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Charging Lithium Ion batteries using a bicycle.

Former Member

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!

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  • 0

    So after watching some videos on Lithium Ion charging, I've learned a lot. I am wondering if this would be an effective way to regulate the power coming from a generator on the bicycle. What voltage ranges would this system be safe at? Does this have any limiations?

    http://makezineblog.files.wordpress.com/2009/06/circuitdiagram-thumb-600x346-30527.jpg?w=400&h=231

     

    If I used a large capacitor, I could minimize the effect of speed changing, and even stopping. Does anyone have an input on this plan, anything I need to keep in mind? This is all fairly new territory for me.

  • 0 in reply to Former Member

    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

  • 0 in reply to Former Member

    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.

  • 0 in reply to johnbeetem

    Just to clarify the details a little bit. Putting aside the idea of charging my phone directly from the generator, could I use something like you linked to be the power source for a Lithium Ion battery charging ciruit? I want to design a project that will charge high capacity lithium ion batteries, and then I can use that power to do whatever I need to later on (i.e. charge my phone). From what I am researching, making a charging circuit is pretty straight forward using an IC like these. Powerstream has LiIon batteries that are upwards of 4Ah, and batteryspace has some upwards of 21Ah+. The batteryspace batteries seem to be missing protection circuits, which I know essentailly nothing about, but do know they are very important. Would it be a logical goal to try to take one of these large batteries, and charge it while im riding? Keep in mind, I ride up to 50 miles sometimes, so charging time shouldnt be a huge deal, I understand that it would take awhile to charge such a large battery.

     

    Also, thanks for all this insight so far, you guys are help me a lot to understand all of this!

  • 0 in reply to Former Member

    Mason Adkins wrote:

     

    Just to clarify the details a little bit. Putting aside the idea of charging my phone directly from the generator, could I use something like you linked to be the power source for a Lithium Ion battery charging ciruit? I want to design a project that will charge high capacity lithium ion batteries, and then I can use that power to do whatever I need to later on (i.e. charge my phone). From what I am researching, making a charging circuit is pretty straight forward using an IC like these. Powerstream has LiIon batteries that are upwards of 4Ah, and batteryspace has some upwards of 21Ah+. The batteryspace batteries seem to be missing protection circuits, which I know essentailly nothing about, but do know they are very important. Would it be a logical goal to try to take one of these large batteries, and charge it while im riding? Keep in mind, I ride up to 50 miles sometimes, so charging time shouldnt be a huge deal, I understand that it would take awhile to charge such a large battery.

     

    Also, thanks for all this insight so far, you guys are help me a lot to understand all of this!

    Yes, those chips look reasonable.  I'm not an expert in battery changing, but the data sheet I looked at sure does make it look easy. image

     

    I don't know if bicycle generators have evolved since I used one last century, but it seemed like it had a lot of friction loss.  I wonder if you'd be better off with solar panels on a 50 mile trip?

  • 0 in reply to johnbeetem

    Im looking into this really cool dynamo hubs, and from everything I have seen, they have come a long way from the original bottle shaped friction generators.

  • 0 in reply to Former Member

    The chips you suggest are not suitable for this application. (Linear regulator, absolute maximum input voltage 7V). Before you design the charging circuit you must measure what comes out of the generator. Most of the Lithium Ion batteries you refer to are bare cells with no protection. Unless you are very experienced you should not use these. Look for battery packs with built in protection. If you overcharge a big Lithium Ion battery it WILL catch fire and may explode. You want a lot more than cycling shorts between you and a burning 67WH lithium battery !

     

    Measure the generator first (you might find enough data on the web to know what to expect but  aquick Google didn't turn up anything).

     

    MK

  • 0 in reply to michaelkellett

    Alright thanks Michael, once I get into the generating aspect, I will measure and report back.

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