Hacker News Reader

Comment by BenoitEssiambre | Hacker News Reader

Conspicuously missing is any mention of energy density. Who cares about power density. That's juste how fast it can discharge. We don't need capacitors that can discharge faster. All capacitors are already much better than batteries with respect to power. We need capacitors that can store more energy. Capacitors typically suck at that and this article doesn't give any indication that these new ones are better.

View on HN

In the comments they mention about 1/4 the energy density of a lithium ion battery, so yes they don't compete on that measure. But full charging can happen in a minute or two, and you could top up a charge in seconds. Perhaps public areas would have inductive chargers on the wall; you'd just walk over and hold your phone or tablet up to it for a few seconds. Airline tray tables could have an inductive charger built in.

Higher energy densities aren't without issues, as Boeing found out with their 787. A battery like that is basically an explosive and it can be tricky to manage.

Your inductive charging tray tables are terrible for my magnetic tapes and floppy disks. Also, where does it get its energy from? It's much more efficient to walk around with wearable solar panels or to embed them directly into the devices. It may not work well on the north pole half the time but batteries aren't very conductive in those environments, anyway.

When was the last time you walked around with a magnetic tape or floppy disc?

Now - what happens if you put your wallet full of credit cards on it?

To be honest, magnetic strips on credit cards are redundant in a fair proportion of the world as well, especially Europe, although with a notable exception of the US which seems to be seriously lagging behind in this area. I live in the UK and it has easily been over 6 years since I have been anywhere where the magnetic strip has been read rather than using chip & pin (EMV).

I visited the UK late last year and can confirm that your card readers can still read the magnetic stripes on our outdated American credit cards if they absolutely have to.

Unfortunately both types die with exposure to magnetic fields - I accidentally too mine into an MRI scanner. This may not Oruro at a lower field strength with the chip type however.

SYNTAX ERROR UNEXPECTED TO... Oh, :s/Oruro/occur/

Wow. That was a bad one, sorry, and I can't edit it. It was supposed to say: Unfortunately both types die with exposure to magnetic fields - I accidentally took mine into an MRI scanner. This may not occur at a lower field strength with the chip-type card however.

I'm pretty sure he was being sarcastic.

There isn't enough upward facing surface area on your body for solar cells to power anything non-trivial. Also floppy disks?!?

Power usage is going down with every generation. A few watts is quite a bit for a machine with an e-ink display.

For others, maybe nothing but charge sustaining purposes at first.

Guess we're not going to be able to work with our SX-64 portables on planes anymore.

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.

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?)

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.

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.

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.

> 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.

>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.

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.

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.

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.

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

Excuse me? This from the linked article (and in the paper abstract):

"These micro-supercapacitors demonstrate a power density of ~200 W cm−3, which is among the highest values achieved for any supercapacitor."

Granted its not a legitimate energy density (wrong units) but lets guess it is 200 Ws per cubic centimeter. I make that guess based on the comment in the video that they ran an LED for 5 minutes. So an LED is like 15mA and with a forward drop of a couple of volts so 30 mW. For 5 minutes your looking at 9000 mW-seconds, or 9 Ws for the small capacitor they showed in their video which could have been about a cm ^ 2. So if the cell they had made was 1/2 mm thick then a stack of 20 of them would be 1 cm^3 and 180 W-seconds (in the ball park of the abstract). There is a fun presentation on Supercaps [1] that was given to DoE in 2011. This computation does suggest that 200 Ws for this material would be a decent jump in capacity.

That said, I immediately dug an old LightScribe CD recorder out of my junk bin to start playing around with making graphene sheets :-)

[1] http://www1.eere.energy.gov/vehiclesandfuels/pdfs/merit_revi...

>That said, I immediately dug an old LightScribe CD recorder out of my junk bin to start playing around with making graphene sheets :-)

You serious? Sounds interesting, do tell us more.

Of course I am serious! Still trying to figure out what they used as a 'base' graphite oxide slurry (pencil lead in solution it isn't)

That's still power density. Power is Watts. Total energy is joules. You haven't addressed the parent's concern. We are interested in how long it can keep up that wattage, ie. joules/sec.

Here's a page which provides a bit more information than the OP: http://www.engineer.ucla.edu/newsroom/more-news/archive/2012...

And here's a paper on the subject: http://evmc2.files.wordpress.com/2013/02/laser-scribing-of-h...

ok so here is the only real data I could find. At the end of the paper:

"The LSG-EC can exhibit energy densities of up to 1.36 mWh/cm3 , a value that is approximately two times higher than that of the AC-EC"

1.36 mWh/cm3 = 1.36 Wh/L

If we assume that 1 L of it weights very roughly 1kg. We get 1.36 Wh/kg.

From wikipedia:

"The amount of energy stored per unit weight [in ultracapacitors] is generally lower than that of electrochemical batteries (3 to 5 W·h/kg, although 85 W·h/kg has been achieved in the lab[12] as of 2010 compared to 30 to 40 W·h/kg for a lead acid battery, 100 to 250 W·h/kg for a lithium-ion battery and about 0.1% of the volumetric energy density of gasoline."

This is still less than 1% of the energy density of lithium ion batteries.

There is a big miscalculation: Graphene seems to be extremely light weight. 1 liter of graphene will definitely not weight 1kg. Wikipedia [1] only gives measures for m²: Graphene is very light, weighing only about 0.77 milligrams per square meter.

[1] https://en.wikipedia.org/wiki/Graphene

Graphene is also very thin. If you created a box of graphene, it would naturally have about the same weight as diamond or coal (plus or minus 100%, maybe, but certainly not orders of magnitude less).

Probably not minus 100% :-)

Here[1], the density is said to be as low as 0.03 gm/cc which is the same as kg/liter. That gives a multiplicative factor of 33 over the above calculations, i.e. this could be 33% of the energy density of lithium ion batteries. Not bad.

[1]: http://www.strem.com/uploads/resources/documents/graphene_na...

Energy per liter also is an important factor. Your mobile would float, but it also would be the size of your lower arm. Maybe, you should wear your battery as a jacket or a pair of trousers?

A square meter of a single-atom-thick layer of graphene isn't very relevant. Expect the final product to have a volumetric density in the neighborhood of graphite or diamond.

Probably closer to graphite, as to my knowledge graphite is literally just layers of graphene.

You need to separate each layer with an insulator to be effective, otherwise you could just use graphite.

I have the article printed out somewhere, the energy density is in there, about 10 times that of conventional activated carbon supercapacitors and 3 times that of a comparable thin film lithium polymer battery off the top of my head, but I could be wrong. You can find the original paper at the AAAS (American Association for Advancement of Science) website, but its behind a registration wall, and they do mad data mining, so registering might take a while. Also, I have to remind you that this is an article on io9, not in the Scientific American. There are several other articles about this which likely came prior to io9 picking up on the story, and im pretty sure they provide some additional numbers which you so or so will find in the original paper on the subject. Here's the Kady paper that is referred to by the way. http://www.sciencemag.org/content/335/6074/1326.abstract

Who cares about power density.

Well, cars do. As evidenced by the use of lead-acid batteries, power density > energy density for automobiles.

Of course, better power density is probably not the main obstacle to using capacitors in a car to replace the battery.

Reply on news.ycombinator.com