# supercapacitors instead of battery?



## tomsko (Jul 6, 2014)

Hello! Newbie here 
Firs of all I'd like to say hello to all members here. I'm 20 y/o dude from Latvia thinking of doing my first electric drag car. yes, a drag racing car not a daily driver.

I have very low electrical knowledge so please dont flame me . I did a little research and was thinking... is it possible to make electric car based on supercapacitors instead of batteries? I mean, for a drag car this could really work i think. they probably wont do well for long journeys, but for a 10-20sec instant power run they should be fine. has anyone ever done this? Is it even possible? Also they should weight and cost waay less.


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## Hollie Maea (Dec 9, 2009)

Can a car be propelled short distances with supercaps? 
Yes.
Is it easy?
No.
Is it cheap?
No.
Is it the best solution?
Not even close.

If you want a small high power pack, LiPo is your friend.


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## tomsko (Jul 6, 2014)

Thanks for your fast reply!

Why is it more complicated? Please explain. Seems like you know a thing or two. Can you please list some pros and cons of doing caps?


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## Hollie Maea (Dec 9, 2009)

tomsko said:


> Thanks for your fast reply!
> 
> Why is it more complicated?
> 
> Please explain. Seems like you know a thing or two. Can you please list some pros and cons of doing caps?


The voltage of a capacitor is a direct function of its state of charge. That's slightly true for batteries as well, but their voltage curve is nearly flat for most of the state of charge--close enough that you can have a fairly simple controller (a standard DC Controller is basically a buck converter). With caps, you need a full fledged and complicated boot/buck converter so you can keep control your output voltage over the wide input voltage range.

Pros:

Well, there aren't very many. They have good power output. They have high cycle life. You can charge them fast. They are exotic, which makes them seem cool.

Cons:

VERY low energy density. Lower than you are imagining. Very expensive. The voltage thing I mentioned. It's hard to get all the energy out of them.


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## major (Apr 4, 2008)

tomsko said:


> ...... supercapacitors instead of batteries? I mean, for a drag car this could really work i think. they probably wont do well for long journeys, but for a 10-20sec instant power run they should be fine. has anyone ever done this? Is it even possible? Also they should weight and cost waay less.


Yes, it has been done. Worked fairly well. Search for BYU EV1 drag racer. http://www.altenergymag.com/articles/04.04.01/dragster/contents.html

You can also search this forum for ultracapacitor and find threads like this: http://www.diyelectriccar.com/forums/showthread.php?t=25994&highlight=candy


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## Ziggythewiz (May 16, 2010)

Keep in mind batteries have come a long way since BYU was building ultracap dragsters.


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## Duncan (Dec 8, 2008)

To add to Tesseracts calculations

1 watt hour of energy is 1 watt for one hour = 1 watt for 60x60 seconds = 3600 Joules (The correct unit for energy)

My Cells have 16Ah x 3.2v = 51 watt hours each = 184,000Joules - 184 KiloJoules 
or 0.184 MegaJoules

Tesseract was talking about 2.025 MegaJoules

2.025/0.184 = 11 

So 11 of my cells have the 2.025Mega Joules

Each cell cost $20 and weighs 0.5Kg
11 of these would be $220 and 5.5Kg

Compared to 
_The cost of the capacitors will be ~$6600 (plus shipping). Each one is 60.7mm in diameter, 138mm long and weighs 510g so the total pack will weigh at least 61.2kg and take up ~61L of volume._

$6600+ and 61.2Kg


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## major (Apr 4, 2008)

From Tess's post #7 (for some reason that post will not allow me to quote it  ): 



> Capacitor: E=*0.5*CV²
> Where E = Joules, C = Farads, V = Volts
> 
> Mouser- 2.7V 3000F
> ...


Tess appears to have dropped the 0.5 factor. So the actual number of caps would be double or 240. I thought that energy figure (2.25MJ) was high for 120 caps based on a system I built years ago. It was 156 caps of 2600F giving about one MJ usable energy from 200 to 400V. 










And yes Duncan, batteries have a much higher energy density. The pictured capacitor bank stores about the same energy as a normal car cranking battery. However the cap bank can be totally charged or discharged in like 10 seconds and do that for a cycle life of a million. Don't try that with a car battery


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## Tesseract (Sep 27, 2008)

major said:


> ...
> Tess appears to have dropped the 0.5 factor. So the actual number of caps would be double or 240. ...


I sure did...

[EDIT] and when I go to correct my post it comes up as blank... which is weird, and maybe explains why maj can't quote it. Hmm....

[EDIT #2] I just deleted my post.


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## Tesseract (Sep 27, 2008)

major said:


> From Tess's post #7 (for some reason that post will not allow me to quote it  ):


Yeah, something weird happened when I tried to edit my post... it came up blank. So I deleted it and am trying again.



major said:


> Tess appears to have dropped the 0.5 factor. So the actual number of caps would be double or 240.


I dropped the 0.5 multiplier from both the kinetic energy and capacitive energy equations, so the number of capacitors required ended up being correct, but the calculated value of energy was wrong. FWIW, I often drop the 0.5 multiplier when figuring out the charge balance between inductors and capacitors in power conversion circuits and that bad habit carried over into my previous (now deleted) post..

So to summarize the previous post, but with the correct multipliers in place, I postulated getting a 1000kg vehicle up to 45m/s (~100mph). The relevant equation is:

E = 0.5Mv², where E is in Joules, M is in kg, v is velocity in m/s

That would require 1.0125MJ (0.5 * 1000 * 45²) of energy (previously incorrectly reported as 2.025MJ).

The equation for energy stored in a capacitor is similar:

E = 0.5CV², where E is in Joules, C is in Farads, V is in volts

Since drawing energy from a capacitor results in the voltage across it dropping, and few (read: no) motor controllers can operate down to 0V, there needs to be an excess of capacitive energy storage. And since few motor controllers for EVs will operate on more than 350V, let's try to keep the voltage below that. As a first try, assume 300V for the maximum operating voltage at start of discharge, and approximately 100V for the end of discharge voltage.

Total energy is derived from the modified equation:

E = 0.5C(Vstart² - Vend²)

Rearrange to find C:

C = E/[0.5*(300² - 100²)]
C = ~25F

Perusing the Mouser site, we find a Maxwell supercap rated for 2.7V and 3000F. If we wire 120 of those in series we get a 300V/25F capacitor (ie - 2.5V each) which is pretty close to what we need. Each of those caps costs ~$55 ea. in quantities of 100+ for a total cost of around $6600. Each cap weighs 510g and takes up a rectangular volume of ~0.51L so a bank of 120 will weigh ~61kg and take up ~61L of volume. 

Personally, this doesn't make a lick of sense to me... nor to too many other people.


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## major (Apr 4, 2008)

O.K. Tess. I'll buy that story 

You notice for that cap bank of mine the usable energy is quoted from 200 to 400 Volts. That is within the range of the inverter used. Energy was matched to a commercial vehicle at 35mph, like a UPS or FedEx delivery truck. So for a parallel hybrid, the caps provided excellent energy recovery (regen) and launch assist. Duty cycle was up to several hundred cycles per day. Some of these trucks are used for 26 years. The ultracaps projected cycle life could handle that whereas batteries could not without being grossly oversized.

And back to the OP intent concerning drag racing with UCs. I've often thought of pushing these caps past the upper voltage spec. You see racers do that with motors. Why not caps? Take these same caps and run them to 4 or 5V, maybe higher  That'd push your energy density up a bit.


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## Karter2 (Nov 17, 2011)

major said:


> . However the cap bank can be totally charged or discharged in like 10 seconds......


 Maybe,..but you would need a pretty expensive power source/charger. !


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## major (Apr 4, 2008)

Karter2 said:


> Maybe,..but you would need a pretty expensive power source/charger. !


Just use the motor and the kinetic energy of the vehicle.


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## Karter2 (Nov 17, 2011)

major said:


> Just use the motor and the kinetic energy of the vehicle.


 Yes... but you need a motor and regen controller capable of those power levels.
A bit af an upgrade on your average DC drive EV ?


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## major (Apr 4, 2008)

Karter2 said:


> Yes... but you need a motor and regen controller capable of those power levels.
> A bit af an upgrade on your average DC drive EV ?


I never said or implied it was intended for your average EV  



major said:


> You notice for that cap bank of mine the usable energy is quoted from 200 to 400 Volts. That is within the range of the inverter used. Energy was matched to a commercial vehicle at 35mph, like a UPS or FedEx delivery truck. So for a parallel hybrid, the caps provided excellent energy recovery (regen) and launch assist.


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## Karter2 (Nov 17, 2011)

Yes, i am with you Major, it works well for the F1 and LM racers also (Toyota are using Caps in their LM racers)



Duncan said:


> My Cells have 16Ah x 3.2v = 51 watt hours each = 184,000Joules - 184 KiloJoules
> or 0.184 MegaJoules
> 
> Tesseract was talking about 2.025 MegaJoules
> ...


 Yes 11 of those cells would store the same energy....
BUT.. 11 of your cells can only supply 35 volts max and possibly 160 amps ..(10C ?)
..IE around 6.0 kW of POWER ..
Whilst the same energy from the capacitor bank could be released @ 300volts and 600+ amps, as 200kW .....for a few seconds !


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## PStechPaul (May 1, 2012)

It thought you could get used ultracapacitors for much less than the Mouser catalog price, but what I found on eBay was about $5000-$7000/MJ. They definitely have much higher power density than even LiPo, and virtually unlimited life (unlike the racers and their cars), so they may be ideal for drag racing. 

Using my EV calculator, a 1000 kg vehicle accelerating at a constant 1G (22 MPH/sec) and 100 MPH top speed (4.5 seconds) uses a peak power of 618 HP and has a terminal inertia of just about 1 MJ. But I'm not sure how this compares to drag racing figures. About the best I found for a stock car is 7.47 for 1/4 mile with 179 MPH top speed. This is 23.8 MPH/sec, or just over 1G. For this, I calculate energy for a 1000 kg vehicle as 3.2 MJ.


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## Karter2 (Nov 17, 2011)

PStechPaul said:


> Using my EV calculator, a 1000 kg vehicle accelerating at a constant 1G (22 MPH/sec) and 100 MPH top speed (4.5 seconds) uses a peak power of 618 HP and has a terminal inertia of just about 1 MJ. But I'm not sure how this compares to drag racing figures. About the best I found for a stock car is 7.47 for 1/4 mile with 179 MPH top speed. This is 23.8 MPH/sec, or just over 1G. For this, I calculate energy for a 1000 kg vehicle as 3.2 MJ.


 Hmm ?..
I seem to recall that most of the top EV drag cars are trying to feed 2000 amps to each of 2 Warp motors at 400 volts.
Ref.. Warp Factor 3, BlackCurrent, SW 37, and John Metrics Assault and Battery, etc..
So that would be 1600kW..... if they can find a pack to do the job.
Which.. ( if my maths and physics is correct )... would be about 16MJ of energy for a 10 sec run !
That is one hell of a lot more caps than your estimate !
..8 times as many as those previous 61kg packs ... 490 kg !! 

That same POWER (1600kW) can be delivered by less than 200kg of top spec LiPo cells !


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## PStechPaul (May 1, 2012)

The power needed for a constant acceleration increases according to the square of velocity, so the 1600 HP is only at the end of the ten second run, assuming it is still accelerating at the same rate. The energy required depends on the mass of the vehicle and the top speed. If the acceleration is constant, the average speed will be at the halfway point, where the energy will be 1/4 that at the end. So I think the total energy will be about half of your figure, or 8 MJ. And my 3.2 MJ figure was for a 1000 kg vehicle on a 7.5 second run. At 10 seconds the velocity would be 33% higher and the energy thus 78% greater, or 5.7 MJ. 

Also, my figures are just for the actual kinetic energy of the vehicle, while the energy expended depends on the efficiency of the motor, drivetrain, and other components. When running a series wound motor balls-to-the-wall for 10 second survival before burnout, the efficiency is likely less than 50%, and maybe even 20%.


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## Karter2 (Nov 17, 2011)

Is it constant acceleration ??
My understanding is the initial accell is much greater and it tails off as the speed increases.

I seem to recall some of the data logs from Warp lll , showed the motors pulling the full 2000 amps right from the lights, and the battery voltage only dropping due to sag.
But yes, it's only an estimate though it does suggest that the LiPo is still a better option when you need big power......
.....unless there are ultra caps with much better energy density these days ?


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## Tesseract (Sep 27, 2008)

major said:


> ...
> And back to the OP intent concerning drag racing with UCs. I've often thought of pushing these caps past the upper voltage spec. You see racers do that with motors. Why not caps? Take these same caps and run them to 4 or 5V, maybe higher  That'd push your energy density up a bit.


Attempting to charge a UC to a voltage higher than ~2.7V will result in a rapid increase in leakage current and dissociation of the solvent(s) and/or the ionic electrolyte. Hijinks ensue.


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## major (Apr 4, 2008)

Tesseract said:


> Attempting to charge a UC to a voltage higher than ~2.7V will result in a rapid increase in leakage current and dissociation of the solvent(s) and/or the ionic electrolyte. Hijinks ensue.


You could get in a few passes  Is it 2 or 200? And what is the failure mode? You can find out in the lab with a few cells. A drag racer doesn't need a million cycles.


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## PStechPaul (May 1, 2012)

I think the risk of catastrophic failure from overcharging the UCs would outweigh the additional energy density that might be obtained. If one of the series string of capacitors failed, unless it becomes a solid short, it may seriously compromise the current capacity of the pack. 

The optimum conditions for drag racing should be maximum torque, which would be delivered just under the point where the tires slip. At start-up, the power at the wheels is zero, because there is no rotation, and the power delivered to the motor would be essentially just enough to overcome the losses at the point of maximum locked rotor torque. Toward the end of a 10 second run, the motor will probably have much higher losses due to increased resistance of the copper windings as well as damage to the commutator and brushes. Also, the available voltage of the mostly depleted battery pack will be lower when it is needed most, and the top speed of the motor may be limited as well.


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## major (Apr 4, 2008)

So it is acceptable to run a 10hp motor at 300kW but you don't approve of running a 2.7V capacitor at 4V


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## Tesseract (Sep 27, 2008)

major said:


> You could get in a few passes  Is it 2 or 200? And what is the failure mode? You can find out in the lab with a few cells. A drag racer doesn't need a million cycles.


My understanding is that UCs fail in a similar fashion to lithium-ion cells when overcharged. That is, the solvent/electrolyte starts to dissociate with the evolution of large volumes of gas. 

I can tell you that large cylindrical Maxwell UCs start losing charging efficiency around 2.85V and show serious inefficiency at 3.10V, as determined by timing how long it takes to achieve a certain dV at a constant charge or discharge current.

For example, a 2000F Maxwell Boostcap I have on hand will take 20s to change 20mV at 2A of charge or discharge current below 2.70V, exactly as the capacitor equation predicts (ie - dt = [C*dV]/I). At 2.830V ithe charging time increases to 24 seconds while the discharging time decreases to 18s (all other parameters the same). At 3.100V the charging time increases again to 28s while the discharging time plummets to 13s; in other words, well over half the charge delivered to the UC was not stored by it.

Furthermore, the self-discharge rate above 3.00V is quite high, declining by about 1mV every 8s, whereas below 2.70V it takes *several minutes* for terminal voltage to decline by 1mV.




PStechPaul said:


> ...The optimum conditions for drag racing should be maximum torque, which would be delivered just under the point where the tires slip....


This is a common misconception; maximum traction occurs when there is some slip, usually in the range of 10-15%.


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## PStechPaul (May 1, 2012)

A motor has a rather predictable and linear (or logarithmic) characteristic for overload of voltage and current, speed and torque, and power. There may be a point where there can be a sudden catastrophic failure, such as where the voltage exceeds the insulation breakdown capacity, but that is probably rarely reached, especially on low voltage motors which are wound with the same type magnet wire used in 600V class machines.

I don't really know what the failure mechanism is for UCs, but it is likely to be a rather sudden and dramatic breakdown of the very thin dielectric that allows them to have such high capacitance. 

Here is some information:
http://batteryuniversity.com/learn/article/whats_the_role_of_the_supercapacitor
http://www.ultracapacitors.org/index.php?option=com_content&Itemid=68&id=123&task=view
http://www.tecategroup.com/app_notes/0_Tecate_Group_What%20is%20an%20ultracapacitor.pdf
http://www.tecategroup.com/ultracapacitors-supercapacitors/ultracapacitor-FAQ.php
http://gigaom.com/2011/07/12/how-ultracapacitors-work-and-why-they-fall-short/
http://web.ing.puc.cl/~power/paperspdf/dixon/66a.pdf

The last link examines a fire in an EV caused by failure of one or more UCs but I have not found anything definitive about the effects of overvoltage except that the life is reduced. So I think perhaps the 2.7V could be extended to 3.5V or so, which is a doubling of energy. If the lifetime is still reasonable, then perhaps UCs could directly replace LiFePO4 cells which have just about the same voltage characteristics and the same charger and BMS might be used with little or no modification.

[edit] Tesseract posted while I wrote this, and provided valuable information that refutes my offering the possibility of a 2x doubling of capacity at 3.5V, and probably shows that 4 or 5 volts would be quite destructive.

I did not know that optimum thrust can be achieved at 10-15% slip, but that may be due to uneven texture of the track surface and/or heating of the tires caused by friction and the resulting increased coefficient of friction. But certainly there is much more torque/thrust available below the slip point than well above it as seems to be the case in drag races I've seen in videos. The squealing tires and smoke are probably more crowd-pleasers than an effective strategy to win the race. Just as in golf, drive for show, putt for dough!


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## Karter2 (Nov 17, 2011)

Over volting or not, I still cannot see a logical reason to choose UC,s over LiPo for competitive drag racing.
UC,s may technically be feasible , but they still seem to have a huge weight penalty at those power levels.
...or am I missing something ?

They are obviously a serious option for circuit racing, where multiple high regen levels are needed and cycle life comes into play,
.....but therein again they are up against other technologies. ..IE...."flygen"..?


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## nimblemotors (Oct 1, 2010)

Did someone say LiPO has 97% efficiency? I'd like to see that data please.

Try discharging and charging a Lithium battery at 100C over and over at see how long you can do that until they are destroyed or catch on fire.

Unlike drag racing, a road racer can go from 100 MPH to 10 MPH in seconds,
and then right back to 100 MPH. Without regen all that energy is used to destroy the cars brakes.


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## Karter2 (Nov 17, 2011)

Drag racing or circuit racing, efficiency is rarely high on the priority list.
(..though fuel efficiency is a major factor in F1 and endurance racing.)
But the F1 teams are certainly using LiPo in their ERS systems at very high regen and discharge power levels.
Of course they won't worry about efficiency or life cycles as long as the pack lasts 2-300 cycles.


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## PStechPaul (May 1, 2012)

Impressive video of the glowing brake disks! But it might take something like 10,000 watts to get them that hot in 5 seconds, and that is just 50,000 J or 14 Wh. That is about 0.1% of a typical battery pack. Regen has its place, but braking like this rarely occurs in ordinary driving, so batteries can usually accept the energy under typical braking and hill descending conditions.


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## Karter2 (Nov 17, 2011)

> it might take something like 10,000 watts to get them that hot in 5 seconds, and that is just 50,000 J ..


 I dont know if that is a calculation, but it occurs to me that if you need 500+ kW to accelerate from 10 to 100mph in 5 sec.. ( a guess !) , then you would have to dissipate a similar ammount to stop again in the same time frame ?
Also, the only actual data i know of on this is from the F1 ERS MGUK where they actually recover 2MJ per lap ( regen in effect),..but that is limited by regulation..not capability.
Without regulation , it is believed they could possibly recover double that from a relatively light vehicle braking for only a few seconds ( <10) per lap
Many of the teams still use Lipo to adsorb that energy.


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## PStechPaul (May 1, 2012)

I think you may be right. I'm more than an order of magnitude off.  My own calculations showed that the vihicle would have about 1 MJ of energy, and thus the power is more like 200 kW. So under such extreme conditions, with frequent stops, 1 MJ (277 Wh) is much more significant, like 5% of the pack energy.


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## major (Apr 4, 2008)

Tesseract said:


> Attempting to charge a UC to a voltage higher than ~2.7V will result in a rapid increase in leakage current and dissociation of the solvent(s) and/or the ionic electrolyte. Hijinks ensue.


Ya know Tess? I couldn't resist. So I took one and progressively overcharged it up to 3.3V. I didn't even get killed, not even once 

It was a Maxwell BCAP0010, 2600F, 2.5 Volt cell. I discharged into a fixed 0.5 Ω resistor until ~0.15V. Voltage measured at cap's terminals with a Fluke multimeter. Energy (Wh) and charge (Ah) measured with a $20 hobby meter.

Discharge from 2.499V measured 2.3Wh and 1.817Ah. Calculated energy is 2.26Wh.

Discharge from 3.298V measured 4.5Wh and 2.703Ah. Calculated energy is 3.93Wh. I repeated the test and got the same results 

Like I said, nothing bad happened. And I was able to get nearly twice the energy. 

Disclaimer: I do NOT recommend anybody use a product like this beyond the manufacturer's specifications. Don't try this at home. I am a professional and took the appropriate safety precautions.


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## PStechPaul (May 1, 2012)

I searched again and could not find any definitive answer about what may happen as a supercapacitor is charged to a voltage greater than its rating. I would expect that there is some point at which the insulation breaks down and dissipates rather than stores energy, but I don't know if this happens as a spectacular short circuit that creates a violent explosion, or if it simply causes localized heating and perhaps vaporization of the metallic coating, which would be more of a "soft" failure that would cause loss of capacitance. One would think there would be somebody on YouTube who would do a destructive test on these, and perhaps the lack of same indicates that the failures may be relatively non-eventful. There must be some point where a low impedance voltage source applied to such a capacitor might be destructive, but that also depends on the components of the capacitor and whether or not it contains water (and thus could produce high steam pressure), or other substances that could enter into an exothermic reaction that could quickly release a lot of stored energy. 

I'd be tempted to try a similar experiment myself, with adequate blast protection, while running a camcorder (also protected) to see what happens. My feeling is that a limited current applied over time would eventually stabilize at some voltage where additional current would just be dissipated with some temperature rise, so something like 10 amps would create at most, say, 5 volts at 10 amps or 50 watts, which would cause significant heat but probably not a violent explosion.


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## piotrsko (Dec 9, 2007)

In my experience with electrolytic's that they arc around the ends of the sheets of insulation inside when overvolted. Had one or two that arced to the can, some overheat and pop like a firecracker. Never used one for high current cycles. I don't know how super caps are made, but based on the generally low working volts, suspect traditional methods.

Ever notice when working voltage goes up, capacity goes down?

Dissipate 50 watts from a relatively small footprint and it should get really hot really fast.


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## Tesseract (Sep 27, 2008)

major said:


> Ya know Tess? I couldn't resist. So I took one and progressively overcharged it up to 3.3V. I didn't even get killed, not even once
> ...
> Disclaimer: I do NOT recommend anybody use a product like this beyond the manufacturer's specifications. Don't try this at home. I am a professional and took the appropriate safety precautions.


You're kinda sending a mixed message here, maj... 

FWIW, I did my overcharge test at a rather low rate of current to better tease out any increase in leakage (ie - from uA to mA). You didn't specify the charge rate but if it was above a few 10's of mA you might have had a hard time seeing any evidence of charging inefficiency.

That said, you have presented one datum point in refutation to my one datum point; not exactly a lot of data, in other words.


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## major (Apr 4, 2008)

Tesseract said:


> You're kinda sending a mixed message here, maj...


Yeah, but who takes my advice anyway 

I charged the cap at about 10A and tapered it down near the top end.


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