Hacker News Board Hacker News, browsed like a classic message board
Live data from Hacker News

Can You Power a Phone With a Capacitor?

wired.com

Post reply on HN
Print view View original

31–40 of 54 posts

Re: Can You Power a Phone With a Capacitor?

#31

Post by aidenn0 »

leephillips wrote: post #3

Amidst the general decline in quality of publications on the Web, it's great to see Wired , which put out lots of fluffy, inaccurate stuff in the past, publishing so many solid, interesting articles. This was fun to read, had actual equations and graphs, and made sense. I remember reading the headline about charging a phone in 20 seconds and not bothering to look at the details, because, no way.

Is my sarcasm detector broken? This is a fluffy inaccurate post masked by a bunch of equations any apt high school physics student could have gotten from their textbook.

Nobody uses mica capacitors for high energy-density applications, electrochemical EDLC's have much higher energy densities.

Furthermore switching regulators that can operate the phone relatively independent of input voltage are cheap and efficient.

Yes the other articles about this were bizarre, no, cell phone battery replacement (where nobody seems to mind plugging them in at night, and ED is king) are not a good application for any capacitor in the foreseeable future, but this article is hardly better.

More nitpicks: "The news reports don’t actually state how much energy the storage device can store."

when the story showed up on HN, I searched google news for a few words from the headline and found the ED in one of the top 5 links. I don't remember the exact number, but it was on the order of a SLA starter battery (so lower than a deep-cycle SLA battery). Clearly not in cell-phone territory.

Re: Can You Power a Phone With a Capacitor?

#32

Post by benjamincburns »

Dylan16807 wrote: post #29
benjamincburns wrote: post #16

Earlier quoted context omitted.

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 ch…

The point is that you don't need to round up. 2.7 volts is pretty close to 3, so you need pretty close to 2x2=4. Bump it to five to compensate for the rounding and look it matches the actual math. Half of your initial suggestion.

Caps tend to explode when they fail, and high energy caps explode violently. You always, always, want to round down when spec'ing caps, not up.

But forget all that. Lets do it differently and fix our math toward the author's original comparison, the 1500mAh battery, not his weird, somewhat arbitrary choice, of a 6V-rated cap which drains to 2.5V after ~10 hours. And instead of talking about electricity, let's talk about what we're really after: energy.

A 1.5Ah battery stores about 22.68kJ of energy when fully charged to 4.2V (1.5Ah×3600C×4.2V=22,680J). From above, At 2.7V, a 1200F cap stores 8.478KJ of energy. 22.68kJ/8.478kJ = 2.675. Meaning we need at least 3 of these caps to meet or exceed the energy storage of a 1500mAh battery.

So we're both wrong in the context of the original comparison. And more importantly, so is the author.

Edit: This 3000F supercap rated for 2.7V would get us damn close to the 22.68kJ requirement, coming in at 21.87kJ of total storage: http://www.mouser.com/ProductDetail/Maxwell-Technologies/BCA...

Edit again:

I messed up again! The battery energy capacity is actually less than 22.68kJ, because the energy equation I used assumes that the voltage stays constant during the battery's discharge cycle. It doesn't. For a constant-current discharge of a LiPoly battery, the mean voltage is something like 3.4-3.6V (estimating from the discharge voltage curve, assuming discharge stops at 2.7V). So really we're talking something closer to 19kJ of energy stored within the battery. By the figures in her submission, her cap would weigh 262.6g and would occupy 30.8m^2 (no figures on thickness to calculate volume). Titanium density is 4.5g/cm^3. Assuming titanium dominates the density of her material, it'd be about 58cm^3 in volume - something like 10cm by 5.8cm by 1cm. If the density matched that of copper, it'd be about half that size.

Re: Can You Power a Phone With a Capacitor?

#33

Post by Dylan16807 »

benjamincburns wrote: post #32
Dylan16807 wrote: post #29

Earlier quoted context omitted.

The point is that you don't need to round up. 2.7 volts is pretty close to 3, so you need pretty close to 2x2=4. Bump it to five to compensate for the rounding and look it matches the actual math. Half of your initial suggestion.

Caps tend to explode when they fail, and high energy caps explode violently. You always, always , want to round down when spec'ing caps, not up. But forget all that. Lets do it differently and fix our math toward the author's original comparison, the 1500mAh battery, not his weird, somewhat arbitrary choice, of a 6V-rated cap which drains to 2.5V after ~10 hours. And instead of talking about electricity, let's talk a…

Whoa, whoa, nowhere did I suggest applying extra voltage. I was saying you could use them at 2.7, or 2.6 if you want a safety margin. You don't have to use pure RLC and drop down to 2 volts.

It is amusing that the author screwed up.

Re: Can You Power a Phone With a Capacitor?

#34

Post by leephillips »

aidenn0 wrote: post #31
leephillips wrote: post #3

Amidst the general decline in quality of publications on the Web, it's great to see Wired , which put out lots of fluffy, inaccurate stuff in the past, publishing so many solid, interesting articles. This was fun to read, had actual equations and graphs, and made sense. I remember reading the headline about charging a phone in 20 seconds and not bothering to look at the details, because, no way.

Is my sarcasm detector broken? This is a fluffy inaccurate post masked by a bunch of equations any apt high school physics student could have gotten from their textbook. Nobody uses mica capacitors for high energy-density applications, electrochemical EDLC's have much higher energy densities. Furthermore switching regulators that can operate the phone relatively independent of input voltage are cheap and efficient. Y…

I was not being sarcastic, so don't worry about your detector. This was the way a physicist would take a first look at the problem, so I could relate and I enjoyed it. You and some other critics here are taking the author to task for not writing a detailed electrical engineering analysis, but I don't think that's entirely fair. Although the article is not completely accurate in that sense, it's also not "fluffy", and much better than the stuff that caused me to write Wired off until recently. Scoff if you must, but physicists are fond of their spherical cows and mica capacitors.

Re: Can You Power a Phone With a Capacitor?

#35

Post by aidenn0 »

leephillips wrote: post #34
aidenn0 wrote: post #31

Earlier quoted context omitted.

Is my sarcasm detector broken? This is a fluffy inaccurate post masked by a bunch of equations any apt high school physics student could have gotten from their textbook. Nobody uses mica capacitors for high energy-density applications, electrochemical EDLC's have much higher energy densities. Furthermore switching regulators that can operate the phone relatively independent of input voltage are cheap and efficient. Y…

I was not being sarcastic, so don't worry about your detector. This was the way a physicist would take a first look at the problem, so I could relate and I enjoyed it. You and some other critics here are taking the author to task for not writing a detailed electrical engineering analysis, but I don't think that's entirely fair. Although the article is not completely accurate in that sense, it's also not "fluffy", and…

To take the spherical-cow analogy further, this article is a bit like responding to a headline of "New non-spherical cow shape way more efficient than spherical shape, promises free milk for everyone!" by doing an approximation with a spherical cow.

Particularly when a tiny bit of googling turns up actual numbers claimed for the cow in the experiment.

Re: Can You Power a Phone With a Capacitor?

#36

Post by xxpor »

pbo wrote: post #30
benjamincburns wrote: post #12

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 feasib…

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. One application for this is lasers. I'm part of a team that works on a converter that uses wall power as an input to continuously charge a few dozens ultracaps then empties them all at once to fire very short pulses in the hundreds of kW. What I find the most impressive about this…

Holy crap, what can you do with kW lasers besides instantly cut almost anything?

Re: Can You Power a Phone With a Capacitor?

#37

Post by roc »

xxpor wrote: post #36
pbo wrote: post #30

Earlier quoted context omitted.

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. One application for this is lasers. I'm part of a team that works on a converter that uses wall power as an input to continuously charge a few dozens ultracaps then empties them all at once to fire very short pulses in the hundreds of kW. What I find the most impressive about this…

Holy crap, what can you do with kW lasers besides instantly cut almost anything?

Popcorn?

Re: Can You Power a Phone With a Capacitor?

#38

Post by venomsnake »

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…

Why would you need 6v for a phone? I got feeling this was pulled out of his ass. Everything works on sub 2V there if I am not gravely wrong (not sure for the light source for the display)

Re: Can You Power a Phone With a Capacitor?

#39

Post by aidenn0 »

benjamincburns wrote: post #12

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 feasib…

A much cooler (but obsoleted by improving capacitor technology) was flywheel based pulsed energy: http://www.bevoware.com/research/cem/IEEE/PR%2071%20Werst%20...

Re: Can You Power a Phone With a Capacitor?

#40

Post by aidenn0 »

xxpor wrote: post #36
pbo wrote: post #30

Earlier quoted context omitted.

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. One application for this is lasers. I'm part of a team that works on a converter that uses wall power as an input to continuously charge a few dozens ultracaps then empties them all at once to fire very short pulses in the hundreds of kW. What I find the most impressive about this…

Holy crap, what can you do with kW lasers besides instantly cut almost anything?

kW isn't that high for pulsed lasers. Particularly since with Q-switching you can get very short pulses and mode-locking ridiculously short pulses. Total energy for a 1ps pulse at 100kW is negligible.
Post reply on HN