SuperCapacitors growing in popularity

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I've been seeing a lot of stuff lately that features supercapacitors which is good because for a while it seemed like the technology was going to waste.
So if you're looking at cool power solutions for your robot here are the pros and cons of supercapacitors:

If you know what a supercapacitor is don't read this part. Since you're still reading I'll now explain what they are. Supercapacitors are capacitors that are made out of special materials which allow the storage of vastly larger amounts in energy while still functioning like capacitors. If you don't know how capacitors work then google it as you're already on the internet and not doing much else. So without further adieu,

Advantages:
- Easy to charge, simply apply any voltage less than the maximum and wait.
- No special charging equipment needed, just two wires.
- Near infinite number of recharge cycles, especially for hobbyist electronics
- Almost no limit to current so you can charge/discharge as fast as you're system can handle

Disadvantages:
- Large volume/watt when compared to lithium batteries
- Charging/discharging curves are a bit tricky to work with
- Relatively small amount of energy stored (0.1Ah if you have a couple big ones, don't even think about 1Ah)
- Impractical for anything over ~10V*
- The lack of a discharge limit means you can kill yourself if you're not careful.**
- Noticeable amount of self-discharge (do not misinterpret as "high amount of self-discharge")

So, where should you use supercapacitors?

You should use them anywhere where you need:
- Very fast charging
- Bursts of very high power longer than normal caps can deliver
- 5V or lower electronics systems. (again, there's a way around this ***)
- Power storage without complicated or expensive charging systems
- Solar power since you can charge a bank up completely in half an hour with a very small panel. (not if you're doing this method though ***)
- something that is ultra 1337 so you can be supercool

If you think this looks more like a vague tutorial than a commentary on how supercapacitors seem to be growing in popularity then you're right. I kinda got carried away while writing this but let me justify everything by saying that I wish for supercapacitors to become even more popular so I wrote this to inform in addition to comment.

* Each supercapacitor is 2.5V. If you want 5V you need two in series. Putting capacitors in series reduces the total storage capacity (notice the root word?) by half so now you need four in order to double the voltage. So every time you want to double the voltage (you will at least once) you have to square the number of capacitors (not counting 1^2 of course) which given their relatively large size can get out of hand very quickly. Couple all that with the fact that you need special circuitry to balance more than 3 capacitors you can see how high voltage is not where supercapacitors excel. There is a way around this however.***

** Just as a side note: the situation where these would be deadly is if you make a relatively high voltage system and accidentally shorted it with your body. Whereas a normal capacitor will give you a bad shock for a milisecond or whatever, supercaps will give you that same shock for almost half a minute depending on how big of a bank you make. This obviously is not good.

*** You can get around this by charging the capacitors in parallel and discharging them in series which will give you both double voltage and maximum capacity. This is a little trickier than just hooking them up but not very hard at all. Basically just modify a charge pump circuit as this is basically what they do already.

[madsci1016]

Woah, hold on there cowboy, we've got some issues.

Easy to charge, simply apply any voltage less than the maximum and wait.
Almost no limit to current so you can charge/discharge as fast as you're system can handle

Maybe you found some where this may be true, but in general for current commercial super cap technology, this is wrong.
Any cap will charge and discharge at the same rate. If you get a low ESR for faster discharge, you need to regulate the charging current. If you get a high ESR, it will be able to charge without current regulation, but won't be able to dump it's energy very quickly. 

An old engineer's (Soeren should appreciate this) trick is to use an incandescent bulb in series with the caps to regulate charge and discharge. As the circuit first starts charging, the bulb lights up bright, get's hot and increases resistance. As the voltage starts to equal, the bulb cools and loses resistance. During discharge, as long as the current stays low, the bulb won't light and stay very near 0 resistance, so very little power loss, unless your load starts drawing too much; then the bulb will light up and increase resistance.

The caps Sparkfun is selling will work for projects for powering watches for hours, not firing a rail gun in a split second.

You should use them anywhere where you need:
Bursts of very high power longer than normal caps can deliver

Remember, the equation for energy stored is 1/2 * CV^2. So really, if high energy is your goal, increasing voltage as apposed to capacity will increase energy exponentially more. A 1Farad cap at 2.5V is the same energy as a 4700uF at 35V, but the 4700uF has a higher discharge current limit due to much lower internal resistance and therefore more power dumping.

Super caps are good for short term battery backup, if used correctly. But you won't be building a rail gun with them any time soon.

[TrickyNekro]

Also last time I checked ( a.k.a. used) the got some self discharge issues....

I really don't think that supercapacitors is the way to go.... Theoretically they are good....
Practically.... They got many issues...

From manufacturing and pricing to usage....

[z.s.tar.gz]

@Madsci: I know what you're talking about with the discharge rate but I got some from mouser that melted quite a few high gauge wires while I was shorting them for storage. That's for the ones I have at least.

On that note let me say that all those "findings" aka "opinions with personal experience and moderate research" are all based around the supercaps I've used which are 2.5V 150F so-and-so brand from mouser.

still @madsci: compared to normal caps they are very slow and have very high current limits. my point was that compared to batteries they are very fast and have low limits. (or at least I hope it was, I wrote that a bit late  )

@tricky: yes they do have a noticable amount of self discharge (again the ones I had weren't too bad) but given the quick charge time I'd say the effect is lessened. I'll definitely add that to the list of cons.

Anyways, here's the bottom line: are supercapacitors right for your project? probably not. are they something you should seriously consider when starting a new project? definitely.
And again, solar power is really the best medium for these given the quick charge time and the fact you don't have to come up with 13V just to use them. (which as we know will make your robot the size of a small dog if you have a panel that big.)

edit: again @madsci: supercaps are horrible for railguns. that's what soup can sized capacitors are for. 

[madsci1016]

@Madsci: I know what you're talking about with the discharge rate but I got some from mouser that melted quite a few high gauge wires while I was shorting them for storage. That's for the ones I have at least.

Lol,. that doesn't mean you aren't tittering on the edge of destruction. 

still @madsci: compared to normal caps they are very slow and have very high current limits. my point was that compared to batteries they are very fast and have low limits. (or at least I hope it was, I wrote that a bit late  )

I looked, all the super caps i could find data sheets on had ESR values 10s to 100s times worse then standard metal caps, meaning they have lower current limits. What is the ESR for the Cap you have?

Anyways, here's the bottom line: are supercapacitors right for your project? probably not.

I wouldn't say that. When you want your microcontroller to be able to write some data to EEPROM during (after) poweroff, Super caps are perfect for the job. It all depends on what your project is. What super caps have is better capacity at lower voltages; but it comes with costs.

edit: again @madsci: supercaps are horrible for railguns. that's what soup can sized capacitors are for. 

Well, then don't say things like

So, where should you use supercapacitors?
You should use them anywhere where you need:
Bursts of very high power longer than normal caps can deliver

 

[z.s.tar.gz]

Lol,. that doesn't mean you aren't tittering on the edge of destruction

From what I can remember a few amps for a few seconds will definitely kill you given the voltage is decent (not 2.5V) so it is very reasonable to say that supercapacitors can deliver this magnitude of current and therefor can be considered to be very harmful if not deadly. they are not the most dangerous thing out there but are also a no-joke safety concern. Agree?

I looked, all the super caps i could find data sheets on had ESR values 10s to 100s times worse then standard metal caps, meaning they have lower current limits. What is the ESR for the Cap you have?

I don't remember off the top of my head but from a bit more research I've found some supercapacitors to have esr values similar to ni-cd batteries which isn't that bad but as you said is not even in the same order of magnitude as normal caps.

I wouldn't say that. When you want your microcontroller to be able to write some data to EEPROM during (after) poweroff, Super caps are perfect for the job. It all depends on what your project is. What super caps have is better capacity at lower voltages; but it comes with costs.

I forgot about that. for the sake of running a microcontroller you can't beat them as 20F will give you several weeks of run time on a charge. I'll add that to original post.

As far as railguns are concerned, you're looking at fast bursts of energy. for the sake of my former statement I meant "bursts of energy that are high but not as high as normal capacitors for long periods of time when compared to normal capacitors but not to batteries" but that is a mouthful which I think doesn't need saying but I guess I'll say it anyways.

[madsci1016]

Lol,. that doesn't mean you aren't tittering on the edge of destruction

From what I can remember a few amps for a few seconds will definitely kill you given the voltage is decent (not 2.5V) so it is very reasonable to say that supercapacitors can deliver this magnitude of current and therefor can be considered to be very harmful if not deadly. they are not the most dangerous thing out there but are also a no-joke safety concern. Agree?

I'm not talking about electrocution. If your caps have high ESR and you are just flat out shorting them, you risk them going explosive.

I meant "bursts of energy that are high but not as high as normal capacitors

Bursts of very high power longer than normal caps can deliver

You are getting your self all tongue tied. Let me help.

Super Caps give you small amounts of energy fairly easily, but cannot dump much power due to high ESR limiting current.

Regular caps give you smaller amounts of energy without going to high voltages, but can dump massive amounts of power quickly.

Batteries give you tons of energy, but less power capability then either type of caps.

Hopefully everyone knows the difference between energy and power.

[Soeren]

Hi,

[...] If your caps have high ESR and you are just flat out shorting them, you risk them going explosive.

So, the more resistance, the more explosive??  Interesting thought 

At the debate in general: Supercaps (bad name IMO) have their places, so does batteries and so does "regular" caps of the many existing varieties.
Problem is some may see them as a cure-all, which they're definitely not. If you need to dig a hole, a screwdriver is a horrible tool, so is a shovel when you need to turn a screw.

Hopefully, the Supercap won't get the same hype and then die screaming like the E-Cell (anyone remember that?)
I guess it won't though, as they have been in existance for numerous years - it's just the falling prices that now expose them to a larger audience.

[z.s.tar.gz]

also supercaps are more safe when it comes to overvoltage than normalcaps. I accidentally charged my bank at double the rated voltage and nothing happened other than expanding of the supercap (the same phenomenon that makes normalcaps explode)

this is not a rule or even a theory, it's just what I've seen.