[mbell]

I've heard this argument a few times: "It charges so fast that the density doesn't matter".

What often gets missed is that for it to charge fast, you need to provide a lot of power, a lot more than any current changer and laughably more than any inductive system can provide. Lithium Ion batteries can already max out the power offered by the 10W charger that comes with the iPad and charging off computer USB is often slow (USB is current limit). See Telsa's car charge times on normal wall outlets vs superchargers for another example.

To really reap the benefits of this quick charge technology you either need an infrastructure of ~1000W DC chargers throughout the world or carry something about the size of a desktop computer power supply with you at all times.

[Someone]

Also, you would need customers brave enough to charge that tiny device they are holding in their hands at 1000W. A mobile has about the surface area of a 100W incandescent, so if charging is 90% efficient, your mobile would have to radiate about as much heat as such an incandescent would do in about 3 seconds. It won't get as hot as that lamp, because your phone has a larger heat capacity, but I doubt it would stay cool, either. Given the effect of heat on current batteries, I guess designers would want that phone to irradiate waste heat rapidly.

And that's when everything goes well. You would also need companies brave enough to risk the potential lawsuits if something goes wrong (user wears a pacemaker? Has some metal in his hand, e.g. in a tattoo? Charging accident releases heat that lights clothing?)

[ScottBurson]

Charging capacitors is normally much more than 90% efficient. They're not like batteries, where a chemical reaction occurs during charging. Resistive losses in capacitors are normally negligible. In this case they will probably still be negligible, I'd guess, because of the high conductivity of graphene.

[mbell]

True but you've got to consider the numbers we're looking at here.

As an example an iPhone 5 has a 5.45Wh battery, if you wanted to replace that with a super-cap and charge it to 5V in 20 seconds you'd need to provide ~1000W of power or 200A @ 5V. Even if the super cap had very low ESR, call it 1mOhm, which is extremely low compared to current super-caps, you'd still end up dissipating 40W as heat with ~96% efficiency.

[gallerytungsten]

Perhaps phone makers can start making their cases out of aluminum, with enough mass to take the heat. 40W over 20 seconds doesn't sound too daunting.

If you split such an aluminum backplate into two parts you could use them as the power contacts. Then you could have a "coffin" type charger where you put in the phone, then closed a cover to run the charge cycle. Kind of like the cover of a washing machine, to reduce the danger level.

[mbell]

> 40W over 20 seconds doesn't sound too daunting.

It would be 40W _for_ 20 seconds, or 800 Joules, which is a lot of heat, enough to boil 3 grams of water that started at 20C or increase the temperature of 30g of aluminum by ~30C over ambient.

[AnthonyMouse]

>200A @ 5V

I don't think so. Have a look at the comparison image around 3/4ths of the way down the page here:

http://www.interfacebus.com/Copper_Wire_AWG_SIze.html

14AWG (the small one) is the one rated for 20A which you might find in the power cord for a desktop PC, and is significantly bigger than the wire on your current phone charger. The big one (1/0) is rated for 125 amps. You have to go to 3/0, two sizes higher than that, for 200 amps. 3/0 gauge wire is what they commonly use for the main electrical service for a commercial building.

There is no way they would use a 5V charger if it had to draw that much current. But then you have a different problem: High voltage DC is extremely dangerous because it causes your muscles to contract if you come into contact with it, so your heart stops and you can't move to separate yourself from the electrical source.

20 seconds for a full charge is just unrealistic. Make it 60 seconds, and use a 24V charger, and now you're well within reason.

[mbell]

Your right that the way to solve the current issue is to increase voltage, but that creates a list of other problems, the most notable of which, death, you've already touched on, but here some others:

1) You likely still want USB charging support, so now you need a boost converter in the phone or cable.

2) Your PMIC needs to accept the higher input voltage, and you have to be willing to accept the reduced regulator efficiency from the increase in Vin - Vout.

3) 24V running around in a phone creates a lot of possible problems that you don't have with 3.7V cells. Increased moisture sensitivity, gradient induced oxidation, etc.

4) You still need a ton of power. If you jump to 24V then you still need ~42A to hit a 20 second charge, go to 60 seconds and you need ~327W or 13A @ 24V. That is still a massive charger with 10 gauge wiring (1 conductor is 2x + the diameter of the entire lightning cable).

Keep in mind these are lower bound numbers all around. Reality could be 50-100% higher for power needs. The ESR value I threw out above is also a very optimistic minimum hoping that this new tech has much better ESR characteristics than current super-caps which for large capacity models can be up in the hundreds of mOhms which causes a huge thermal issue.

Its still a long ways from being even remotely reasonable for super caps to replace batteries in high power devices like phones and tablets.

[derekp7]

Maybe the charger could also have a super capacitor in it, and would charge it up over time. Then when you hook up your device, you would be getting capacitor to capacitor charging. After all, if these things can charge up fast, then they can also discharge fast.

[mbell]

That would work, but you'd need to use about 0 gauge wire between the devices. If you tried that with something the size of a lightning cable you'd have a decent chance of vaporizing the wire/connectors.

[jodrellblank]

So now all we need is this: http://en.wikipedia.org/wiki/File:CERN-cables-p1030764.jpg