Use dynamo to charge battery pack

[ormo]

Hi, I'm looking to build a device for my bike that will:

Use a dynamo to charge a central battery pack that will then be used to power some LED lights. I wish for the charging to be continuous: the battery is constantly being charged (provided I am pedalling) and the light's can be turned on or off. The battery must be sufficient to power the lights for at least 10 minutes without pedalling.

So far my rough idea is to use a small motor and some kind of grip that runs along the wheel edge to act as the dynamo. After this I'm completely stumped.

I'm not looking for circuit diagrams or anything (but feel free to provide them if you want), just a general summary of how you think I could arrange this project.

Thanks!

 

[ormo]

Would I be correct in assuming that the power drawn by some 10 LEDs (5 x white; 5 x red) wouldn't be very high?

 

[blueroomelectronics]

Depends on the leds, not much generally unless you're trying to build a headlight.

 

[Super_voip]

Could be using this 53 LED headlight
**broken link removed**

 

[pc88]

Do you want something like this: **broken link removed**

 

[James1986]

What is your battery type going to be? Li-ion? NiMH? or super cap ?

Also, could you give a little more information, like the above posts ask, how many LED's do you want to power, and what is your experience with electrics? I don't want to be giving you advice on circuit setup and current draws when you already have your degree in electronics

Do you have any dynamos in mind? Or a total cost for the project?

 

[bfree]

there is already a dynamo for bikes, u can use one of them and with its max voltage u can use a zener diode to limit the voltage

 

[ormo]

Ok... I'm fairly good with electronics - give me your best shot and I will Google what I don't understand. I plan to use about 10-20 LEDs.

I'm not sure what battery type to use, what you you recommend?

I would like to keep costs for this project as lot as possible ideally.

 

[James1986]

Okay, there are a couple of battery ideas. I'll try to give you a brief overview.

You have, alkaline, NiMH and NiCD, Li-ion and Li-poly.

Basically, in that order, you have cheapest to most expensive. As for quality you can pretty much use that order too. Things to take into account are battery life (Li-poly being the longest lived), cost (Li-poly being the highest cost), and number of recharges before it becomes pretty useless (once again Li-poly is the best). I'm sure you can research this on the internet using your preferred search engine! Find the right one for you!


I would, on the other hand, try using a super capacitor. In a nutshell, these capacitors are capable of storing a huge amount of charge, without loosing their charge carrying capacity. For example, a Li-poly battery will live for around 1000 charge cycles, whereas a super cap will live for well over 500,000 charge cycles, with no max charge loss!
I suggest you look these up too, as they are probably just as good for what you want to use them for as batteries, but the circuit design might be a little more complicated. And they are about $30 as compared with a Li-ion pack, for about $10


As for power drain, it depends on how you connect them (series, parallel, or combination of both) Either way, you will have lots of volts and little current, or lots of current and little volts. I think, depending on colour, that you will need at least 1.5 - 3v before the LED comes on, but only 15mA current draw. So for 10 LEDs in series, you could be using as much as 15-30V to push through. Much better to have say 4 parallel lines, of between 2-5 LED's.
That way you only need to supply 3-7.5V and 60mA which is right in the range of a Li-ion battery pack!

For a 1700mAh 7v battery you can get roughly 28 hours out of your light before you need to recharge. Might not even need the dynamo


Also, If i have got any of this wrong, someone please tell me

Hope it helps!

 

[ormo]

So if I put 5 x 5.5v 1F supercapacitors in parallel to give 5 farads - would this be enough to reliably power those 10-20 LEDS for a while? How would I go about simultaneously charging the cap array and using it to power the LEDs?

 

[audioguru]

The voltage of a capacitor drops sharply as it discharges. You might actually see the LEDs dimming.

The voltage of a battery drops a little as it runs down. You won't see dimming until the battery is almost dead.

 

[James1986]

Like audio guru says, you might see it dimming. BUT, if , as you said previously, you will be constantly charging it, (with your pedaling) then the capacitor will act as a buffer for all the extra charge you are throwing at the LED's and store it. Then, when you stop pedalling it will continue to discharge (at the rate you want, with some form of control mechanism; you can use a resistor I believe to control current flow and charge and discharge speed)

The higher the capacitance, the longer it will keep the light on. However, the more voltage you put in, the brighter the LED. Find the right balance.

If on the other hand you did want to go with the Li-ion battery, it would be a bad idea to charge constantly. Instead you could attach the dynamo to your brakes and it only charges the light when you slow down? Provided the dynamo gives more than 5.5v...

The choice is yours!

 

[ormo]

The capacitors will still charge ok if the voltage supplied to them is varied, correct?

 

[James1986]

Im pretty sure, yes. Time taken will differ though.

For capacitance T=RC

Time (taken to charge to 67.5%, I think) = Resistance x Capacitance

So you need to choose a mid value resistor to add in series to the circuit. Although this will increase charge time, it will also prolong the discharge time.
It's probably best for you to do some experiments before you go and buy everything, do you have access to a lab at School or Uni or Work?


Also, I think you would connect everything in series; that is to say, your array of capacitors (in parallel), would have it's negative terminal connected to the positive of the motor, and the positive terminal to the negative of the LED's array. Then the positive of the LED's array to the negative of the motor (your dynamo). Hope that makes sense!

 

[ormo]

Are you suggesting a circuit like this:

       v  A SWITCH
  |---o/ o-- - DYNAMO + ---------|
  +                              |
LEDs                             |
  -                              |
  |---------- + CAPS - ----------|

I have excluded the resistor that would stop me from burning out my LEDs

Would I need additional diodes to prevent the capacitors from powering the motor in reverse? How would I be able to charge the caps even when a switch is turning the LEDs off?

Thanks!

 

[James1986]

Yes you can do everything you said.

You would need to design the circuit so that when you push the switch to the off position, it shorts (bypasses in parallel) the LED's out. Yes you should probably add a diode to stop reverse current flow. With most capacitors reverse polarity is very bad, and damages the capacitor.

Like I said above, you will need the resistor to limit the amount of charge flowing out of the capacitor, otherwise as you said, you would release it all super quick and probably blow your LED's.

 

[flat5]

Add a diode ...
Sorry. Already mentioned, but I was thinking you don't want the dynamo to be a load when the bike is not moving forward.

 

[flat5]

...diode...