The following three circuits can be used to charge a battery off of a bicycle generator. Since bicycle generators are designed to not overload whatever they are powering, these circuits do not include any kind of power regulation. The best type of battery to use with this type of setup is either Nickel-Cadmium battery or a Lead-Acid. Newer battery types such as Nickel-Metal-Hydride and Lithium-ion require "smart" chargers.
I do provide a (now obsolete) Regulated Battery Charging Circuit discussion as well if you do what I had done at one time, and build a much higher power DC generator. (In my case, made from the spindle motor of an old daisy-wheel printer)
The first two circuits are really simple (even for those who don't know anything about electricity) and the last is an attempt to get the best characteristics of the previous two. You might also want to look at my Bicycle Generator Overview to understand the basics of these devices.
The Bridge-Rectifier Circuit
|Simple Bridge Rectifier|
This circuit works quite well for charging a battery from a bike generator. Assuming that a standard 6 volt output bike generator is used, it can easily charge a battery up to about (oh, say) 7.2 Volts. When a bike generator is running under no load, it will put out way higher than its rated voltage. Since the battery does not provide a load until the generator voltage gets just above the battery voltage, plus two diode-drop voltages (usually about .7V each) the '6 volt' generator will easily and efficiently charge the battery. The current coming from the generator is limited by its construction, however. Most bike generators are designed to 'saturate' at about 1/2 amp. A nicad battery can easily accept a charging rate (in amps) as high as its rated ampere-hour capacity, so the circuit does not need to provide any regulation.
As described above, this circuit will work great with a generator that is designed for 6v output and 3-6 watts total power. The battery can be any capacity, at a voltage up to a couple of volts above the rated generator voltage. If you wanted to use a 12V battery, there are some 12V, 6w bike generators out there, but they are a lot rarer than the 6V kind. You CANNOT string two generators in series (like you would batteries) to get higher voltage, unless you found some way to keep the generators in phase. You can, however, have multiple generators, each with its own bridge rectifier, charging the same battery. (At one point, on an earlier version of my bike, I had no less than six generators hooked to one battery in this fashion).
With this circuit, and using 'matching' generators and batteries (roughly matching rated voltages), you can expect to start charging the battery from the generator when you are going 10-12mph. (assuming 26" bike wheels) You can lower this speed by using a higher voltage generator with the same voltage battery, a lover voltage battery, or by using a smaller diameter bicycle wheel. The first two options lose efficiency at higher speeds, since the battery will limit the generator voltage, and in both cases the battery voltage is significantly less than the generator voltage. Doing this gets your minumum useful speed down to about 8mph. The last option, using a smaller bike wheel, works because at a given speed, the wheel is spinning faster, so it turns the generator faster. Of course, this might require massive reconfiguration of your bike. If you have a recumbent with a 16" wheel, then you are in business, though. (Use a hub generator in the 16" wheel, it should start working at less than 8mph with a 7.2V battery, you might even be able to get away with a 10-12v battery in that case)
Of course, when you are going slower than the minumum speed necessary to charge the battery, the generator is still making power, and that power is being wasted. The next circuit solves that problem, but creates a bunch more.
This circuit starts charging the batteries when the generator output is still only about half of the sum of the two battery voltages (which should be the same). It does this be charging each battery alternately. The top battery gets charged off the top half of the generator's output waveform, and the bottom off the other half. Since the batteries are hooked in series (like batteries in a flashlight) you still get the full voltage out of the circuit, across the + and - terminals. This circuit has another advantage over the above circuit in that it has fewer diodes. This means not only are there fewer parts, but also the generator need only overcome one diode-drop voltage instead of two, to start charging the battery. This circuit starts working (assuming two 3.6 volt batteries and 'matching' 6V rated generator) at well under 10mph.
The problem with using only this circuit is that, since the generator is current limited, the maximum power that the generator can put out with this scheme is around 4.3V, 1/2A (same battery, generator as above). With the previous circuit, the generator was able to put out 8.6V at 1/2A, twice as much power. Therefore, once you are riding fast enough to use the bridge rectifier circuit, it is much better. Another possible problem might arise if you want to use a 'canned' lead-acid gel battery. You can't get to the individual cells in this kind of battery, so you would have to get two separate gel batteries each at half the voltage you want. With Nicad batteries, it is usually not hard to get at individual cells to put in the center wire.
On the other hand, if you were to set this circuit up so that Each battery had the same rated voltage as the generator, then this circuit becomes more efficient than the bridge rectifier circuit at any speed where the bridge rectifier works, since the generator is allowed to run at full blast, but there is still only one diode drop to contend with. If you want a 12V battery and you have a 6V generator, this is the way to go. (Of course, then your minumum speed would go back up again.)
This is the unregulated charging circuit that I have installed in my bike (with the green portion included). It can be switched between the bridge rectifier and split-battery charging schemes using the black switch. When the switch is open, the circuit works as a bridge rectifier, when it is closed, it works as the split battery scheme. The top (green) section is a way to automatically switch the circuit. The green diodes and capacitors form a second, independent split-battery scheme which provides power to the relay coil, through a resistor. When enough power is coming from the generator to open the normally-closed relay contacts (green switch), it switches over to the bridge rectifier circuit. When the generator slows down enough, it goes back to split-battery charging. This way I can improve overall efficiency.
The relay coil has to be fairly low-power for this to be useful, otherwise enough power is sucked up by the relay to cancel out any gains from this switching scheme. The relay I used is a DIP-Relay (the width and height of a pencil, and about 1" long) and it draws only about 2% of the generator's power. It has a 12V coil. The resistor is to control the speed at which the circuit switches over. With no resistor (0 ohms) it will switch at 3 mph, way too slow. I used a 20 ohm resistor, this moved its switch-over speed up to 12-13 mph, right about where the bridge rectifier becomes better than the split-battery scheme.
If you do not want the automatic portion, do not build anything in green, and replace the green switch with a wire (short it out). Then the black switch will control which charging method is used. With the optional circuitry, the black switch enables / disables it. When the black switch is open, the automatic section is disabled and the circuit works as a bridge rectifier.