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Modelling of Time Constants for Carbon Based Supercapacitors

howroyd

Shameless plug to my recently published paper!  It's open source so you should be able to view it.

 

Key findings:

Supercapacitors do not behave like other capacitors (obvious if you've used one).  Mainly in that their charge profile is not linear due to an internal distribution of time constants.  This means that you can charge a SC up to 1V, say, for 1s, then go open circuit and the voltage will seemingly drop, or sag.  If you do the same test but hold the 1V charge for 2s the sag will be less.

Voltage sag

 

Conversely, and more importantly from a safety perspective, you can short out a SC, i.e. discharge it to 0V, leave it open circuit for a minute, and the voltage will have rebounded positively.  So that SC you thought was "flat" has seemingly recharged itself!  If you have a decent bank of these that could give you "a wee tingle" to quote Big Clive!  Interesting since there is no electrochemistry in a SC like there is in a battery.

Voltage rebound

 

What does this mean for us though?  Well as a researcher I am obviously very low in the TRLs so bear that in mind, but we actually know very little about how these devices actually work.  Modelling simulations that you may get in software may not be representative of real life (e.g. Matlab, LTSpice, etc).

Also, do not assume that the charge characteristics are linear, in that if your 5V SC is at 2V5 it is at 50% SoC.  It might be if you have held it at 2V5 for a long time (mins, hours, days, we don't know?) but certainly not if it has only just reached this voltage, unlike, say, a ceramic or electrolytic.

 

Food for thought, hope you find this an interesting insight to some of the work I am doing here at Loughborough University!

  • Searching the Internet for 'inherent time constant' (in electrolytic power caps) turns out to be a rather frustrating exercise - until one stumbles upon this post and reads the

    corresponding  paper that you co-authored. I hope you don't mind me bothering you with the subject more than three years later ?

    First of all I am curious if there has been follow-up research, at your university or elsewhere ?

    Furthermore I wonder, if similar phenomena and/or consequences have been established or could be predicted for parallel and/or series-connected electrolytic power caps ?

    (From a materials science point of view, high-tech electrolytic power caps could be considered the closest thing to supercaps i.m.o.).

    Looking forward to your reply, Ray

  • in reply to shabaz

    Hi Shabaz

     

    To an extent yes you are correct, however because the numbers are so small in normal caps it is less noticeable and less of a concern I guess.  However, we think the mechanism is different.  Getting down into the science, and as you suggest, my understanding is that dielectric absorption relates to the orientation of the molecules and their dipoles.  This may exist in SC as well (I will check with our resident chemist) and get back to you.  The much more dominant mechanism in these SC is more to do with to "viscosity of the ions travelling through pores in the activated carbon.  I suppose the observations by the end user is similar, but the science is completely different.

     

    I should add that it is still very early days here so we are uncovering new things almost daily, and our publications are months behind our knowledge, however, I am being careful not to release this knowledge until it is published for our own protection.  Sucks because I want to tell the world everything right now, it is so exciting!

     

    I can give you one nugget to wet the appetite; we've observed a gaussianish distribution of time constants. Wouldn't it be nice if we could design the materials and the ionic liquid such that we could meet a specification as to what this distribution looks like; e.g. more fast states, wider or narrower....  To do this we need to know what is happening at the molecular level to know what to change or aim for. ;-)

     

    Cheers

     

    Si

  • Hi Simon,

     

    Interesting paper, will check it out. I had a question, don't all caps have the bounce-back effect you mention? (at least plastic caps as I understand, i.e. dielectric absorption). Maybe the underlying phenomenon is different in super-caps however, I don't really know enough about them.