Power Your Smartphone Forever! Charge It Instantly! Or Not

In the past, we've touched on the topic of charging batteries for smartphone applications (and other similar portable devices) in columns and comments. However, a recent award to an up-and-coming engineer gave me a new reason to pick up the issue again and ask how far we have really come when it comes to battery life in our mobile-centric world.

This all started when my eye caught a recent column in Mashable: “Teen’s Invention Could Create 20-Second Phone Charge.”

The story involves some really great news about Eesha Khare, a high school student and one of the winners of the prestigious 2013 Intel International Science and Engineering Fair. She won for her work on the development of a new super capacitor. The abstract for her research is available here.

In an interview from the science fair, Khare said:

The super-capacitor I have developed uses a special nanostructure, which allows for a lot greater energy per unit volume. It can charge very quickly, and it can last for 10,000 cycles, compared to batteries which are only like 1,000 cycles.

Khare added that the device is far more environmentally friendly than rechargeable batteries.

So far, so good, right? It’s a great story about a young woman and up-and-coming scientist. However, there was something that I couldn't get past.

I don't doubt her claims regarding the number of charge cycles that one can expect from a typical rechargeable battery. I also don’t dispute her claims regarding the number of charge cycles that one can expect from her Hydrogenated TiO2 -Polyaniline Nanorod super cap. And the fact that it's flexible is an added bonus.

My quibble with the article is how the device was used. Khare told Mashable that she charged it up and used it to light an LED for some amount of time. I would have preferred something a bit more sophisticated, like a blend of a load resistor plus active current sink. And some meters would have been good, too. But she's a scientist, not an engineer, so OK.

I have a much larger concern with the use of this super cap for a cellphone, and the claim in the Mashable headline. I expect it would work OK in the phone application based on the information provided in the research paper. However, the claim regarding the time to charge this capacitor when used to power a cellphone is a little bit suspect.

Let's use my cellphone battery as a starting point and do a little analysis. Its specs are 3.7V (nominal VOUT ) and 1230mAh. So, in broad terms, it could supply 1.23A for 1.0 hour or 123mA for 10 hours, and then it would be completely drained. Well, actually, it wouldn't be quite so neat towards the end. It's not perfect, so its VOUT would taper off somewhat at the end-of-charge.

Let's not argue over that detail (there are other details about which we can quibble). Let's instead wait till it's discharged completely and then recharge it. With conservation of energy still in effect (in spite of what the politicians in Washington want you to believe), and ignoring losses and imperfections, it'll need to be charged at 123mA for 10 hours. Or, we could charge it in an hour at 1.23A. Or, we could charge it in a minute at 1.23A X 60 — better yet, in 30 seconds at 1.23A X 120. That works out to a charging current of 147.6A. I predict (even without having one of these super caps on my bench) that I'll have difficulty drawing that current from a typical wall-wart or USB port.

Least I be accused of being curmudgeonly towards our crop of new scientists (Hey! You kids get off my lawn!), my crankiness is really directed more towards Mashable and the way the writer presented the story. I would have thought they'd have had someone on staff who knew how to multiply and divide. But, perhaps not.

Still, kudos to Eesha Khare for her work. There is hope yet for us as long as we have a crop of clever scientists coming up through the ranks.

22 comments on “Power Your Smartphone Forever! Charge It Instantly! Or Not

  1. vbiancomano
    September 26, 2013

    @BA—An article that appeared in the science section of a major newspaper a decade or two back focused on a mathematician who had solved “The Traveling Salesman” problem. Unfortunately, that report wasn't quite right. It was later reported that the mathematician had solved a portion of that problem. That's quite a difference.

    A great editor is the difference between make and break in stories like these, assuming he/she gets to read his copy before it goes out. Your analysis for this one seems reasonable enough. Nevertheless, we hope the editors got it right for this one and can clarify the nebulous passages. Because batteries could use a lot of help right now.

  2. eafpres
    September 26, 2013

    @Brad. I had much the same reaction. I also see from time to time claims of new EV batteries that will charge in minutes. I pointed out to one group on LinkedIn it would require anywhere from 40A to a few hundred A depending on the battery. I'd not want to see 500A being pulled from my main panel!

  3. Davidled
    September 26, 2013

    Well, young engineer is very creative and passion. To get it in the market, it might require more testing for risk factor such as over-charging. Regardless of this matter, it is a great project. I wish Super-Cap might be in the market in future.

  4. Vishal Prajapati
    September 27, 2013

    We might not get that much simultaneous current to fully charge a super cap in a minutes but the charging time of cap will definitely be less than batteries. So, it will be very much welcoming by laymen just by seeing its promisingly shorter charging time as compared to battery.


    And even if the charging time doesn't decrease, super cap would still win the game by its charging cycle. Which is one step progress from battery.


    Waiting to see some commercial implementaions soon.

  5. samicksha
    September 27, 2013

    You are right Vishal, as a laymen even i will welcome if some thing can charge my phone in just minutes rather hours, on other part i read about graphene-based battery technologies, wherein they planned to use low-cost materials such as titanium dioxide to replace the more expensive materials used today. But titanium dioxide on its own doesn't perform well enough to serve as a replacement.

  6. Brad_Albing
    September 27, 2013

    @eafpres – Exactly! That's a disaster waiting to happen.

  7. Brad_Albing
    September 27, 2013

    @Vince – my opinion here is that any info from the science fair is OK. It was the editors at Mashable that got it wrong.

  8. Brad_Albing
    September 27, 2013

    @VP – Quite right. You can expect shorter charge-times, limited mostly by your power supply that is doing the charging. And you can expect more charge-discharge cyles before the device shows significant degradation. Combine that with the fact that the device is flexible and environmentally friendly, and you have a quality product.

    I should mention that the part about being environmentally friendly is according to its inventor, Eesha Khare. I'm not a chemist, so all I can say is it looks right….

  9. vbiancomano
    September 27, 2013

    @BA—I'm with you. Sorry to say the editors seem to have put themselves in a bit of a fix this time.

  10. eafpres
    September 27, 2013

    @Vishal–I'm not sure super caps have all the desirable properties to replace a battery.  There are issues of holding charge (self-discharge rate), total energy capacity to volume and weight, etc.

    The best use of super caps I have seen so far is some car fanatics made a system that would run an auxillary supercharger using an electric motor for a short time to increase the output of the gasoline engine.  Since it was powered off the caps, it didn't draw horsepower to run it.  You could use it for a short burst to pass a competitor, or win a drag race, for instance.  Good engineering, sensible application where a high discharge rate is needed.

  11. jkvasan
    September 28, 2013


    The young scientist has done well and we welcome such bright kids. At the same time, the invention is probably a Proof Of Principle. To promote it to a Proof of Concept level takes much more and who knows one day, it may happen. However, your analysis is very valid.

  12. Vishal Prajapati
    September 28, 2013

    @eafpres, I would agree with you that what ever options are available right now for super capacitor have the problem of self discharge for sure. Which probably could be the main hurdle to replace a battery. I am not sure about capacity to Volume and weight for the supercaps.


    But as day by day new materials and their properties are changing and improving e.g. by neno technology for perticular application. I hope there would be soon some option available which will put the super caps to be direct replacement of battery or even better.

  13. Netcrawl
    September 28, 2013

    There's always some new developpment, with semiconductor manufacturers pushing the boundary of limits, not only the speed of our mobile devices, but their battery life as well, the latest development is a tiny supercapcitor that is small enough to fit inside a smartphone battery and charge it fully within 30 seconds.

    Besides just smartphones and tablets, this technology has huge potential it can be applied to even higher capacity batteries like those found in automobiles.

  14. David Maciel Silva
    September 28, 2013

    When we think of batteries, remember the electric car … This is the great challenge of automakers increase battery life … But on the other hand, imagine a long distance trip where there is like recharging the vehicle and in the middle of nowhere … to the damned car, goes without energy …

    Imagine u take from his pocket a few minutes SuperCap and “resurrect the car,” I see this happen with cell phones today, maybe in the future for all who need a good battery and a load that is not possible.

    Some informations powerfull about the supercaps:

  15. Vishal Prajapati
    September 30, 2013

    Thank you sir, for your pointers they are really helpful to understand the basics of the super cap. I want to know is the double layer caps are super caps or stacking of these double layer caps become a super caps?

  16. David Maciel Silva
    September 30, 2013

    The basic principle in my view and has the 2 layers so you can store energy for longer, being more efficient, makes it very close to a super cap.

  17. jaybus
    October 2, 2013

    Sure. To stop a 1000 kg car from 20 m/s dissipates 200 kJ by heating up the brakes. If the supercap can truly take a charge current of around 150 A, then they could probably be used as intermediate storage buffer for regenerative braking.

  18. WKetel
    October 2, 2013

    Part of the claim is certainly valid: Supercaps are much more enviromentally friendly, since the main ingrediant is carbon. And they are good for a whole lot more discharge-recharge cycles, there is no question about that.

    Now the fast recharge is a bit more undecided. Batterys recharge by reversing a chemical reation, which winds up producing heat. And I have seen batteries exploded from excessive charging input. So there is a very definite limit as to how fast a battery can be charged if it is going to last very long. Now supercaps charge by polarizing a capacitor, and the rate of charge is limited primarily by the heat produced as current passes through the internal resistance. This heat is quite a bit less than the heat produced charging a battery.

    But the shortcoming of a supercapacitor is exactly the same as the shortcomings of using a regular capacitor, which is that the output voltage drops as the thing discharges. That is different from a battery, where the voltage does not drop much until most of the reacting chemicals have reacted. So the system can be designed to only utilize the top of the RC discharge curve, leaving a very large portion of the stored energy unused, or else the system can have additional circuitry, such as a step-up voltage regulator, to compensate for the dropping input voltage. The penalty is greater complexity, additional expense, and somewhat lower efficiency. But the advantage s being able to utilize a much higher portion of the energy stored in the supercapacitor. So there really is the potential of using a supercap to power a cell phone. Self discharge during storeage is a reality, but most cellphones are switched on most of the time, so it may not be an issue for most people.

  19. JackGrat2
    October 2, 2013

    I was surprised at the broad claims and the lack of any critical analysis. The abstract does have some useful data describing the charge density as 203.3mF/cm2 and 238.5F/gram. How they arrived at 20.1Whr/kg is not explained. The article does not claim a 20 second recharge of a cell phone, just that 20 seconds of charge was used to illuminate a LED. The extrapolation to the cell phone was hyperbola. Getting a capacitor to provide operating power for a cell phone is not trivial. QuantumScape sounds like they are going to try. 

    Lets return to your 1230mAh battery. Q=It=CV=4.428kCoulombs . The cell phone typically operates from 3.7V down to 3.3V at discharge. That delta of 0.4V is your working V. A similar capacitor would need to be 11.07kFarads if operated over the same voltage range. That capacitor would weigh 46.4 grams, and cover 54.45kcm2 of surface.

    Assume we charge this supercap from a another supercap. This 11.07kF capacitor would be precharged to 4.1V. When connected to the phone's capacitor at 3.3V, the two then reach balance at 3.7V. Let's assume that the current is some how regulated and charges the phone in 20 seconds. That would be an average current of 221.4A. 

    At the start of the charge cycle, the source capacitor had a stored energy (E=0.5CV^2) of 93.04kjoules, and the cell phone had a 60.27kjoules residual charge. After the charge cycle, both capacitors are at 3.7V, and 75.77kjoules in each. The source gave up 17.27kjoules, the phone absorbed 15.5kjoules, and 1.77kjoules was radiated into the ether. That is 88.5watts for 20 seconds. Someone can call the FCC.


    That doesn't account for any resistive and switching losses. Obviously a capacitor is not equivalent to a good battery. We could assume some perfect form of DC/DC conversion and squeeze as much residual charge out of the capacitor, but we will still lose power to radiation every time we move charge from point A to B.

  20. SunitaT
    October 29, 2013

    Hand Turbine Smartphone Charger is a sort of charger. Charge your smartphone with a casual hand turbine, and this comes with an AM/FM weather aware radio too. This is perfect for outdoor usage.

  21. JefW
    November 14, 2013

    You can solve this problem easily with a supercap pretty much as described.

    Stop focussing on charging the phone with electricity.  Charge it instead with a small chemical package (a lithium ion battery).  Now you just need a supercap that can run the phone for 20 seconds to prevent any outage during the break-before-make battery swap.

    Now the problems have moslty been solved and the key technology required is mechanical design to facilitate battery swap without risk of incorrect insertion etc.

  22. JackGrat2
    November 14, 2013

    As an engineer, we call that small chemical package a “Primary Cell”. Most people prefer “Secondary Cell” over single use batteries. Supercaps are used anytime volatile memory is used and it is desired to hold the data store. Many systems use flash memory to avoid a bulky supercap. Systems with time clocks that can't be stopped are an example of the remaining holdouts.

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