If you only want to learn how I built the USB Bike Generator and not why the BikeGen system wasn't the best
, you can skip this step.
In order to set a bench mark for the efficiency of BikeGen system I decided to do some testing. In my searching for switching regulators one of the best resources I found was this article http://www.dimensionengineering.com/switchingregulators.htm
It explains how switching regulators work and how you can calculate the power lost through heat generation. The power lost to heat can be calculated using this equation:
Power lost = (Input voltage – output voltage) * load current
So in order to find the power lost you need to know the input and output voltages as well as the load current. Luckily, I already had the tools for this job which required 3 digital multimeters and a small drill press.
The test setup was fairly simple, I tightened the drill press chuck onto the shaft of the stepper motor and loosely clamped the motor in a vice. This was the same stepper motor that I have used for both the Bike Generator and and BikeGen Instructabes. This actually works out really well for two reasons. First the speeds that the drill press can spin are labeled and from these speeds I calculated how fast I would need to ride the bike based on how fast the generator is spinning. So using the tire size, rim size and the diameter of the small wheel mounted on the stepper motor for the BikeGen instructable I calculated the following speeds:
Drill Press RPM Bike Speed (MPH)
This range of bike speeds seems very reasonable. The second reason that using the drill press is a good representation of a bike rider is the power rating of the drill press motor. This drill press is rated at 1/3 horse power which is roughly 250 watts, this is attainable by an average person while riding a bicycle.
The next step was to connect 3 digital multimeters into the circuit. I had cut some of the wires and use a few sets of alligator clips to make this happen. Check out the picture or the Test Setup pdf to see how I connected the meters. Basically the current meter was placed inline between the regulator and the charging circuit and the voltage meters connected after the diodes and after the regulator and were grounded at the same spot. This measures the values I need to calculate the power lost.
Once everything was ready I plugged in the drill and turned it on. I tested the generator at every speed the drill was capable of and found out that at the first 3 speeds (620,1100,1720 RPM) there was not enough power to consistently charge the batteries. This was confirmed by the blinking LED on the charger. At 2340 RPM and 3100 RPM enough power was provided by the generator to charge the batteries. The values I measured at these two speeds are listed below:
Motor RPM 2340 3100
Input Volts 15.2 20.2
Output Volts 13 12.68
Output Amps 0.29 0.28
Power Lost (W) 0.638 2.106
Output Power (W) 3.770 3.550
Total Power (W) 4.408 5.656
Efficiency 85.5% 62.8%
The power lost was calculated from the equation I showed earlier, the other values were calculated from the following equations:
output power = output voltage x output current
total power = output power + power lost
efficiency = output power / total power
So looking at these results I think there is something to be desired. First there was not enough power to charge the batteries until 2340 RPM which is 17.7 mph on the bike. This is a high speed and doesn't seem reasonable for a casual bike ride. Of course this speed is attainable but it would be a lot of effort to maintain the speed for an extended period of time. The efficiency seems very good at 2340 RPM (85%) but it drops to 62% at 3100 RPM. This is because the input voltage goes up at higher speeds which means more power is lost to heat. So unless I want to ride the bike at and average speed of 18mph everywhere I ride I will not achieve the best efficiency possible.