Charging Lithium Ion batteries using a bicycle.

Question

So, I am working on my own bicycle computer project, and I want to make it entirely self contained. Ive been browsing different way to generate power on a bicycle, and the common denominator of all of them is that the power is very inconistently generated. If you're moving fast, you are generating more power then when you are riding slow. I am trying to come up with a way to take this variable input, and effectively, and safely charge some Lithium Ion batteries.

One of the first things that stood out, was this solar charging circuit from adafruit. (http://www.adafruit.com/products/390) I assume the output ofa solar panel does vary a lot, so it would seem logical that I could modify this to work. They also have this slightly more basic version, but I am not sure how it would function in my project. (http://www.adafruit.com/products/259)

Lastly, I have seen a few very minimalistic options, but not specifically tailored to what I am doing.http://www.youtube.com/watch?v=5nGrcR030R8) I assume in this case that the phone handles the charging phases of the Lithium Ion battery, so hooking one direcctly to it would be a bad call.

I was hoping someone might be able to give me some insight on what I need to do. Thanks for the time guys!

michaelkellett · Answer

Possibly not a (very) good way to do it. You should consider using a switching regulator because the 7805 just wastes any extra voltage by getting hot. The 1000uF capacitor won't store much energy. If you are fit and your generator is good it might not matter that you are wasting some energy. Can you set up the rectifier and capacitor and measure the voltage across the capacitor at typical and max pedalling. Then connect a load resistor across the capacitor (about 10R 10W would be right) and repeat. Then you can work out if you need care much about efficiency to charge your phone. There are better linear regulators than the 7805 which is a VERY old design. (Newer ones don't waste so many volts so will work better at low pedalling rates.) MK

johnbeetem · Answer

I agree with Michael that a switching regulator would be much better. I don't know what sort of voltage range you get from a typical bicycle generator or alternator, but it will vary a lot with how fast the wheel is moving. The switcher efficiently switches higher voltages down to your 5V output voltage. You might be able to get by with a lower output voltage for your phone or bike computer. The eBay unit you show is the right idea, but it requires a minimum input voltage of 7V. You should be able to find something with a lower input voltage. You might also see if you can find a switching regulator module based on a "buck-boost" regulator such as one from Linear Tech . A "buck-boost" regulator very efficiently converts from a higher voltage to your desired output voltage in "buck" mode and automatically switches to a less-efficient "boost" mode if your input voltage falls below your desired output voltage. Some have very wide input voltage ranges, such as 2.7V to 40V for the Linear Tech LTC3115-1. A 1000uF (1 mF) capacitor only stores 1 mC of charge per volt. This is enough to provide 1 mA per volt for one second, or 100 mA per volt for 10 msec. A larger capacitor increases this time linearly. A capacitor has a limit as to how fast it can charge or discharge without damage. I think this is called the "ripple current", but that's starting to exit my level of expertise regarding capacitors. I suspect that a bicycle generator and your circuit are far from this limit.

Charging Lithium Ion batteries using a bicycle.

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