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.