Saturday, 25 May 2013

Charge your mobile phone in 20 seconds.

It's all over the internet, and I was pretty much ignoring it, because we all know that just because something is on the internet, doesn't mean it's true. But today it was in the Times!

"The waiting is over as teenager invents gadget to charge a phone in 20 seconds"

I've noticed that many people, especially journalists, fail to apply the "should I believe this?" test. So let's give it a go.

A typical mobile phone battery is 1200 milliamp-hours, 3.6 volts. That means that if you take 1200 milliamps out of it, it will last an hour. Or, if you put 1.2 amps (that's 1200 milliamps) into it, it will take an hour to charge up.

If you want it to charge up in a minute, you'd need to put 72 amps into it. If you want it to charge up in 20 seconds, you'll need to put 216 amps into it.

Your house wiring is rated at 13 amps. Some parts of your house wiring (for cookers, for example) will be rated at 30 amps. The wiring coming into your house is probably rated for 100 amps, and is VERY THICK. To carry 200 amps, you want wire that's about as thick as your little finger. Two of them, one for positive, and one for negative. But then you're going to connect that to the tiny connectors on your mobile phone?

I find it extremely difficult to believe that someone is going to put 216 amps into a little mobile phone battery. I also find it extremely difficult to believe that if someone does try to do that, that the battery, the wiring (or even the intrepid phone user) will survive the experiment.

But now let's think about the internal impedance of the battery. If the battery has 200 milli-ohms, it's in good condition ( That's 0.2 ohms. To get 216 amps into it, you'll have to give it 43.2 volts. If you give a 3.6 volt battery 43.2 volts, the results will be pretty sad, maybe even spectacular.

OK, let's dispense with the battery, and let's just put this "super-capacitor" into your mobile phone.

Same problem. To charge it up in 20 seconds, you need to put 216 amps into it. You still need those gorilla-sized wires.

So. What's going on here?

I believe that the girl in question has made what the media call a "super-capacitor", which is what I would call a "capacitor". And that you can charge that capacitor in 20 seconds; that I believe, because you can charge pretty much any capacitor in 20 seconds. But I doubt if it's 1200 milliamp-hours, and someone has leapt to an unsupportable conclusion, and no-one, pretty much no-one, has asked anyone who knows anything about electricity (and really, you don't need to know an awful lot, it's just simple arithmetic) whether this exaggerated claim is possible.

And Eesha Khare, the teenager in question, has just learned an important lesson about the media. Namely: "Don't believe anything you read in newspapers, because they don't check facts".

So what's the truth?

Ultra-capacitors have been around for a while. They can indeed be charged really fast. But they'll store only 5% as much energy as a Lithium battery. So you'll need a REALLY BIG mobile phone to use them.

My guess is that Eesha has come up with an improvement on that 5%. But we'll probably never know, because the media have so distorted this news, I donn't know how to find out.

1 comment:

  1. But you must know you can have a fully charged battery on a Nokia 6310i in less than 20 seconds. Just change the dead battery with a pre charged battery.