How to Build a Bicycle Generator

Step 1: Bike Stand

First you need something to hold your bike. You can either build your own bike stand or buy them. We used a bought stand for the back and made our own for the front.

Buy a stand: These stands are especially nice for the back wheel because some of them are adjustable from side to side (right and left to the rider). This variation makes aligning the connection to the motor easier.

Step 2: Make a stand for the front wheel

For the front bike stand, we used a few blocks of wood. The base was created by a 4x4x24” wood block. Using two 2×4” planks, we created the bolt-holding blocks by making a ⅜” hole high enough to be comfortable when you ride. For us, this ended up being about 12 ½” from the ground, but this is variable depending on the size of your bike.

The support blocks sandwiching either side of the two high blocks were made by sawing about 4 inches off of the 2x4s. These support blocks were attached into the base block with 2.5” screws, allowing enough space in between the blocks to fit the tall bolt-holding blocks.

Finally, 3” screws were drilled in diagonally from each side at the support blocks, through the bolt holding blocks, and into the bottom block. We threaded the ⅜” bolt through the holes in both 2x4s to create a place where the front fork of the bike could be rested. A good idea when drilling screws is to pre-drill your intended location with a slightly smaller bit than your screw. This makes the process a lot easier.

This is for those who only have the bike frame. If you have a front wheel attached, don’t worry about this!

Step 3: Bicycle Frame

Step 4: Bicycle to Motor

Here you again face a choice:  you can use the back wheel to spin the motor, or you can go more directly from the chain to the motor. Using the back wheel wastes some energy in friction and spinning a mass. But getting the correct gear ratio for the chain-to-motor strategy proves difficult.

This step is the most hands-on and difficult of the process. We recommend that you use the back wheel as the connection to the motor. However, if you want to have a more efficient connection, we also have a more complex option.

Why you need a motor: the motor converts movement of your legs into DC electricity.

Choosing a Motor: A stepper motor, car alternator, or an electric scooter motor will all work. We used a scooter motor. The motor produced voltage proportional to its RPM . The motor produces current based on the load attached.
For reference, a mountain bike tire going at 20 mph spins at 250 RPM. Additional RPMs for the motor come from the ratio of the wheel size to the frictional cylinder on the motor.

Step 5: Back Wheel Option

Making a bike generator using the back wheel is the more common method. Find a motor that can mount a cylinder that can grip well to the back wheel of the bike. Using a hinge and various plates of aluminum, you can construct an adjustable mount for the motor that will allow you to vary the amount of contact between the cylinder and the wheel. You attach the motor to the upper plate, and adjust the position or angle of the plate with a bolt or screw.

The back wheel option will give you all the RPM that you need-the gear ratio between the wheel and the cylinder in the back creates plenty of RPM and thus more than enough voltage.

Additional RPMs for the motor come from the ratio of the wheel size to the frictional cylinder on the motor.

Step 6: Chain to Motor Option

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Step 7: Motor to Charger

Why you need a charger:
To charge, batteries need a voltage slightly higher than their output voltage. Putting in too high a voltage can damage the internal circuitry of the battery, reducing its lifetime. Usually, circuits trickle a little bit of current in a battery. But with a bicycle cranking out watts, you want to put whole amps. Battery chargers hold the voltage steady at the appropriate point, and then increase the current allowing higher than normal transmission of power.

Picking a Charger:
Remember that the voltage of your motor will be varying with the speed of your pedaling. The charger we used takes anywhere from 12- 24V. Though chargers may brag outputs of 10s to 20s of amps, batteries cannot stand such current. For example, the battery we used has a maximum charging current of 5.4 amps. Check that the current of your charger matches the limit of your battery.

Connecting:
With a multimeter, measure the voltage coming out of your motor. Connect the positive output of the motor to the positive input of the charger and vice versa with the ground wire. Depending on the direction you spin the motor, the positive wire may not be the red wire; the motor works both directions but gives inverse voltage.  If you can adjust the output current. As you may expect, larger current charges the battery faster but makes pedaling harder.

A word of warning: Do not overload the charger! Depending on your gear system, it can be very easy to put out more than 24V. Doing so will break your charger. If you will not be the only one using the system, consider adding zener diodes in case of excess voltage.

Some numbers for thought:
An iPhone 5 battery has a capacity of about 1440 mAh. Let’s say you output 2 Amps from the bicycle into the 12V battery, and use a socket on the inverter to charge your phone. Then it would take 40 minutes of pedaling to create enough energy to charge your iPhone from nothing to full capacity. Likewise, at 4 amps, only 20 minutes.

To charge the entire battery, it would take about 9 hours when outputting 2 amps.

Step 8: Charger to Battery

Why you need a Battery:
Charging your laptop could take a few hours, but you probably do not want to be on your stationary bike for that long. The battery holds your generated watts to be dowled out on an as-need basis.

Choosing a Battery: 
If a traditional car batterys are called lead-acid batteries; You do not want lead-acid dripping from you battery if you tip it over. Furthermore, we heard that if a car battery is tipped over, it can short circuit and explode. .
Marine batteries or sealed batteries can withstand the tipping of a tumultuous world. Make sure your battery is rechargeable. And finally, choose the capacity of the battery to match your needs. We chose a 18 Amp-h battery because it holds about three laptops worth of energy.

Connecting: Use the same caution as you do when jumping your car. Connect the positive terminal first for added safety. The voltage across your battery will be different when you are charging, when it is sitting, and when it is discharging; they will be about 14V, 12.5V, and 11 V respectively. The spec sheet for our battery warned to stop charging when the voltage reached 14.4 V.
Check your battery’s spec sheet for its max voltage point.

Step 9: Battery to Inverter

Why you need an inverter: 
The AC inverter converts the DC voltage from the battery into AC voltage, which is what comes out of most electrical wall sockets. You’ll often see inverters on a small scale in car adaptors, where they take the power from the cigarette lighter (which is hooked up to the car’s battery). Most general purpose AC inverters are Modified Sine Wave inverters. If you want to know more about how these inverters work,here is a good reference source.

Choosing an inverter: 
When shopping for inverters, you want to look for a few features. First, make sure that the output AC voltage is at the level of wall plugs. Wall sockets usually put out about 120V, but it isn’t absolutely necessary to have your voltage match that; anything from 110-130 Volts AC will be fine. Be sure that the frequency of the output is at 60 Hz, which is standard in the United States.

Another thing to consider is the watts that the inverter can output. The power needed from the AC inverter will depend on the type of electronic appliance you are trying to use. For some reference, cell phone recharging takes less than 5 watts, while a microwave will consume 1500 watts! Since price goes up with the power output, you will need to make some decisions on how much you want to spend and what appliances you expect to power.

Another important feature to have is an inverter that can take a range of voltages. Many general purpose inverters will only take in a 12 V DC input. Since the actual output of a standard recharging battery can vary from less than twelve to just over 14, it is important to find an inverter that will be able to take that range of voltage inputs.

Finally, to protect your appliances it would be important to keep the inverter in an open location. Transforming DC to AC will create some heat, and circulation is important to keep the inverter functional.

As for our choice of inverters, we decided to go with the Wagan 400W converter with two additional 5V USB ports, from McMaster-Carr (model 6987K22) . We knew that we weren’t going to be attaching high power appliances to our generator, yet we needed enough to power something like a desktop computer and monitor, which combines to about 250 watts of power. This inverter will recognize if there is an overload of input voltage and shut off, protecting your appliances from surges. It also came conveniently with battery clips, which we used to hook up the battery to the inverter.

How to hook it up:
Using the battery clips, hook up the positive and negative leads to the matching leads on the battery. When attaching the second clip, expect a small spark as the circuit completes. Make sure that you’re holding the rubber ends of the clips when hooking up the battery.

Step 10: Videos!