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Forum Index : Electronics : battery charging circuit?

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mahela007

Newbie

Joined: 06/07/2008
Location: Sri Lanka
Posts: 29
Posted: 05:16am 08 Jul 2008
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i have a motor that generated DC current which i want to hook up to my windmill. It provides a good current flow as well. I got it out of my old VCR. Anyway can someone pls tell me how to build a circuit to charge batteries with this motor? I checked out the minimill but it has an alternating current motor. (I'm just a newbie to all this windmill stuff).
how come its not very windy where i live???
 
davef
Guru

Joined: 14/05/2006
Location: New Zealand
Posts: 499
Posted: 10:12am 09 Jul 2008
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I'd start off by measuring a few things like voltage output versus RPM for a selection of fixed load resistors. Then work out the current you have in each case.

You may find that the voltage levels are not appropriate for direct charging of your chosen battery.

You then can either opt for non-optimal loading of your generator or find information on buck/boost/SEPIC converters.

Good luck
 
mahela007

Newbie

Joined: 06/07/2008
Location: Sri Lanka
Posts: 29
Posted: 11:02am 09 Jul 2008
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could i please have a little more infomation on these converters? What execatly do they do?
how come its not very windy where i live???
 
oztules

Guru

Joined: 26/07/2007
Location: Australia
Posts: 1686
Posted: 12:48pm 09 Jul 2008
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In it's simplest form, a blocking diode between the generator and the battery is all you need for a tiny system.
This consists of connecting your generators negative output to your batteries neg terminal, connecting your generator's positive output to the anode of a diode, and then connecting the cathode of the diode to the positive of your battery.

In this configuration, no current will flow when the generator is stopped. The diode blocks the battery from trying to drive it as a load.
When the generator gains sufficient rpm, it will have enough voltage to overcome the diode drop and the battery potential, to push current through to the battery.
As the generator slows, it drops below the battery "cut in " voltage and no current flows again until the rpm drives the voltage beyond cutin again.

The next level of complexity is a dump load control to stop overcharging the battery. (probably not needed for a novelty device.

If you wish to play with dc dc voltage converters in their simplest form for small batteries (AA C and maybe D size ), then find an old solar garden light with 1 nicad cell in it.

If you look carefully inside it , you will find a circuit board inside that converts the 1.2v into a higher voltage to drive the LED when the sun goes down. It usually has a tiny 2 transistor flip flop with inductors that look like resistors.

The transistors oscillate at about 1000000 cycles per second, and use the back emf in the inductors to generate higher voltages which then drive the led from the 1.2v battery.

When the sun comes up, the potential from the solar cell holds one of the bases low, and charges the battery. The 1.2v is not enough to run the led, so it stays isolated until the sun goes down and forward biases the transistor, it starts to oscillate again and the cycle repeats itself until the battery is exhausted.

An example of back emf, is when you get a decent auto solenoid, hold it on the 12v battery of the car, while holding both the positive and the negative bare ends of the wires in either hand to the battery terminals.

Nothing happens to you until you disconnect the battery (while still holding the wire ends going to the coil)

Then you get a high voltage shock as the magnetic field collapses through the coils. This high speed collapsing field generates the high voltage back emf that gives you the jolt.

So we have generated a higher voltage from a lower voltage. The faster you can collapse the field, the higher the generated voltage. The two transistor flip flop does this kind of thing for you.

dc dc converters can become very complex very quickly. It is best not to involve bigger ones of them into your design unless you have a considerable mental pain threshold.

Entire careers can be spent on designing and perfecting these things. Considerable frustration is part of the territory for the newly initiated.

Best to avoid at this stage.

Better to match your battery voltage to your generator at the expected rpm range, than fiddle with converters.





........oztules
Village idiot...or... just another hack out of his depth
 
mahela007

Newbie

Joined: 06/07/2008
Location: Sri Lanka
Posts: 29
Posted: 03:00am 10 Jul 2008
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will a simple set up like the one you suggested have a significant effect on battery life? and just one more question.. is there any other way to prevent overcharging?
how come its not very windy where i live???
 
oztules

Guru

Joined: 26/07/2007
Location: Australia
Posts: 1686
Posted: 09:29am 10 Jul 2008
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This is a "how long is this piece of string I have in my pocket" type question.

Sadly we won't know anything more until you actually put prop to motor, motor to battery and whole lot to the wind.

Until then the whole thing is just supposition.

Once you have some figures of how/if... it charges what voltage battery, we can't go further.

Now it is time to do the real world testing.



........oztules
Village idiot...or... just another hack out of his depth
 
mahela007

Newbie

Joined: 06/07/2008
Location: Sri Lanka
Posts: 29
Posted: 09:40am 10 Jul 2008
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the battery is 300 mAh at 3.6 volts. My motor can generate about 5 volts and 100 mA.
how come its not very windy where i live???
 
oztules

Guru

Joined: 26/07/2007
Location: Australia
Posts: 1686
Posted: 10:19am 10 Jul 2008
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and the performance in the wind?
Village idiot...or... just another hack out of his depth
 
mahela007

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Joined: 06/07/2008
Location: Sri Lanka
Posts: 29
Posted: 10:54am 10 Jul 2008
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that was the performance in the wind.
how come its not very windy where i live???
 
oztules

Guru

Joined: 26/07/2007
Location: Australia
Posts: 1686
Posted: 11:06pm 10 Jul 2008
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My first try would be a 5w 4.2v zener placed across the battery. This will limit charge voltage to float voltage, and act as a dump load as well.

If you want you could put an isolating switch in series with the zener, to allow faster charging if wind is available, and you monitor it.

I'm not good at all with lithium batteries, but I think 100ma may be about the max that the 300mah battery will handle. 4.1 - 4.2v should be a compromise float voltage to keep long life versus max charge state.

Hopefully someone with experience with charging lithium technology will correct me in this.

..........oztules
Village idiot...or... just another hack out of his depth
 
mahela007

Newbie

Joined: 06/07/2008
Location: Sri Lanka
Posts: 29
Posted: 01:00am 11 Jul 2008
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thanks for all your help.
how come its not very windy where i live???
 
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