Solar/capacitor powered RC car.
As you can see in my previous post, I believe that the super capacitors are possibly the way to go with the future cars. To test my theory, I've moded a very small RC car to use a capacitor instead of a battery.
The results are pretty good, and the car moves for an estimated 20 meters. I can play with it for about 2 minutes before it runs out of juice, while not in the sun.
Except for the smaller run time, the capacitor version is much better than the battery equivalent, because the battery was 1.2V @80 mAh.
This capacitor, while rated at 2.5V, the open circuit voltage it can store is over 3.6V . So basically it provides (in the beginning) 3 times more voltage than the battery. I was a little afraid this might burn the motor, but since the cost of that car is about 10 USD or less that was an acceptable risk.
It turns out that the motor was quite happy with 3.6 volts, and the car was so fast that it could even climb on the kitchen carpet (I was running it on linoleum). With just a battery, it can't really do that.
The turns are very abrupt, and the can can stumble over itself, and turn upside down. Since this won't cause any damage, I think it's kind of cool.
One thing worth mentioning is the fact that a capacitor, unlike a battery, will drop it's voltage linearly. Which means, after half the power was used, the voltage is also halved.
A battery, on the other hand, doesn't drop it's voltage (much) until it's drained.
That means that only 2/3 of the capacitor power can be used, after the voltage drops under 1.1 volts the car it's not going to work.
Ok, enough for the theory, now let's go into the construction details.
What you will need:
a. A small RC car. You can find them in a local toy store, Radio Shack, eBay, etc. For example, you can get 4 of them and free shipping for 29 USD, from this guys.
b. A super capacitor, preferably 10 Farads or more. They are kind of hard to find, I got mine from a broken digital camera, but this company has some nice selection for affordable prices: www.alliedelec.com
Notice how my capacitor is flat, and the capacitors here are round. A minor inconvenience, but shouldn't affect the performance of the car.
c. A 3V solar panel. I highly recommend a flexible one, they are very lightweight, and being flexible you don't have to worry about them being damaged if the car crashes. Some good source is this company, although you might be able to find them cheaper on eBay.
d. Soldering gun/iron, soldering supplies, and some wires.
e. Glue, some screwdriver, wire cutters, etc.
1. Take the RC car cover off. It's just getting in the way.
2. Open the battery compartment (bottom) and remove the battery. You might want to put it aside for other projects that need small batteries.
3. Solder two wires on the bottom, where the battery terminals were. The polarity should be marked on the bottom.
4. Glue the capacitor on the back of the car, and connect the wires from the battery compartment to the capacitor. Pay attention to the polarity of the capacitor.
5. Glue the solar panel[s] to the capacitor, and connect another set of wires from the solar panel[s] to the capacitor.
After everything is completed, turn on the RC remote (some car models will leak a lot of voltage while the RC remote is off) and place the car in such a way so that the sun shines on it. If no sun is available, you can try to recharge it from a light bulb, but be careful, if you place an incadescent light bulb to close it might damage your solar panels.
A multimeter would be very useful to determine when the car is fully charged (a voltage of over 3 volts it's good, 3.40 is better, but some capacitors might not be able to charge that much).
With one flexible solar panel (3v, 25 mAh) it should fully charge in about 20 minutes, under the sun.
That's it, now you are ready to play with your car.
The reason there is no diode between the solar panel[s] and the capacitor is because the car itself will drain all the voltage if idle, as they do not have a on/off switch. Another reason is that a diode will lower the voltage by about 0.6V, and you want the capacitor to be as charged as possible.
Let me know if you have any comments, questions, etc.