# charging a battery with a bicycle dynamo.

Discussion in 'The Projects Forum' started by qwerjo, Nov 21, 2008.

1. ### qwerjo Thread Starter New Member

Nov 21, 2008
6
1
This is my first time on this website,

I have a bottle dynamo connected to my bicycle which produces a voltage of 6 to 25 volts depending on how fast I'm going.
The dynamo is supposedly 5.5 watts but I haven't measured this.
I want to turn this into a constant voltage of 12 volts so I can charge a 12 volt rechargeable battery, and use the battery to power some speakers or my mobile phone.

Any ideas on what I could stick in between the battery and the dynamo would be much appreciated.
I'm not too good with electronics, I know capacitors, resistors and diodes but that's pretty much it. thanks

Tikam likes this.

Apr 5, 2008
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Hello,

At first you will have to rectify the signal from the dynamo.
(the output of the dynamo is probably not sine-shape).
Then place a buffer capacitor behind it (this is to avoid hum, as you want to use it with an audio application) and regulate the charging current for the battery.
This dependent on the type of battery you are using.

Greetings,
Bertus

Last edited: Nov 21, 2008
3. ### qwerjo Thread Starter New Member

Nov 21, 2008
6
1
By rectifying the signal, do you mean turn it into DC?
I don't really understand what buffer capacitors do, but won't it just block the circuit when the dynamo produces high voltages of say 20v, so that when I'm going down hill the battery isn't charging.

I have an old li-ion laptop battery but can also get hold of a lead acid battery, which i think will be more appropriate.

4. ### mik3 Senior Member

Feb 4, 2008
4,846
63
Because the output voltage of the dynamo is not always greater than 12 V you will need a buck-boost DC-DC converter to regulate the output voltage at 12V. However, before the buck-boost converter you will need a rectifier circuit with a filter, a capacitor filter is enough for most applications.

Tikam likes this.

Apr 5, 2008
15,805
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PackratKing likes this.
6. ### davebee Well-Known Member

Oct 22, 2008
539
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Generators like that which I'm familiar with are designed to power light bulbs of about 6 volts, so the generator may limit its own voltage under load - you might want to test its voltage while driving a load of moderate power before going too far with this project, to make sure the generator is able to properly drive whatever circuit you design.

To charge a lead-acid battery all you need is a diode and a resistor. The diode rectifies the current to pulsating DC and the resistor limits the current into the battery. There is no need for external voltage regulation because the battery regulates its own voltage; as long as the current is limited to a safe amount then the applied voltage does not matter. I don't think that you need a capacitor either; the battery should charge equally well from pulsating DC as steady DC.

I've heard of Li-Ion batteries catching fire or exploding from improper charging; these need carefully designed charging circuits to operate safely. Not to say to totally avoid them, just don't try to charge them casually.

7. ### Bernard AAC Fanatic!

Aug 7, 2008
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I thought a Dynamo produced DC, alternators produce AC, but then im here in the the US of A.

Dec 4, 2008
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9. ### qwerjo Thread Starter New Member

Nov 21, 2008
6
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Davebee. i have a 12v 1.3Ah lead acid battery, the dynamo outputs 500mA, what is a safe current for this kind of battery? Dyanmo's always give a constant current, but will the battery be ok with up to 40volts going across?

10. ### SgtWookie Expert

Jul 17, 2007
22,183
1,728
For maximum life, lead-acid batteries should be charged at no more than C/8, where your battery capacity (C) is 1.3. So, 1.3/8 = 162.5mA. If you tried to charge it at a 500mA rate, you may cause damage to the plates and/or cause the battery to overheat and/or gas excessively.

11. ### floomdoggle Senior Member

Sep 1, 2008
217
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First of all make sure your battery is actually 12v. If it is a car battery, then it is 13.3v or greater, up to 14.4v. To charge a 12v battery, you must have at least 13v from the charger. The amperage only dictates how long it will take to charge your battery. The slower the better. Also, what type of battery do you have? Charging systems are different for different types of batteries. And, what is your charger?
Also, ask Sarge about dump and float charges, he is pretty good.
Dan

12. ### Søren Senior Member

Sep 2, 2006
472
28
Hi,

The battery is most likely an SLA type (SLA=Sealed Lead-Acid), meaning it should be charged at minimum 10% og its nominal capacity (maximum differs between manufacturers, some go with 20%, others up the ante to 30%).
Slower charging should be avoided with SLA's.
So, best rate is between 130mA and 260mA (or 390mA).

A bicycle "Bottle Dynamo" (which is a generator) doesn't give 500mA if you go slow and there is nothing constant to the current!
Bottle Dynos are spec'd at 6V, around the speed of ~15..20 km/h (which is what the average untrained rider usually goes).
If you ride it up to 25V (loaded value), it means you go 60..80 km/h - why not sign up for the next Tour De France

The fairly perfect sine output of a bottle dyno rises proportionally in amplitude with speed. The nominal power of the majority of bottle dynos is 3W at 6V (i.e. 500mA), while some are even less (just like my hub dyno which comes in 3W and 2.4W and I got the latter of course ).
I have yet to see a regular bottle dyno putting out a nominal 5.5W!
This might also indicate to you, that you'll never get 25V out of a loaded dyno, since it would commit seppuko (getting 4 times the voltage means you get 4 times the current as well and that means your power will rise vith 4^2 or 16 times - a wee bit optimistic to think they spend wiring capable of 50W in a simple 3W dyno.

As Mik3 mentioned, you need a buck/boost regulator (a KUK or SEPIC topology could be used as well), since you really don't have power to throw away - rectification will eat some of that precious juice too (use Schottky diodes or even MOSFETs for a lower drop).

Since the maneuver is just to get some sound, I'll suggest finding more efficient speakers (if at all needed), build a small amplifier (0.5W to 1W should be plenty to be a nuisance) and run it off 4 AA's NiMH which will be lighter in weight, easier to charge and if you go with the 2900 mAh types, will have more power than your SLA.

80 dB for 8 hours a day is the limit if you dont wanna harm your hearing.
For each 3 dB over that, you need to halve the listening time.
100 dB means you can only listen to a single track (less than 4 minutes) unless you want to be a hearing aid user prematurely (good news is, that you can get hearing aids that you can stream music to wirelessly - when music has made you wear them )

13. ### qwerjo Thread Starter New Member

Nov 21, 2008
6
1

I assure you that my dynamo has an output of 12V 6W. I am currently testing it for my physics coursework and I can get an EMF of 35V at speeds of 30km/h.

I'm not sure what you mean when you talk about getting 4^2 power but at the moment i have a bridge rectifier on the output of the dynamo, a bunch of 12V zener diodes in parallel about 4 1000 microfarad capacitors in parallel and a 3.5w amp with a quality speaker. Which all work great.

I have yet to set up a charging system for my 12v lead acid battery, but i think it will the same setup just with zener diodes with higher voltages. then i will be able to charge phones and mp3 players and shit when i cycle to amsterdam in the summer!

I tell you man, i am living the life