What's all this supercapacitor stuff anyhow?

[dnyberg2]

Anyone on this site that happens to be a supercapacitor expert?

I'm looking for a real supercap genius. I see a lot of "super capacitor" parts for purchase online from various sources like Digikey and Mouser but what I don't see is an informative guide on how to figure out what type is the best for my application...

I understand that there are some exotic varieties but don't see them for sale necessarily. If I can't buy one for dollars its of no use in my application. Are there such things as super caps that can be charged much faster than others based on construction or materials? Is it insane to consider contacting super cap suppliers for a "custom" super cap design that's the best I can afford in the size I need?

Yes I realize I'm probably talking about hundreds of thousands of dollars but in the end it might be worth it to me. Any knowledge over what I glean off the net would be most appreciated.

 

[berkeman]

Sorry to put on my Captain Obvious hat here, but what have you found in your reading so far at Wikipedia and similar websites? Can you narrow down your questions a bit based on that reading? That would probably help us to answer your questions.

For example, what range of ESR do you see discussed at Wikipedia? What changes the ESR and allowed ripple currents? What are the loss angles of the different types of supercaps?

 

[dnyberg2]

Unfortunately those are some of the basic questions I need answered. This is strictly a DC charge-pot type storage application. A charging source has some capacity to dump energy into the cap and a voltage regulator dishes it out up to a specified max current draw set by me. What I need to know is what effects like ESR have on my ability charge and or discharge. Un-connected leakage is another concept. If I disconnect the cap, how long can it hold that charge? So many questions, so little expertise. Its not a battery, but I want to use it like one for very low power applications...

 

[Averagesupernova]

Knowing that ESR is Equivalent Series Resistance you can answer the charging and discharging question better than anyone else here since you know more about the application than anyone here.

 

[dnyberg2]

Perhaps but, what I don't know is if there are exoctic technologies availabel that don't act the same in both directions. If ESR is the prevailing threat to charge and discharge rate, which I realize can affect the life of these things, are there materials that allow faster charging than the discharge current I need? I can't imagine it but I don't work in a university supercapacitor lab... If I want a discharge rate of something on the order of 50 mA, is there any technology out there that can allow me to charge the same cap in an instant with much less resistance on the charge cycle? I dunno...

 

[Averagesupernova]

ESR is going to be bidirectional.

 

[dnyberg2]

Let me give the forum a good example. I can take an AVX 3F 3 Volt supercap off the shelf and charge it up with a power supply 100% to 3 Volts in a few seconds using amps of current. The cap gets warm. If I put a 50mA load on it and drain it down to 1 volt, it takes about 4 minutes. It take about 24 minutes to charge it back up to 3 Volts @ 10 mA. Sure would be cool if that same 10 mA of charge current I have could charge it at a MUCH faster rate...

 

[dnyberg2]

OKay then. If there are no new cutting edge technology to these caps in that respect, then it is what it is.

 

[Averagesupernova]

Let me give the forum a good example. I can take an AVX 3F 3 Volt supercap off the shelf and charge it up with a power supply 100% to 3 Volts in a few seconds using amps of current. The cap gets warm. If I put a 50mA load on it and drain it down to 1 volt, it takes about 4 minutes. It take about 24 minutes to charge it back up to 3 Volts @ 10 mA. Sure would be cool if that same 10 mA of charge current I have could charge it at a MUCH faster rate...

-
That's like trying to fill a five gallon bucket from empty with from a source that is only able to flow 1 gallon per minute and not wanting to wait the 5 minutes.

 

[dnyberg2]

Yea, but A guy can ask right? A guy can dream... If you don't ask, you'll never know.

 

[CWatters]

Perhaps use a battery instead.

 

[dnyberg2]

Can someone with pity please tell me the relationship between ESR and the ability to charge and discharge a supercap using DC? In the first place, why do they stipulate ESR for a supercap when I read they are not appropriate for use in any AC CKT? If I am using it as a DC power storage device, does a lower ESR mean its "easier" to charge and discharge it or is a higher ESR easier to charge and discharge?

 

[mfb]

Yea, but A guy can ask right? A guy can dream... If you don't ask, you'll never know.

You can dream, but dreaming about violating conservation of energy isn't going to help.

A lower ESR will normally lead to faster charging and discharging and lower heat in the capacitor.

Knowing what you need the capacitors for (or at least some specifications) would help.

 

[dnyberg2]

I've been giving the examples throughout the thread. But to be as specific as I can, The application is a short term, low power, DC power storage application. I have a CKT that can provide a very low current charge up to 3 volts, the max voltage of most of these caps. I only have room for one. I can physically fit one 3F, 3 volt cap into my chassis or container along with the charging and regulator CKT's. The output is fixed in this case @ 1.5 Volts. This "Thing" I'm working on is able to supply up to about 60 milliamps of DC current @ 1.5 volts for minutes before its out of juice. (The LDO regulator cuts off) The low current "Charging" circuitry (<mA's) is always working to refill the empty bucket as best it can. The easier that job is, the faster the bucket (Supercap) can be refiled. The point (use case) is not to ever empty the supercap below 1.6 volts so this "Device" I'm tinkering with is only useful for very low power / intermittent applications. When not in use for long periods, say over night, you wake up with a full bucket of charge to use as you see fit. Almost any supercap out there can be used for what I'm tinkering with, my question to the forum was, is there a better cap - exoctic composiotion, more expensive technology with "The Lowest ESR" (Thanks for that). I was hoping to catch a gurue that works in a supercap lab that knows something I maybe can't even buy now or maybe someone that's in on some cutting edge research that might be able to tell me something like, hey, if you wait another year, this new exocti supercap will be on the market with blah, blah less ESR than any other cap on the planet at three time the present cost. What would be nice is if I had some idea about the various technologies and science that go into the electrolyte and electrode make-up. Maybe then I cold find a better cap than the crappy AVX parts I'm tinkering with now. What about those supercaps that advertise to have gold in their makeup? Do they have lower ESR's than other supercap technologies? IS that the very best man can buy?

 

[dnyberg2]

The AVX supercap I am using right now has an ESR of 95 mOhm @ 1kHz. I still have no idea why they have to spec a cap that can't be used in an AC application at some frequency when I'm only using it at DC.

 

[dnyberg2]

I guess at a very basic level, size is much more important than anything. The volume I have for any solution is no bigger than the size of this part... https://www.digikey.com/products/en?keywords=478-11284-ND
That CAP has an ESR slightly better than the 3.3F I'm using https://www.digikey.com/products/en?keywords=478-11283 Is 85 mOhms the best I can buy?

 

[dnyberg2]

Are there any tricks I can play? Is it possible to charge a supercap faster by using higher current pulses rather than a steady low current DC my present CHG'ing CKT can muster? I could dump my tiny charge current into another smaller vessel until full (Faster) and use that to trigger some sort of higher current pulse using an inductive CKT or something like that... Just trying to think outside any box here...

 

[dnyberg2]

I wonder how much impact a few mOhms will make? Using this part: https://www.digikey.com/products/en?keywords=478-11283 I can drain that cap down to .9 volts from fully charged @ 3 volts in about 4 minutes @ 60mA and it takes about 24 minutes to charge it at 10mA... I guess I need to buy a bunch of caps and build a supercap tester machine.

 

[dnyberg2]

Seems like the lower voltage supercaps have half as much ESR but I wonder... Does cutting my ESR in half cut my charge and discharge time in half?

 

[CWatters]

The AVX supercap I am using right now has an ESR of 95 mOhm @ 1kHz. I still have no idea why they have to spec a cap that can't be used in an AC application at some frequency when I'm only using it at DC.

If you charge or discharge a capacitor the voltage changes, so it's not really "DC".

 

[dnyberg2]

Duh... See, I told you guys my sometimers is working! Seems the best I can do in the size I can stand is 1/2 of my present ESR, twice the farads but down to 2.7 volts from the 3 volt part I was using. Mouser instead of Digikey. https://www.mouser.com/productdetail/powerstor-eaton/hv0830-2r7605-r?qs=sGAEpiMZZMuDCPMZUZ%252bYl5pHcBUbMqmZ5zf%2F2mJtcQY%3D Almost $3 each in large QTY... I wonder at the end of the day, if this cap will charge faster and last longer at the same CHG & drain current?

 

[Vitro]

@dnyberg2, your question is fundamentally flawed. Using the bucket analogy again, if you have a 5 gallon bucket and a 1 GPM water source, you are asking if there exists any exotic high tech bucket that can be filled faster than 5 minutes. There answer is no, there cannot exist one even in principle regardless of how advanced and exotic the technology of the bucket, because the limiting factor is not the bucket but the water source.

Supercapacitors allow fast charging and discharging but only if you have a high power source or consumer. It sounds like you don't have either, so your application is very far from the limitations on (dis)charging rate imposed by ESR. A bigger problem of a supercap for your application is self-discharge. A small LiPo cell is much better suited for your needs, but you need an efficient charging circuit for it. Are you trying to make an RF energy harvester or some-such?

 

[dnyberg2]

Yes, it is an RF energy harvester or some-such...

 

[dnyberg2]

One of the prevailing target objectives is NOT to use any chemically active based storage device.

 

[CWatters]

How much current are you trying to take out of the capacitor? As far as I can tell your charging current is low enough that the ESR is irrelevant.

 

[dnyberg2]

No more than 60 mA or so... I think I understand your point about the ESR not not being an issue for charging but can you expound on that for me?

 

[CWatters]

95mR*60mA =60mV so if the cap is charged to 2V the output voltage on a 60mA load would be about 2V-60mV = 1.994V

 

[CWatters]

If your charger delivers 60mA at up to 3V the capacitor would charge to about 3V-6mV or 2.994V. close enough?

 

[dnyberg2]

Nice. The output gets jacked into an LDO of 1.5V right now but I'm considering a design that works down to .9V

 

[dnyberg2]

That's the rub. I can generate up to about six volts at 1 to 4 mA. Takes a long time to charge a supercap like that. I can charge a very large STD cap much quicker but it drains out of juice in seconds at 60 mA and that won't work for this remote sensor application.

 

[CWatters]

I calculated that your source should charge a 3.3F cap from 0V to 3V in about 160 seconds ignoring losses. In the charged state it would hold about 14 Joules of energy.

If your load works from 3V down to 0.9 V the capacitor could deliver 60mA for about 115 seconds.

Useful theory...
Q=CV
dQ/dt=CdV/dt
dQ/DT=I
So...
I=CdV/dt

You can rearrange that last equation to calculate a lot of things such as the charge or discharge time given the current, capacitance and allowed change in voltage.

 

[CWatters]

That's the rub. I can generate up to about six volts at 1 to 4 mA. Takes a long time to charge a supercap like that. I can charge a very large STD cap much quicker but it drains out of juice in seconds at 60 mA and that won't work for this remote sensor application.

Ah I used your earlier figures or perhaps missunderstand post #14.

 

[dnyberg2]

Then somethings wrong somewhere. With a load of 60mA I get about 4 minutes of power down to about 1.5 volts. With a charge current of 10mA, it takes 24 minutes to charge the cap back to 3 volts...

 

[dnyberg2]

I find it interesting that the low current charger I have can be six volts @ no load but the that supercap pulls that down until it gets up near full voltage. Then it seems that the "load" of the cap itself can hold the low current CHG CKT down around 3 volts. I was surprised to see that behavior but I've never really worked so closely with supercaps before.

 

[dnyberg2]

I'm going to to have to do a side by side experiment. CW DC at as much CHG current as I can get versus storing small amounts of it faster in a CKT that bursts a higher current pulse into the supercap...