Re: Charging a battery using piezoelectric material
Originally Posted by jijith
hiiiiiiii....i just want to know......whether we can charge a battery using piezo electric material?????
The piezoelectric device will generate small amount of electrical energy from mechanical forces. And the voltage will be fairly high, but the current will be very low. (Another way of saying this is that the material has a very high impedance).
The circuit you'll need is just a little more complicated than the simple capacitor. First let me give you some background info:
The atoms of a piezoelectric material are arranged in a crystalline structure. The atoms are covalently bonded to each other (meaning that they share electrons to form a stable compound). Since some of the atoms give up electrons while others receive those electrons for part of the time, the former end up with a net positive charge while the latter end up with a net negative charge. From a macroscopic point of view, this is all irrelevant under equilibrium conditions. The positive charges exactly balance the negative charges within the material, and both are dispersed evenly throughout. In fact, the same can be said of basically any chemical compound, crystalline or not.
What makes a piezolelectric material special is that when you deform it (such as squeezing it or bending it) along certain directions, the crystalline lattice gets deformed in such a way that the positively charged atoms shift mostly in one direction, while the negatively charged atoms shift in the opposite direction. This creates a small dipole moment (a net separation of charge within the material), which results in a voltage from one side to the other. If you were to put metal plates on either side of the crystal, you could measure this voltage.
We're not talking about a lot of charge here. You can't just squeeze a piezoelectric material with a clamp to create a battery or anything like that! Sure, the initial squeeze will generate a voltage, but as soon as you hook your piezoelectric "battery" to any sort of load (say the bulb of a flashlight, for example), electrons will flow from the negative side of the piezoelectric, through the wires and the light bulb, and into the positive electrode. Your flashlight would light up for a brief instant (okay, not really, since an incandescent bulb needs a little bit of time to heat up before creating light, but you get the point) as those electrons flowed, but after a few microseconds, enough enough electrons would have moved from one side to the other to counteract the initial voltage. You'd be back to equilibrium conditions again, and your flashlight bulb would remain dead forever more. Hence it generates micro-amps in "small bursts".
Once your piezoelectric battery has reached equilibrium and died out, if you then released the clamp that was squeezing it, the material would spring back to its equilibrium shape, and the internal dipole moment would go away. Now all those excess electrons that previously had flowed from one electrode to the other would be creating a voltage of their own! (In the opposite direction as before). Assuming the device was still hooked to your flashlight bulb, the current would then flow back in the opposite direction as before to get back to the "unsqueezed" equilibrium, and the bulb would once again light up in the process.
Thus, you can see that if you were to rapidly and repeatedly squeeze and release the piezoelectric device, you'd cause current to flow back and forth through the bulb, and you could actually cause it to light up. In other words, a piezo device can be used as an AC generator (alternating current, as in back and forth). This works great for something like a light bulb which doesn't care what direction the current is flowing. It cannot be used, however, as a DC generator. That means you can't just hook up a piezo transducer to a battery or capacitor and expect it to charge it up. Since the current from the piezo is going back and forth, every time it charged up the battery or the capacitor, it would turn right around and suck the charge back out, over and over again.
Therefore, if you want to use the piezo device to ultimately generate DC current, you need to rectify it. You can build a very simple full-wave rectifier with four diodes. If you were to then use that to charge a capacitor (smoothing capacitor), the voltage on the cap should slowly increase over the course of time if you repeatedly squeeze and release the piezo device. Using something to vibrate the surface would be good for demonstrating the effect (Power drill? Muscle massager? Blare loud music at it?)
As far as the capacitor goes, the larger the value, the more charge will be able to be stored. However, larger values mean that it'll take more vibration to demonstrate an appreciable increase in voltage. You may need to experiment a little with some different values. I'd try a plain old electrolytic cap to begin with (or whatever else you can find that's cheap; I doubt leakage will matter much here), maybe something in the 1 uF range. Hook a voltmeter to it and watch it charge up. Assuming that works, you should be able to hook a dead rechargeable battery to it and actually charge it up.