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Can You Power a Phone With a Capacitor?

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Re: Can You Power a Phone With a Capacitor?

#11

Post by benjamincburns »

guan wrote: post #8

This guy doesn’t seem to know much about really existing supercapacitors. Assuming that 1200F is enough, you can get a 1200F supercap (Maxwell BCAP1200 P270 K04) rated for 2.7V that’s 60.7 mm in diameter and 74.30 mm tall. He’s assuming 6V, so let’s say you need two of them. That’s still way too large for a phone, but nowhere near as huge as he suggests.

You'd actually need 9 of them. Capacitor networks don't behave like resistor networks. Placing caps in series increases their voltage tolerance but reduces their overall capacitance. You'd need three parallel sets of three of these caps in series to get the voltage and capacitance you're after.

Edits: It turns out that 3*3=9 these days, not 6.

Re: Can You Power a Phone With a Capacitor?

#12

Post by benjamincburns »

Here's her actual submission [1]. She created a novel dielectric and achieved an energy density of 20.1 Wh/kg which is about half of that of a lead-acid battery, and something like 6%-10% of a Lithium Polymer battery. She lists a capacitance per volume, but nothing about breakdown voltages, so I can't say what the size of her capacitor would theoretically be for 1200F rated at 6V. Nor can I say if it'd even be feasible without knowing how difficult/expensive the dielectric is to manufacture.

Also, here's a page [2] with a list of common battery chemistries and ballpark figures for their energy density values (about halfway down in green). Newer Li Poly tech is somewhere in the 180Wh/kg range.

The sad thing about this is that we're missing a bigger point. Capacitors are useful because of their low internal impedance. They're able to dump (or receive) a tremendous amount of power incredibly quickly. It's useful when you have a circuit which needs a high-current burst which is powered from a low current (but high capacity) power source.

1: http://www.usc.edu/CSSF/Current/Projects/S0912.pdf

2: http://www.allaboutbatteries.com/Battery-Energy.html

Re: Can You Power a Phone With a Capacitor?

#13

Post by finnh »

freehunter wrote: post #10
samatman wrote: post #5

Earlier quoted context omitted.

I used to TA undergraduate science courses. I successfully convinced at least one generation of young minds that the SI prefix G is pronounced hard, except for power, where it is pronounced soft like J. I blamed the French for this historical oddity. They bought it.

Giga actually has an acceptable pronunciation with a soft g. The American National Bureau of Standards actually has this pronunciation formalized.

gigga what? gigga who? Switcha flow, getcha dough

EDIT: apparently downvoters don't like Jay-Z.

Re: Can You Power a Phone With a Capacitor?

#14

Post by zackbloom »

benjamincburns wrote: post #11
guan wrote: post #8

This guy doesn’t seem to know much about really existing supercapacitors. Assuming that 1200F is enough, you can get a 1200F supercap (Maxwell BCAP1200 P270 K04) rated for 2.7V that’s 60.7 mm in diameter and 74.30 mm tall. He’s assuming 6V, so let’s say you need two of them. That’s still way too large for a phone, but nowhere near as huge as he suggests.

You'd actually need 9 of them. Capacitor networks don't behave like resistor networks. Placing caps in series increases their voltage tolerance but reduces their overall capacitance. You'd need three parallel sets of three of these caps in series to get the voltage and capacitance you're after. Edits: It turns out that 3*3=9 these days, not 6.

You can use a DC/DC to boost the voltage.

Re: Can You Power a Phone With a Capacitor?

#15

Post by ISL »

benjamincburns wrote: post #9
pejoculant wrote: post #6

There most certainly are capacitors available that meet his specs without being unreasonably large. For example this one has a capacitance of 1200F and is only 8 cm long: http://www.mouser.com/ProductDetail/Ioxus/RSC2R7128LR/?qs=cf... Modeling it as a parallel plate capacitor with a mica dielectric is basically totally ignoring the part about it being a supercapacitor. He may as well have been seeing if it would be f…

Most supercaps are rated for very low voltages (this one is only rated for 2.4V), as dictated by the breakdown voltages of the dielectric and its manufacturing tolerances. A 6V-rated 1200F supercap made from the same materials will be considerably larger. Further, most electrical engineers will enforce a factor of safety beyond the manufacturing tolerances . This is doubly true for capacitors, since dielectric failur…

High efficiency DC/DC converters are now commonplace. Energy density (J/m^3 and J/kg) and internal resistance are the really important quantities in energy storage capacitors.

Re: Can You Power a Phone With a Capacitor?

#16

Post by benjamincburns »

zackbloom wrote: post #14
benjamincburns wrote: post #11

Earlier quoted context omitted.

You'd actually need 9 of them. Capacitor networks don't behave like resistor networks. Placing caps in series increases their voltage tolerance but reduces their overall capacitance. You'd need three parallel sets of three of these caps in series to get the voltage and capacitance you're after. Edits: It turns out that 3*3=9 these days, not 6.

You can use a DC/DC to boost the voltage.

Sure. In fact you'd want to use a high-efficiency buck/boost converter to take advantage of the entire discharge curve. However you're only storing about 20% of the energy at 2.7V.

Edit: I'll show my work.

Farads are Joules (energy) per Volt (electric potential) squared. That means that the formula for energy stored (E) in a capacitor is E=CV^2.

A 1200F cap charged to 2.7V stores 8478 Joules of energy. A 1200F cap charged to 6V stores 43,200 Joules of energy. That means that at a charge of 2.7V there's ~80% less energy stored in the capacitor than at 6V. Incidentally this ratio holds no matter value of the capacitor.

Re: Can You Power a Phone With a Capacitor?

#17

Post by vasco »

zackbloom wrote: post #14
benjamincburns wrote: post #11

Earlier quoted context omitted.

You'd actually need 9 of them. Capacitor networks don't behave like resistor networks. Placing caps in series increases their voltage tolerance but reduces their overall capacitance. You'd need three parallel sets of three of these caps in series to get the voltage and capacitance you're after. Edits: It turns out that 3*3=9 these days, not 6.

You can use a DC/DC to boost the voltage.

At the cost of power which will increase your necessary total capacitance.

Re: Can You Power a Phone With a Capacitor?

#18

Post by benjamincburns »

vasco wrote: post #17
zackbloom wrote: post #14

Earlier quoted context omitted.

You can use a DC/DC to boost the voltage.

At the cost of power which will increase your necessary total capacitance.

Digital circuits require some minimal voltage in order for their internal transistors to switch. This is because semiconductors only conduct in the presence of an electric field with a high enough potential to excite and free up the electrons hanging out near their junctions. That means that if your voltage drops below, say 1.8V (common for most low-power digital circuits these days), you won't be able to use your device any more.

There are some very efficient buck/boost converters out there. Like 95% and above efficient. Typically the cost of the power to use a buck/boost dc-dc converter is far outweighed by the ability to convert more of the stored charge into a potential that's suitable to operate the circuit.

Put differently, picture a barrel with a tap on it's wall somewhere near the bottom, and a different barrel with a conical-shaped bottom which tapers into a drain-like tap. Which is more useful? The latter is what you get from a good buck/boost converter.

Re: Can You Power a Phone With a Capacitor?

#19

Post by carlob »

I think it's much better to think of a capacitor as a spring:

* they store energy;

* they can release energy quickly;

* when put in parallel they get stronger;

* when put in series they get weaker;

* the RCL circuit differential equation is the same differential equation as the damped harmonic oscillator.

Re: Can You Power a Phone With a Capacitor?

#20

Post by leephillips »

samatman wrote: post #5
jtoeman wrote: post #4

how about a flux capacitor? 1.21 gigawatts!

I used to TA undergraduate science courses. I successfully convinced at least one generation of young minds that the SI prefix G is pronounced hard, except for power, where it is pronounced soft like J. I blamed the French for this historical oddity. They bought it.

You used your powers for evil.
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