
Can You Power a Phone With a Capacitor? - ColinWright
http://www.wired.com/wiredscience/2013/05/can-you-power-a-phone-with-a-capacitor/
======
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>

~~~
pbo
_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 is that the mechanical engineers
have managed to fit this in a 2U rack.

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

~~~
aidenn0
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.

~~~
pbo
Exactly. Since the output is not continuous, the average power remains quite
low. This is why you can still power it from regular domestic power :)

------
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.

~~~
ijager
That is better given one is more comfortable with mechanical systems. Since my
field is EE, I tend to do the opposite: translate a problem to the electrical
domain to be more intuitive about it. You can use Bond graphs[1] to easily
translate between different domains (Mechanical translation,Mechanical
rotation, Electromagnetic, Hydraulic and Thermic in a more limited way). This
technique generalises force and voltage to "Effort" and velocity and current
to "Flow".

[1]:<https://en.wikipedia.org/wiki/Bond_graph>

------
leephillips
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.

~~~
aidenn0
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.

~~~
leephillips
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.

~~~
benjamincburns
> You and some other critics here...

You're probably referring to me, since I've been posting all _over_ this
thread and I did nothing to preface my criticisms. I really didn't mean to
"take the author to task," and I was as astonished and happy as you were to
see this piece in _Wired_. This was just something I became interested in when
the news first hit, and for whatever reason I've enjoyed talking about it and
breaking it down more than usual.

The only thing I'd criticize him for was that he's basing his back-of-the-
envelope on some arbitrary things (the 6V rating and choice of a ceramic/mica
dielectric, for instance) rather than trying to base it on an appropriate
lower bound. Had he done the latter he would've reached the same conclusion
but with a far less outlandish example. It's only bad because relatively small
supercaps which would have worked for his example do exist (much to the
chagrin of the news stories he's criticizing), and his example makes it look
like he's suggesting otherwise.

~~~
leephillips
"You're probably referring to me"

No, your comments were informative while politely avoiding direct criticism of
the author. And you make another good point here.

------
guan
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.

~~~
benjamincburns
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.

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

~~~
benjamincburns
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.

~~~
Dylan16807
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.

~~~
benjamincburns
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...](http://www.mouser.com/ProductDetail/Maxwell-
Technologies/BCAP3000-P270-K04/?qs=sGAEpiMZZMuDCPMZUZ%252bYl4vDGR228uRDhgVZbUDv3h0%3d)

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.

~~~
Dylan16807
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.

------
pejoculant
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...](http://www.mouser.com/ProductDetail/Ioxus/RSC2R7128LR/?qs=cfj1DI9zbtZQF%252bmdKfgsJA==)

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 feasible to get to the moon using a wood fired
rocket.

~~~
benjamincburns
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 failure on high energy caps can
be somewhat catastrophic (read: explosive).

~~~
venomsnake
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)

~~~
benjamincburns
You don't. I'd bet most of the voltage rails on a modern phone are 1.8V and
3.3V. Then the display backlight usually uses a constant-current regulator,
and you'll probably have some oddball low voltages for the CPU core and other
various things. Given Ohm's law you can generate all that using DC-to-DC
converters.

------
ColinWright
This in reference to the recent stream of posts about charging a phone in 20
seconds. Or 30 seconds. Or something.

Here's the most recent, with a collection of links to others:
<https://news.ycombinator.com/item?id=5798189>

------
yazaddaruvala
Can someone remind/explain to me what happens if I were to use both my phone's
current battery but put a super cap in front of it?

ie When I plug my phone in to charge, the super cap is "instantly" charged.
Then a smart charging circuit would use the super cap to charge my battery.

Situation and my expectation: I'm headed home from work, drop off my laptop
and leave for some social gathering, but my phone is about to die. Trying to
charge me phone for the 1 min I'm in my house in futile. However, my
expectation is that the super cap would fully charge. Then while I'm walking
to my destination, it could charge my battery to some reasonable percent.

I'd be really happy even if I could get ~20% or so from the fully charged
super cap. Would definitely be better than nothing.

~~~
solistice
It's kinda like you have a bucket you carry around you for drinking water. And
you get back home, your bucket is about empty, and you have 3 seconds to get
water before you need to leave and you're like "Let me fill up my glass
instead and then slowly pour it into my bucket". Your glass will be full, but
it wouldn't make a dent in you bucket, and frankly if you were to carry around
a glass of neccesairy size to fill up your bucket to even 20%, you could just
carry another bucket around.

It's a really bad metaphor for supercapacitors having worse energy densities
than for example lithium ion batteries, and that having a supercapacitor large
enough to recharge your battery to 10% would esentially entail carrying around
another battery.

Honestly, if you have a phone you can actually open, you'd be better advised
with an exchange battery, and if you can't open it, an external battery pack
you carry around for emergencies like this.

For example, I think this external charger might do what you want it to do
[http://www.amazon.com/Energizer-Energi-To-Go-Battery-
Operate...](http://www.amazon.com/Energizer-Energi-To-Go-Battery-Operated-
Instant/dp/B000IOKLZM) but your requirements might differ.

------
swamp40
In 10-15 years, you will almost certainly be able to charge your cellphone in
30 seconds.

And a supercapacitor will probably be what's inside to allow that.

The incredible charge rates of supercaps will someday (when the energy density
improves enough) become a nice feature.

It's possible some version of LiPo batteries will improve the charge rate
dramatically, but I believe the rate is limited by chemical reactions.

------
scotty79
I can totally imagine phone that has two huge metal contacts on sides that get
exposed and connected as you shove whole phone into the charger. Such
connection could transfer 1kW easily.

------
jtoeman
how about a flux capacitor? 1.21 gigawatts!

~~~
samatman
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.

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

~~~
finnh
gigga what? gigga who? Switcha flow, getcha dough

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

~~~
geuis
You're being downvoted because your comment is inane, off topic, and adds
nothing of value to the conversation. Even the grandparent quotation from Back
to the Future suffers from the same problem and is being downvoted.

~~~
jtoeman
message clear - hacker news: NOT the place for idle joking

