# Charging a battery with an ultra low voltage AC supply

Discussion in 'General Electronics Chat' started by texaskid1993, Apr 22, 2010.

Apr 22, 2010
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I am working on a project to convert energy dissipated during walking, to electrical energy. To convert the mechanical energy to electrical energy I made a linear electromagnetic generator which uses a change in the Area of the circuit to generate a voltage.

The device I have designed is outputting a fluctuating voltage of 20-50 mV AC. I would like to charge a small AAA battery with this but I'm not good with circuits and I know the battery requires a DC voltage of at least 1.5V.

I have done research and I realize I need a rectifier, but every example I run across uses much higher voltages and states that the rectifier loses 1V due to efficiency (this would kill my small voltage). I have considering an amplifier to increase the voltage or a capacitor to store the energy before, but I am having trouble figuring out what to do. Can someone help me figure out how to convert and store this small AC energy? I really appreciate any suggestions. thanks.

2. ### retched AAC Fanatic!

Dec 5, 2009
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Can you change your design? You can use a peizo to convert the action into DC voltage. That will do away with the need for rectification.

Apr 22, 2010
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The due date for my project is near so I can't afford to change my design. I just gotta find a way to make this darn thing work!

4. ### retched AAC Fanatic!

Dec 5, 2009
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Well, the smallest voltage drop from a diode I know of is .6v

You could charge a capacitor (slowly) then dump the charged cap into the charger circuit.

Apr 22, 2010
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Thanks, a problem that arises though is that because of the nature of the circuit (opening and closing) the current changes directions when the circuit is retracting so If all I have is a capacitor, it will drain out the energy that was just put in. I will definitely use a capacitor to store the energy before trying to charge the battery, but I think some sort of diodes must be used

6. ### Jaguarjoe Active Member

Apr 7, 2010
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Apr 22, 2010
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For a precision rectifier I would have to supply an external voltage but because I am trying to design a 'green' device, using additional power to harvest small outputs of power doesn't make sense. What if I could get the voltage higher to 300 mV, would this broaden the chances of collecting it?

8. ### SgtWookie Expert

Jul 17, 2007
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Schottky rectifiers have the lowest Vf of commonly available rectifiers nowadays. But even at low currents, the Vf is still a good bit above your output.

Piezo transducer output still needs rectification, but its' output voltage is much higher.

There was an article in Electronic Design News (edn.com) a while back about using a piezo to harvest sound waves to charge a supercapacitor.

I'm afraid that unless you change your tactics, you won't have a viable project to turn in.

9. ### KMoffett AAC Fanatic!

Dec 19, 2007
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Apr 22, 2010
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Thank you very much for that article. It looks pretty daunting with the pulse width modulation boost converter and feed forward and feedback control, but it looks to be a viable solution for harvesting smaller amounts of energy. I will discuss it with my professor to see if I have to build it or if I could possibly purchase a circuit which already does this. Thanks a lot! any other ideas certainly wouldn't hurt

11. ### KMoffett AAC Fanatic!

Dec 19, 2007
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If you look through the search results you might find more useful info.

Ken