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No, You Can't Charge Your Phone Battery In 20 Seconds

by Eric Hall

May 26, 2013

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Donate Eesha Khare did something pretty incredible. She experimented with materials and developed one which acts like a supercapacitor - a capacitor with a structure which allows for a much smaller package than a traditional capacitor (a higher energy density). To improve on this area of research is a great contribution to science, and certainly demonstrates her drive and her understanding of both the science and the commercial application of this product.

What isn't so incredible is how this story was reported. Headlines such as "Teenage Girl Invents World's Fastest Mobile Phone Battery Charger" or from a site claiming to be geeky - - had the headline, "Student develops supercapacitor that enables 30 second battery charges." The Washington Times reported, "A California teenager has invented a device that fully charges a cell phone in 20 seconds flat," which isn't even in the press release from Intel, where Ms. Khare presented her device at theIntel International Science and Engineering Fair.

The reason this is so misleading is none of these stories talk about the intermediate steps that will be needed in order for this to become a product usable in our phones. A capacitor has a great advantage over a chemical battery in that it can take large amounts of energy in or let large amounts out in very short amounts of time. The problem is the overall energy stored is much lower than with a battery. AsJosie Garthwaite at GigaOM explains:
The ultracapacitor is like a small bucket with a big spout. Water can flow in or out very fast, but there’s not very much of it. The battery is like a big bucket with a tiny spout. It can hold much more water, but it takes a long time to fill and drain it. The small bucket can provide a brief “power surge” (“lots of water” in this analogy), and then refill gradually from the big bucket...
The other significant problem with a capacitor is the voltage drop as the energy is used. A battery supplies a fairly constant voltage through its discharge cycle. This is important because many of the semi-conductors used in these portable devices require a minimum voltage in order to function. For example, my fully charged 17oo mAh battery for my Droid Incredible measures at about 4.2 volts. I use this device as a music player since upgrading my phone, so after running it for a day playing music and sitting on standby, the phone was indicating the battery life was 3%. Measuring the voltage, it is still at a usable 3.6 volts. This means my average current draw was about 70 mA.

To compare to a sample supercapacitor, I assumed using a standard voltage and current available to charge portable devices. Using the iPad standard of 5 volts and 2.1 amps, a 4 Farad capacitor would be 95% charged after 30 seconds. If this capacitor is then discharged at 50 mA, the capacitor will have already dropped to 2.7 volts after just 240 seconds. At this point, many semiconductors would no longer function.

That's not to say there are not ways to overcome this problem. Increasing the capacitance would mean more energy can be stored, giving a longer discharge time. The voltage and current could be increased as well, allowing for more energy to be stored on the capacitor without increasing the time for the increased capacitance to be charged. But now it requires changing the circuitry of the chargers, as well as additional circuitry in the phone itself to step the voltage back down to a usable level for the semiconductors inside.

One reason we still use fossil fuels is that, per pound, materials like gasoline have large amounts of energy that can be easily accessed. Batteries do not come close in the weight to energy ratio, capacitors even less so. But as interest in electric vehicles has increased, finding ways to store electric energy more efficiently is a hot topic. Being able to charge quickly is another issue, and gasoline has excelled at replenishing energy because it takes a couple of minutes, versus hours with batteries. Capacitors can be recharged quickly, but because of the lower energy density they need charging more frequently. It turns out,Ms.Khareisn't the only one who has been thinkingabout this energy storage problem. A graduate student at UCLA created a high energy density capacitor using some fairly simple technology:
Maher El-Kady, a graduate student in chemist Richard Kaner’s lab at UCLA, wondered what would happen if he placed a sheet of graphite oxide"an abundant carbon compound"under a laser. And not just any laser, but a really inexpensive one, something that millions of people around the world already have"a DVD burner containing a technology calledLightScribe, which is used for etching labels and designs on your mixtapes. As El-Kady, Kaner, and their colleagues described in a paper published last year inScience, the simple trick produced very high-quality sheets of graphene, very quickly, and at low cost.
So where these energy-dense capacitors seem to fit is working together with batteries. Like the Slate article linked above points out, examples could include electric buses that grab bits of charge at each stop while passengers get on and off the bus. Highways might have a lane with inductors in them to serve as a "charging lane" to grab some energy for less stops. From a phone perspective, I imagine someone perhaps down to 20% on their phone and needing to go to a meeting could grab a quick 20% battery equivalent of energy in a couple of minutes, which would go into the capacitor, then would work to add some charge back to the battery to keep a more stable voltage.

It is possibleMs.Khare's capacitor is so energy dense that even with the voltage drop, enough of the material could be packed into a phone to run it equivalently with a battery. This would mean much shorter charging times than what is experienced with a battery. But unless run at a high current to charge it, the time (at household currents) to get the energy equivalent of current phone batteries would still be measured in minutes, not seconds. But with the fast charging capability and the larger number of charging cycles a capacitor is capable of, a person might never wait for a full charge. Instead a person may just grab bits of charge wherever they are, so they may never have a full charge nor run out.

Ms.Khare's drive and innovation certainly is valuable to science and to commercial enterprises. The flexibility of the material and its relatively high energy density in the world of capacitors means it has a wide array of applications. It would be easy to see people carrying extra capacitance with them, or even having energy from their movement being stored in these lightweight capacitors, meaning the energy to power devices would never be out of reach. I applaud her innovation.

Science reporters should be ashamed of themselves for not acknowledging the work on whichMs.Khare based her work, and spreading a misunderstanding of how this technology would work. We won't be "charging a battery" in 30 seconds, or 20 seconds. Her innovation may lead to a change in how we think about charging our devices. It certainly will only increase the convenience of these devices. It may lead to further innovations asMs.Khare and other scientists build on her work. That is how science works - and how it should be reported.

by Eric Hall

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