I heard about this some time ago on Buzz Out Loud. They were talking about the possibility of charging your phone through a piezoelectric charger. The basic idea is that by just typing on the screen, your fingers would essentially charge the battery. It seems the idea came from this study by Dr. Madhu Bhaskaran (yes, I just linked to the Mashable cover of that study). Crazy, but I can’t find access to the original paper. Is there anyway to see if this is even a realistic solution? Oh, yes.

What is a piezoelectric device?

This things exist. You probably use one and you don’t even know it. Basically, a piezoelectric material produces a change in potential across it when it is squeezed. Also, if you apply a potential difference across it you can get it to expand a little bit. Why? I guess you could say that when pressure is applied to the material, the material becomes polarized. This polarization creates in internal electric field and thus a change in electric potential across the two sides. If you apply an electric field, you can get the material to change its polarization which will change its size (by just a little). I know it is really much more complicated than that, but I am trying to simplify it some. Two very common uses of these piezoelectric materials are in potato guns and annoying music-playing birthday cards. Potato guns? Yes. One type of potato gun launches the potato using an expanding gas such as hair spray. How do you ignite the hair spray? A piezoelectric igniter from a gas grill produces a spark when you squeeze it. For the annoying birthday cards, they need a super small speaker. Instead of using a coil of wire and a magnet like a traditional speaker, the card has a piezoelectric speaker. A varying voltage is apply to the material causing it to expand and contract to the tune of the desired music. Maybe they aren’t the best speakers, but they work and they are thin. They are thin enough that you can take them out of the card and hide them in the office next to yours. I would never do that though.

How well would they work at charging?

I really have no idea if some materials mastermind has invented a super awesome charging system. Since I have no idea about the device, let me look at something else. How much energy could be in a typing finger. This would be an upper-limit on the amount of energy you could get from some charging device. You certainly couldn’t get more than this, you would probably get less since the device won’t be 100% efficient. As usual, let me start with a diagram. This is a diagram of a finger “typing” on a phone with a touch interface.

The purple thing is a hand (in case you can’t tell). What is important is that the finger exerts a force on the phone over some distance. This means there is work done by this force. The work done would be:

Now, remember I am calculating the work done by the finger. What you would really want is the work done on the piezeoelectric. Would these two be the same? No. The finger is going to move farther than the the piezeoelectric gets compressed. The finger has to do things other than just compress the phone. But hey, let me look at the best case scenario fo for the charging device. What could this force and displacement be like? To experimentally estimate this, I used a Vernier force probe as a “finger”. To simulate the finger, I put a rubber stopper on the end.

Then I just tapped the table as though I were typing on a phone. I am sure I pushed a little harder than I would had it been a finger, but you get the idea. Here is the force as a function of time from that force probe.

Force-time data is nice, but that isn’t what I need. I need force as a function of distance. Ok, I am just going to completely approximate it. The maximum force seems to be around 3 Newtons. Let me pretend like that is constant over a distance of about 0.0015 meters. Where did I get that number? It turns out I made a quick video of the rubber stopper hitting the table. Here is a shot from that video.

Now I am ready to calculate the work done by the finger.

This value of 0.0045 Joules seems small. Really, it is probably even smaller. Let me point out a few things:

• I used the peak force and assumed it was over the whole distance the finger pushed. Probably not true.
• This is the work done by the finger. The work done on the device would use the distance the device is compressed. This would likely be way smaller. Think about it, the screen is much less that 0.1 cm thick so it could not be compressed that much.
• The device is likely to have an efficiency less than 100%.

Now there is one point in favor of the device. It is possible that although the displacement (compression) of the piezoelectric device is much smaller than the distance the finger moves, the force could also be bigger. I don’t think this is the case for normal screens, but for a key on a keyboard it is possible to have some type of mechanical lever that would increase the force but decrease the distance. But still, you couldn’t get more energy than the work done by the finger.

How long would it take to charge a battery?

Suppose you want to charge your battery by typing. How long would this take? The first question is: how much energy is in a battery? Here is a replacement iPhone 4 battery. The battery is listed with a capacity of 1420 mAh with a voltage of 3.7 volts. This rating means the battery could produce an equivalent of 1420 mA at 3.7 volts for 1 hour. This would be an energy of:

Now, how many “pushes” would I need to get that amount of energy?

So 4 million characters typed (or 4 million Angry Birds flung). How long would this take? How fast can you type on a phone? Just from my experience, it seems 2 characters per second would be pretty fast. This means 4 million characters would take 2 million seconds or 23 days. Oh, and this is 23 days assuming you aren’t using the battery and assuming you are constantly typing and not sleeping. Someone check me here. I think I gave the charging technology all the benefits, but it is still coming up short. Even if I double the force and double the distance, it would still take days to charge a phone. Days of continuous typing. Let me be clear, I don’t see how this charging would work for charging by typing on a touch screen. There is another viable option. Charging with a piezoelectric in your shoe. Why is this different? The force exerted on your shoe is WAY higher than the force your finger would exert on the screen. Also, your shoe can easily compress at least 1 cm. Popsci even has instructions for building such a device. Not sure how well it works, but it seems like a better way to go.

What about solar?

It seems like a great alternative for battery charging would be a solar panel. The iPhone 4 is 115.2 mm by 58.66 mm. This means the area on one side is about 6.8 x 10^-3 m². What if a large portion of the back of the iPhone was a solar panel – say 80%. The best position for this solar panel will be if it is completely perpendicular to the sunlight. Of course this never happens, let me just say that on average it is 30° off. This would put the effective solar panel area at:

Ok, now two assumptions. Let me assume a solar flux of 1000 W/m² – that is about correct for direct sunlight. Also, let me assume that the solar panel is about 25% efficient. This means that the maximum power you would likely get (while in direct sunlight) would be:

1 Watt seems much better than the typing. If I consider two pushes a second, the typing would have a power of 0.009 Watts. Big difference. If you could keep the phone in direct sunlight for just 4 hours, you would charge it. This seems plausible.

Some other ideas

How else could you charge a phone?

• Charging by sound. Actually, I already looked at this one.
• What about a thermoelectric charger using a peltier device. The basic idea is that a change in temperature across this material produces a potential difference. Of course, your hands could be warm, but the phone would have to be colder.
• Internal shake-weight magnet. When you shake the phone, it charges like those shake lights. You know the ones.
• What about a phone that taps into your blood. Somehow it uses the oxygen in your blood to power some type of fuel cell. That would be cool, but maybe a little freaky. They could call it the vampire phone. Twilight fans would love it.

Top image: JuditK/Flickr

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