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  1. #1
    Senior Member Frankinbiker's Avatar
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    Converting hand crank dynamo light

    I've read many of the threads dealing with dynamo lights and chargers but still don't fully understand the situation. I just moved to Japan and I currently have a dynamo powering an LED front light that is quite bright. I am upgrading this to a 5 LED light. I use a red LED light in the rear and this setup is more than adequate for riding around town but I want to incorporate a stand light feature for the front and integrate the rear blinky into the dynamo's power supply. I saw an emergency hand crank dynamo at the hardware store and thought that it could work as a charger and stand light battery backup. The charges at the store seem to have a battery pack similar to that of a cordless phone and the light works while the crank is moving or if it is not. I can figure out how to wire this directly to the dynamo and then take the power lead and wire it to my lights, but I don't know a lot about charging circuits. From what I read in the forums there is a danger of overcharging and a danger of undercharging. how can these be avoided? Do you think pedaling around town will blow out the batteries? This would make my bike even more of a get on and go bike than it already is. In America I had two 20 watt headlights, a 54 LED taillight/brakelight, and working LED turn signals, but to power all that I had to charge my battery nightly. No big deal but the thought of having the battery charge during the day really appeals to me. I will soon tear into the project but wanted to ask for advice first.

  2. #2
    Formerly Known as Newbie Juha's Avatar
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    I'm moving this over to Electronics and Lighting Forum (originally in Commuting)

    --Juha, a Forum Mod
    To err is human. To moo is bovine.

    Who is this General Failure anyway, and why is he reading my drive?


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  3. #3
    Scott n4zou's Avatar
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    Those hand crank dynamo flashlights are not very bright. It would be adequate as a "be seen" light only.
    You might consider a DIY dynamo LED lighting system. Here is a simple way to do it.

    The 4 Ni-MH batteries will power both the headlight and taillight for a couple of hours with the dynamo disengaged from the tire or if your using a hub dynamo use switch S2 to disconnect the hub from the batteries. With the dynamo engaged 500mA is feeding the batteries and LED's. The headlight 1-watt LED consumes 350mA, 36mA is being consumed by the taillight LED, with 114mA being sunk by the batteries allowing a very slow recharge. If you need to recharge your batteries faster due to extended stand times or very slow speeds due to traffic you can open S1 turning off the LED's and pump 500mA rapid recharging current into the batteries. If your commute does not have many stops you can overcharge your battery pack. Just disengage the dynamo and run on batteries only for a while to discharge them. If you deplete your batteries to the point they stop working at stops or very slow speed open S1 to prevent damage to the batteries and LED's. You can use an LED driver to do that for you automatically with a circuit like this circuit.

    A very simple battery monitor can be made from a 741 op-amp. I wont go into it here, just go to this link.
    http://www.reuk.co.uk/LM741-OpAmp-Voltage-Indicator.htm

    If you use a TL082 chip with two OP-AMP's you can make both low and high level indicators on one chip!

  4. #4
    Senior Member Frankinbiker's Avatar
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    I did not mean use the hand crank dynamo for a light. I want t o to the electronics and battery charging system from the hand crank dynamo with my hub dynamo and head light and tail light. It seems that for saftey the manufactures would have allready built in overcharging circuits but maybe not.

    I am confused about your system. I didn't think you could just hook up a battery to electricity to charge it unless it was a sealed lead acid battery. Doen't the voltage have to be regulated? I think your system will work except I would love to have the switching automatic. Would having the lights switch from battery power to hub power automatically use too much energy?

    My dream hub setup would be two 5 LED headlights, each with a switch, a rear tail light with about 1-15 LEDs for size and brightness and a battery charging pack for use when the bike is not moving. I might be over my limit of available power but LEDs don't use that much power. The LED on my bike now is almost fully on when walking the bike.

  5. #5
    Scott n4zou's Avatar
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    Quote Originally Posted by Frankinbiker View Post
    I did not mean use the hand crank dynamo for a light. I want t o to the electronics and battery charging system from the hand crank dynamo with my hub dynamo and head light and tail light. It seems that for saftey the manufactures would have allready built in overcharging circuits but maybe not.

    I am confused about your system. I didn't think you could just hook up a battery to electricity to charge it unless it was a sealed lead acid battery. Doen't the voltage have to be regulated? I think your system will work except I would love to have the switching automatic. Would having the lights switch from battery power to hub power automatically use too much energy?
    The batteries work as both voltage and current regulators with the dynamo. The batteries load the dynamo enough to prevent the dynamo from producing more voltage than the rated voltage of the batteries. A bicycle dynamo is a constant current device rated at 500mA. It will pump 500mA into a circuit without batteries. This is not good if your combined load of LED's is less than 500mA. Adding the batteries gives the excess current a place to go, in this case into the rechargeable batteries, which is good! No additional circuits or control chips are required.
    The switching is automatic! I run a cheap bottle dynamo. At 8 MPH my dynamo starts making enough power to equal the load of the LED's so at that point no power is being drawn from the battery pack. At 10 MPH the dynamo is producing enough excess current to start recharging the batteries. At 14 MPH the dynamo saturates and is incapable of producing any higher current. At this point the excess current is 120mA, which happens to be the suggested normal recharging current. Disconnecting the LED's forces 500mA into the batteries. This happens to be the suggested rapid recharging current for Ni-MH batteries.
    My system is perfect for LED lights and a standard bicycle dynamo.

    Quote Originally Posted by Frankinbiker View Post
    My dream hub setup would be two 5 LED headlights, each with a switch, a rear tail light with about 1-15 LEDs for size and brightness and a battery charging pack for use when the bike is not moving. I might be over my limit of available power but LEDs don't use that much power. The LED on my bike now is almost fully on when walking the bike.
    Note the LED I am using, a 1-watt Luxeon star. This LED is rated at 50 lumens! That’s much brighter than 5 5mm LED's. My Taillight is a single 5000 MCD LED. That is the brightness of a typical brake light on a car. My battery pack is rated at 2.4 Amp Hours. I can ride around with the dynamo disengaged for several hours before the batteries are depleted. My problem is a tendency to overcharge the batteries by forgetting to disengage the dynamo on long night rides on an MUP where I rarely have any stops.
    Last edited by n4zou; 09-26-07 at 01:30 PM.

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