For the Professional Maker: Supercapacitors Demystified, Part 1
August 28, 2019
Supercapacitors can store a lot more electrical energy than normal capacitors, but there are a few important differences, however, that might make them a little less fearsome for your next project.
Consider the capacitors that you may use in a system to filter out unwanted noise or store a small amount of charge for later use. These devices, in their simplest state, consist of a pair of separated parallel plates that hold positive and negative charges, though the reality tends to be a bit more intricate. These components are generally rated in farads, which is the capacitance that stores a one-coulomb charge (note that 1 ampere is 1 coulomb/second) with a potential difference between the positive and negative sides of one volt. Capacitors are also rated by breakdown voltage, which can vary between tens and hundreds of volts or more as needed. You’ll generally see ratings in microfarads (μF), or millionths of a farad, or even picofarads (pF), trillionths of a farad.
If you’re experienced with electronics, you hopefully have a healthy fear of these components, as they can hold a charge for some time—even when power is turned off—and can give you a jolt or worse when improperly handled. So then, supercapacitors, which can be commonly found in the multiple-farad range, would appear to be extremely dangerous, as this is orders of magnitude higher than the normal capacitors that you’re used to seeing. Weirdly though, they can still be physically quite small, often resembling the generally benign coin-cell batteries that you might power small electronics or even an LED light with. Perhaps they’re not as fearsome as you might initially expect.
The truth here is that, yes, supercapacitors can store a lot more electrical energy than normal capacitors, but there are a few important differences, however, that might make them a little less fearsome for your next project. In fact, you could argue that they’re not even really the same thing, but the term ‘supercapacitor’ probably sounded cool when it was first marketed by NEC in the 1970s.
First, while capacitors are simply separated charges, there is actually ionic activity going on in supercapacitors, allowing them to form what’s known as a Helmholtz double layer of positive and negative charges. The Helmholtz layer is extremely thin, and since capacitance is inversely proportional to this separation value, this gives supercaps their extremely high charge ratings. Conversely, supercaps are very slow to charge and discharge when compared to traditional capacitors, making them inappropriate for filtering applications, though they are still much faster than batteries.
The second, and most likely most important difference between the two is that their potential ratings tend to be in the single-digits. So, while a ‘real’ capacitor could in some cases be charged to hundreds of volts—easily shocking unprotected skin—supercapacitors normally are charged to single-digit voltages. The danger can be, however, that if you hook the two ends of a supercapacitor up without any sort of resistor, it can discharge a lot of current in a short time, potentially leading to burns or even a fire. Of course, shorted batteries can also be dangerous, so it’s a good rule of thumb not to apply power like this in general.
When thinking about supercaps, consider also a normal capacitor rated at 50V and 2200μF (.0022F). This would give a capacity (at 50V) to store .11 coulombs of charge. A supercapacitor rated at 1F and 5.5V can in turn store 5.5 coulombs of charge. That’s a big difference—the supercapacitor can store 50 times the charge as the normal ‘cap. However, if both capacitors could only be charged to 5.5V, the traditional capacitor could only charge 0.0121 coulombs, while the supercap could still store .11 coulombs. So, charge storage advantages for supercapacitors are exaggerated at low voltages.
In an upcoming post, I’m planning to further expound on supercapacitors, and about my recent experiment with using a supercap and LED as a sort of emergency light. While there are perhaps better ways to do the job, it’s a good “hello world” with the technology, a class of device that I think is perhaps widely misunderstood among many.
Jeremy S. Cook is a freelance tech journalist and engineering consultant with over 10 years of factory automation experience. An avid maker and experimenter, you can follow him on Twitter, or see his electromechanical exploits on the Jeremy S. Cook YouTube Channel!
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