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Name: Jane, Ashly, Julia, and Kaitlyn
Status: educator
Grade: 9-12
Location: NJ
Country: USA
Date: Winter 2012-2013


Question:
We are students at High Point Regional High School, and we are in a Women in Engineering class. We are working on a design project. We would like to figure out how to charge a rechargeable AA battery with piezoelectricity. We would like to demonstrate how a use a pressure pad can power a small light bulb, and we think that a piezo ceramic disks would work. The problem is that we are not sure how much power or energy we will need to charge a battery that could power the light bulb, and we are not sure what the kilohertz are



Replies:
Hi Jane, Ashly, Julia, and Kaitlyn,

To begin, a general (unfortunately discouraging) statement is that piezoelectric discs or other simple piezoelectric devices are incapable of generating the amount of electrical power needed to charge even a small AA battery or power a small light. I say this having significant engineering experience (as well as a few patents) in the field of piezoelectric devices.

A typical rechargeable 1.2 Volt Nickel Metal Hydride AA battery has capacity of 1.2 to 1.8 Ampere-Hours. This means that it can theoretically supply 1.2 to 1.8 Amperes of current for an hour, before it is discharged. To recharge this battery at the usual so-called C/10 rate will require a current of 1/10th this value (0.12 to 0.18 Amperes) for about 16 hours. Note that the charging process is not 100% efficient, which is why you can see that more energy is needed to recharge the battery than it delivers during discharge.

By contrast, typical piezoelectric devices (which convert mechanical motion to electrical charge) produce very little electrical current (no more than pulses of a few milliAmps), but typically generate a very high voltage.

So to summarize, piezoelectric devices, when subject to mechanical stress, typically generate intermittent pulses of high voltage but very little electrical current. Batteries, when being charged, require sustained high levels of current, but at very low voltages. Clearly, trying to charge a battery or light a lamp (both of which require sustained levels of high current, at low voltage) by using a piezoelectric device (which produces intermittent pulses of high voltage at very low current) is not a good "fit".

For more engineering information on piezoelectric devices, you might be interested in trying to look for the book "Piezoelectric Ceramics" (ISBN 0 901232 75 0), in an engineering library. It is considered the authority on this subject.

You should also note that constant repetition of stress on the piezoelectric device is necessary to produce any continuous (albeit, low value of) output current.

I am puzzled by your statement "we are not sure what the kilohertz are". Are you referring to the continuous rate of repetition of mechanical stress needed to generate a sustained electrical output? If this is your question, then the answer is that highest efficiency is achieved when the rate of repetition of mechanical stress applied, coincides with the mechanical resonance frequency of the piezoelectric device.

To summarize, while trying not to be a bit of a wet blanket here, attempting to generate significant levels of electrical output current at low voltage, using a simple piezoelectric device such as a piezoelectric disk, is simply not practical.

Regards, Bob Wilson


Hi Jane, Ashly, Julia, and Kaitlyn,

Thanks for the question. In order to design a rechargeable AA battery using piezoelectricity, you will want to learn more about the piezoelectric effect. Will the piezoelectric effect provide you with AC or DC current? And which type will you need to recharge a DC battery? As a hint, you may want to include a diode in your design.

You will most likely need to discharge a battery (and measure the voltage as a function of time) in order to determine battery capacity and how much energy is required for charging. Will your pressure pad(s) be able to provide this much energy?

Kilohertz is a measurement of frequency. The kilohertz value tells you about the frequencies that the piezoelectric crystal can accept.

I hope this helps get you started. Please let me know if you have any questions. Thanks Jeff Grell



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