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.
Formerly Known as Newbie
I'm moving this over to Electronics and Lighting Forum (originally in Commuting)
--Juha, a Forum Mod
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.
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.
Originally Posted by Frankinbiker
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.
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.
Originally Posted by Frankinbiker
Last edited by n4zou; 09-26-07 at 01:30 PM.