connection between the outside of the metal alternator and the metal bicycle frame (grounding), likewise with the negative side of the battery (use a high-gauge well-insulated wire).

The Large Bolt on the alternator carries the majority of the current, so you need a pretty hefty, well-insulated wire connected to the positive side of the battery, but that's not going to be enough to produce electricity once the alternator's turning.

The reason is because the alternator has no permanent magnet. It's an electro-magnet, which means that it needs just a little bit of juice to start production (lighter-gauge wire should be fine). Thus the idiot light. When you flick the switch to "on", the battery sends a little bit of current through the bulb, which provides resistance and tells us that there's electricity flowing through there. And when the bulb is lit, that means the electromagnet is juiced and its ready to produce electricity on its own.

The alternator is built to cease using electricity from the battery once it's going, so after a few turns the idiot light should go off indicating that it's sufficiently powering itself as it turns. When you stop, however, the light should come back on.

Be sure to turn the switch to "off" when you're done, so you don't drain the battery.

Advantages:

• This system would be great for long-term power use - if you were actually trying to fill a battery with human-generated power.
• It was super cheap - each bike was built for less than $100!
• All the parts are readily available at an auto-parts store.
  

Disadvantages:

• The way we built them ... (heh heh) they were almost impossible to move. Those deep cycle batteries were freaking heavy.
• We ended up attaching a tiny inverter to our battery (with the cigarette-lighter end cut off) so that we could actually plug in some devices for our parent/community exhibition. But what we didn't anticipate was that those inverters were the type of device that is "always on", and as long as it was plugged in they were sucking energy from battery :( thus killing them.
• They did not demonstrate the correlation between the wattage of a device and how hard you would have to pedal to power it. Because of the electro-magnet, you couldn't get around needing a battery. So as an educational tool to demonstrate power it wasn't so good. As a tool to teach circuitry, it was great.

The bicycle generator I lent to the college student was of a different (simpler) design, which I'll notate tomorrow probably.