Recharging With Piezoelectric Discs
Name: Jane, Ashly, Julia, and Kaitlyn
Date: Winter 2012-2013
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
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
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
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
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.
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.
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Update: November 2011