Sunday, March 30, 2014

Bicycle Power Unit - Overview


My bicycle is my main mode of transportation. Depending on where I live and work, I ride 3-25 miles a day. Over the years I have equipped my bike with a few things to make the ride more safe and convenient.

Handlebar bag, u-lock, pump, fenders, lights

The most recent addition is a headlight that consumes 9 watts of power. It came with an 8.4V 4400mAh lithium-ion battery pack, which lasts about 3 hours per charge on the full brightness setting. For a 20 mile round trip I have to charge it every day. More often than not I don't — forcing me to use the lowest brightness setting, limiting my visibility of the road as well as to other drivers. 

There are a few solutions to this problem. I could charge it every night, buy a bigger battery, or... spend hundreds of hours making a badass alternator and charging system! After adding some desirable features the decision was made and I began the project.

In addition to charging the battery to keep my headlight running, I would like to run my other light off the main battery pack. No more worrying about charging or carrying extra batteries. While I'm at it, I'd like to charge my phone too for tours or those days that someone forgot to plug their phone in the night before. 

Project Overview

To achieve this will require more than a simple alternator. First, I need to be able to power my devices whether I am moving or not. Once I am moving and producing a high enough voltage, devices will be switched from the battery to the alternator. Finally, when all the devices are being powered by the alternator and there is sufficient power to spare, the battery will begin charging. The system is outlined in the block diagram below.

Originally I wanted everything to run directly off the battery and the generator would simply supplement the system and charge the battery. The information I could find on lithium batteries suggested that they should not be simultaneously charged and discharged; that's a setup particular to lead acid batteries or complicated packs with a battery management system. 

It was also difficult with my limited knowledge of circuits to design something that wouldn't overcharge the battery in this configuration. If the generator's voltage jumped above the battery's, it could potentially source a large amount of current and kill the battery. 

I settled on switching devices from the battery to the alternator one at a time. Since the voltage produced by an electric motor is dependent on its rotational speed, the plan was to switch devices to the alternator as sufficient voltage was produced. This is not as simple as it seems. Although brushless motors are built to certain RPM/volts ratings—known as KV in the RC world or the back electromotive force constant elsewhere—this is the open circuit or unloaded rating. Once a load is placed on the generator, the voltage will drop. 

This effect can be seen in the two graphs below. Both were from testing a scavenged stepper motor that has been rectified. The first is the open circuit voltage and the second is with a DC fan as the load. 

At 100 RPM unloaded, the stepper motor shows 15 volts. Loaded with a fan for the same RPM, it only produces approximately 8 volts. If a second fan was added it would be even less. 

This will be explored more with the brushless motor purchased for this project. Once the brushless motor has been characterized, the microcontroller can be programmed to switch loads to the generator when it can maintain the minimum voltage for the given load. 

Unfortunately the jig I had set up for the stepper motor does not quite work for the brushless motor, so that's all for now.