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0 pointsmbell13y ago0 comments

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.

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Someone13y ago

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?)

ScottBurson13y ago

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.

mbellOP13y ago

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.

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derekp713y ago

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.

mbellOP13y ago

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.

jodrellblank13y ago

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

j / k navigate · click thread line to collapse

0 pointsmbell13y ago0 comments

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

0 comments

Someone13y ago

ScottBurson13y ago

mbellOP13y ago

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

2 more replies

derekp713y ago

mbellOP13y ago

jodrellblank13y ago

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

j / k navigate · click thread line to collapse