Decision factors and tradeoffs in supercapacitor selection -

Decision factors and tradeoffs in supercapacitor selection

Supercapacitors have emerged as a popular solution for those situations in which high-density back-up power is required, along with high cycle life and fast charge and discharge times.

However, selecting the correct type of device for the task at hand can appear rather complex, even for the most experienced engineer. It requires an understanding of a broad range of technical performance characteristics, along with some trade-offs that need to be considered along the way.

Before we look in further detail at how to choose the right supercapacitor for your application, it is worth setting out the fundamentals of how such devices work, as well as some of the benefits they deliver over and above other technologies such as coin cell batteries. This information can provide some valuable insight into the sorts of roles for which supercapacitors are particularly well suited.

Supercapacitor Fundamentals

Firstly, in terms of how they work, many supercapacitors use what is known as the Electric Double-Layer Capacitor (EDLC) layout featuring two electrodes that are often coated in a carbon-based porous material and separated by an electrolyte that is itself divided by a membrane (Figure 1).While batteries rely on a chemical reaction, the supercapacitor is different in that it stores and releases energy very rapidly through a process of physical adsorption and desorption of ions in the electrolyte contained between its electrodes.

Figure 1: The internal workings of a supercapacitor

These processes are much quicker than the chemical reactions that would be found in battery charging. With supercapacitors having a low internal resistance, the device can be fully charged within a few seconds, whereas a lithium coin cell used in a secondary battery application could take from  ten minutes to several hours to be fully charged due to far higher resistance. Also, there is no theoretical limit to cycle life, whereas a lithium-ion secondary cell has a finite lifetime of approximately 500 cycles.

Recent advancements in carbon-based materials mean porous electrodes can be designed to have a large surface area which results in a high capacitance value and small external dimensions. Supercapacitor construction using aqueous electrolytes are inherently conductive, come with a low environmental impact and offer non-flammable characteristics, and this yields excellent performance and strong safety credentials.

Typically speaking, they also have greater resistance to moisture absorption than organic compounds, resulting in a longer life with better stability (Figure 2). This also means supercapacitors are essentially maintenance-free, whereas lithium coin cells need replacing, with regularity depending on the specific application.

Figure 2: High-Reliability Design: Cell Construction Differences between aqueous and organic-based electrolytes

In terms of energy density, supercapacitors are usually rated at 0.5 to 5 Wh/kg, compared to 30 to 270 Wh/kg for a lithium coin cell. But supercapacitors have a much higher power density, enabling them to deliver large amounts of energy in a very short time.Supercapacitors offer greater flexibility in terms of operating temperature range โ€“ typically performing between -40 and +85 deg-C, compared to narrower parameters of -20 to +60 deg-C for the lithium coin cell.

These kinds of performance characteristics mean supercapacitors are finding increasing use-cases. These include backup power duties in a broad range of equipment ranging from Internet of Things-based devices, smart utility meters, or medical equipment, to automotive electronics and industrial computing for advanced automation.

Typical applications include taking over the systemโ€™s real-time clock or volatile memory when the main system power is cut off, such as during a power outage or when the main system battery has been removed for replacement.

How to choose the right supercapacitor

So, those are the fundamentals of supercapacitors and some of the roles they perform. But how do you go about selecting the right device for the required application?

Figure 3 represents a good starting point, as it illustrates at a high level some of the initial considerations to be made. For instance, if the final application dictates a need for a higher backup time, then a high impedance solution from KEMETโ€™s FG, FY, FC, FM, and FR series would represent the best place to start.

Shorter backup time requires low impedance and that would mean a different set of solutions, primarily from the FA, FE, FS, FT, and FM family of products. Alternatively, if the key application requirement was high power, then a very specific range such as HV Series would provide the answer. These are important considerations from the outset, though, before additional thought is applied.

Figure 3: Performance for selection

In addition to backup time, a more comprehensive list of parameters that need to be defined before choosing a supercapacitor could also include required minimum and maximum operating voltage; operating temperature; required dimensions; and mounting type (surface or through-hole). Then, with these details to hand, a relatively simple formula can be applied to enable the rough calculation of the capacitance needed for the task at hand.

As an example of the calculation process, in the project example below the customer needs a supercapacitor that will be able to withstand 150 hours of backup time under the following conditions:

โžข ๐‘ฝ๐’Ž๐’Š๐’ (minimum operating voltage) =2.5 V
โžข ๐‘ฝ๐’Ž๐’‚๐’™ (maximum operating voltage) = 5.5 V
โžข ๐‘ฐ๐’ƒ๐’‚๐’„๐’Œ๐’–๐’‘ (required back up current) = 540๐‘›๐ด
โžข requested back up time T > 150 hours,
โžข 85ยฐC degrees ambient temperature + optional cooling system (-15ยฐC)

The equation for the requested capacitance is given by:

From this rough calculation, you would need a supercapacitor with a capacitance of around 0.1F. However, there are additional parameters that would need to be considered. These include a full range of parasitic effects such as R series โ€“ DC resistance and ESR; self-discharge characteristics; leakage current and ambient temperature. In each case, KEMET can help design engineers calculate the impact of these parasitic effects to establish more accurate product selection.

Furthermore, it is important to remember the โ€˜life estimationโ€™ of the supercapacitor (Figure 4). End of life is defined as the point at which capacitance is reduced to 70% of the initial value. Typically, the lifetime of supercapacitors at an ambient temperature of 25-degree C would be ten years. However, high-temperature load life tests show that this is reduced by half with an increase of 10-degree C.

So, in many cases, if the supercapacitor is needed for a high-temperature application, it needs to be cooled to reduce the ambient temperature to increase the life expectancy. An informed judgment call needs to be made here, though: for very high-temperature use-cases, other technologies such as lithium battery cells might provide a better answer.

Figure 4: Lifetime of supercapacitors

Flexible solution for all high-power back-up needs

In conclusion, choosing the right supercapacitor for the right application is best served through a structured process that results in the identification of the right product. KEMET offers a broad range of solutions that can meet the requirements of most use-cases. The product range is being extended all the time, with the latest ranges compliant to AEC-Q200 for high-reliability automotive applications.

Ultimately, the benefits of supercapacitors are now well-proven and well-understood. They have no limits for charge/discharge cycles, with quick charge and discharge possible since no chemical charge is involved. They have an open failure mode and simple installation โ€“ with SMD and automatic mounting series available. Supercapacitors also have a wide operating temperature range, and high reliability in humid environments and they are maintenance-free.

In short, these performance characteristics mean that supercapacitors can provide the perfect solution for any application that needs a reliable high-power density back-up solution.

Please visit the ebook for the complete article.

Roya Nikjoo is Field Application Engineer at KEMET Corp.

>> This article was originally published on our sister site, Power Electronics News.

Related Contents:

For more Embedded, subscribe to Embedded’s weekly email newsletter.

Leave a Reply

This site uses Akismet to reduce spam. Learn how your comment data is processed.