Group Actions
Engagement
  • Author Author: Gough Lui
  • Date Created: Date Created
  • Views 2219 views
  • Likes 14 likes
  • Comments 15 comments
Related
Recommended

What’s Super about Supercapacitors? – Part 2: Types, Vendors, Safety & Specifications

Gough Lui
What’s Super about Supercapacitors? – Part 2: Types, Vendors, Safety & Specifications

It’s the first week since I’ve received my supercapacitors, which means it’s time for my first “bonus points” blog. This one will explain how supercapacitors work, the types of supercapacitors on the market, the vendors which sell radial/coin-cell type supercapacitors, delve into some of the safety aspects, collate the key specifications of the supercapacitors we have been sent and do some quick measurements of weight and voltage as received.

Table of Contents

In Brief: What’s a Supercapacitor? What are the Types of Supercapacitor?

Supercapacitors, sometimes also known as an ultracapacitor, are capacitors with much higher capacitance than ordinary capacitors but with a lower voltage capability. They are more energy dense than ordinary capacitors, but much less so than batteries, occupying a space in-between when it comes to energy and power density. They have a much higher cycle life than batteries, although their operational lifetime and temperature capabilities mostly fall behind that of ordinary capacitors.

Such components see applications in memory back-up, power-loss protection in computing, short-term ride-through for UPSes, burst-power requirements for car audio, medical devices, electric vehicles and even wind-turbines and kinetic energy recovery systems. Their use is primarily based on the need for rapid charge/discharge and frequent cycling, whilst maintaining adequate energy density (both in terms of weight and volume) and cost.

There are a number of types of supercapacitor, the most traditional being the Electric Double-Layer Capacitor (EDLC). These use activated carbon electrodes with electrostatic storage of charge in a double-layer between the electrode and electrolyte. Such an arrangement improves energy density above that of ordinary capacitors mainly due to the reduced charge separation distance and increased surface area of the activated carbon. A key downside of this, however, is that the structure of the capacitor acts like an array of parallel connected capacitors with different series resistances – to fully charge (soak) the capacitor takes a long time.

image

Above diagram from the Cornell Dubilier Electronics Supercapacitor Technical Guide

The next most common type, and a more recent development, is the Lithium-Ion Capacitor (LIC) which is a type of hybrid capacitor. It draws upon the structure of the EDLC, but replaces the anode with that found in a lithium-ion battery. As a result, it takes some of the characteristics of a lithium-ion battery (e.g. higher voltage, lower self discharge, higher energy density) and combines them with the characteristics of EDLCs (e.g. high cycle life, lower ESR than batteries, improved safety).

image

Above diagram from Karimi et.al. (2022).

Both EDLCs and LICs have been supplied in the kit, however, there is also the pseudocapacitor which is much more like a battery by storing charge in chemical reactions, but is rarely encountered.

For a more comprehensive review on the technology, I’d highly recommend Karimi et.al. (2022) A Comprehensive Review of Lithium-Ion Capacitor Technology: Theory, Development, Modeling, Thermal Management Systems, and Applications, MDPI Molecules (open-access). It’s quite a recent paper and it goes into quite some detail about the various technologies involved.

There is the Cornell Dubilier Supercapacitor Technical Guide. I have read this and while it is a good primer, it is not that detailed nor comprehensive and there seem to be a few minor omissions or mistakes (e.g. in the ESR DC and balancing sections for example). There are also quite a few inconsistencies between the charge rates and voltage thresholds in the procedures and diagrams, although I suspect this is a reflection that there are multiple ways to achieve the tested parameters, each with their own advantages and disadvantages. There is not much on lithium-ion supercapacitors here either.

Perusing the various electronic component distributor sites including element14 and Newark, it is possible to find supercapacitors ranging from 10mF to 6000F with products being single components for integration into mainstream or portable electronics through to large modules intended for industrial and transportation use. Voltage ratings ranged from 1.4V to 850V where the higher voltages are formed by modules with balanced series-connection of multiple supercapacitors. Typically EDLC have 2.7V or 3.0V nominal voltages and LIC have 2.8V nominal. Lifetimes range from as low as 500h or 1000h at 50°C up to a high of about 6000h at 85°C which is still less than some of the best wet electrolytic capacitors of today, possibly due to issues with “wetting” and “leakage” of internal electrolytes which seem particularly aggressive and are an industry-wide issue according to VINATech. The ESR ratings range from 200µΩ up to 300Ω depending on the product.

My literature review had thus spread to competitors and their documentation in order to obtain answers.  I’ve found that Kyocera AVX and Maxwell Technologies datasheets, seem to have quite good information at the end with regards to protocols. Samxon’s catalogue from 2016 gets a special mention for also being quite similarly helpful as CDE’s Technical Guide but also for bringing to my attention this tidbit –

image

I had no idea that the devices aren’t polarised until they receive their initial charge, but I suppose it makes sense given the diagram of the electrodes and separators are symmetrical. Honourable mention to TDK for their page on EDLCs that mixes a bit of technical details with applications, while Elna has a page directly comparing EDLCs with ordinary capacitors.

Who Makes Supercapacitors?

Assuming one is interested in using supercapacitors and incorporating them into a design, it would be good to know who makes supercapacitors. After spending a day scouring electronics distributors, I found the following companies to make or have such products in their catalogue:

  • EDLC
    • Abracon
    • CDA (Zhifengwei Technology)
    • Cornell Dubilier
    • Eaton
    • Kemet
    • Kyocera AVX
    • Lelon Electronics
    • LICAP Technologies
    • Maxwell Technologies
    • NIC Components
    • Nichicon
    • Nippon Chemi-Con
    • Ohmite
    • Rubycon
    • Samxon
    • Skeleton Technologies
    • Taiyo Yuden
    • Tecate Group
    • VINATech
    • Vishay
    • Würth Elektronik
    • Yongmin Electronic (Ymin)
  • LIC
    • CDA (Zhifengwei Technology)
    • Cornell Dubilier
    • Eaton
    • LICAP Technologies
    • Taiyo Yuden
    • Tecate Group
    • VINATech

Notably, I have omitted ELNA, Panasonic and TDK/EPCOS from the list as all seem to be discontinuing their lines of EDLC. Panasonic EOL’d all their lines on 1st October 2020, while ELNA has issued NRND notices 1st July 2023 for their EDLC products with no alternatives. A banner also indicates NRND status for all EDLC in TDK’s portfolio.  I’m not sure if Rubycon is continuing with their series – even though inventory is available, the DMB series does not appear on their English website. During this deep dive, I also discovered the reason behind the “IC” branding on the capacitors from Cornell Dubilier – they are actually Illinois Capacitor products but since Cornell Dubilier Marketing had acquired Illinois Capacitor in 2015, they now fall under the CDE banding. As a result, it does seem the industry is in a bit of “flux” with companies entering and exiting the space.

Of course, there are also some other companies I have excluded from the list because they make very specific types of supercapacitor products – Capacitech makes supercapacitor cables, while Seiko make “chip”-based supercapacitors which seem to go well with their other timing products and some other manufacturers specialise in module-level products.

From the list, I would say that the leading companies in this space seem to be Eaton, Cornell Dubilier and VINATech.

Specification Comparison of the Supercapacitors in the Kit

In order to understand the range of supercapacitors included in the kit and how they compare with one another, I’ve extracted all of the important parameters from the datasheet (where available) and tabulated it with colour-coding.

image

The range of capacitances is quite large with some overlap between EDLC and LIC products. The EDC stacked cells have a voltage rating of 5.5V, while most of the ordinary EDLCs have a 2.7V/3V rating with LICs having a 3.8V rating. The exception is the EDS224Z3R6H which has a 3.6V rating instead. Current delivery and internal resistance figures generally get better with increasing capacity which also correlates with the size of capacitor. The radial lead capacitors in the set range from 8 to 16mm diameter and 16 to 30mm height, while the coin-cell type ranges from 11.5 to 19mm diameter and 5.5 to 9.5mm height. Rather unsurprisingly, if looking at the energy in mWh, the capacitors have a very small amount of energy even compared to a true-wireless stereo earbud battery (3.7V/40mAh = 148mWh).

Through doing some calculations, it was possible to fill in some additional values, bolded in purple for better comparison. Unfortunately, not knowing how power density is calculated, I wasn’t able to get any more data on that and I didn’t want to start polluting the table with my own measured data just yet.

image

Nevertheless, from the table, the advantages of LICs in terms of energy density are clear as they are almost an order of magnitude better with a higher operating voltage than normal. A more limited voltage window, cycle life limitations with more limited temperature window and internal resistances are, however, the price that is paid for this.

Experiment #1: Weight and Voltage

I started measuring a sample of capacitors for their voltage in Experiment #0, but this time, I was going to be more thorough and measure the voltage across every capacitor. I would also use my rather accurate Muticomp Pro BAL1 scales to measure the weight to the milligram to see how consistent the product is and if they are close to specification.

image

The results are as follows:

image

The weights in bold differ significantly from the datasheet. The DSF705Q3R0 is stated as weighing 2.3g while the DSF256Q3R0 is stated as weighing 7.0g which suggests that the energy density figures for these products may be a little optimistic as the measured weights were 0.3-0.5g higher. On the whole, most weights were close to the datasheet value and the pairs of weights were reasonably close together. Voltage-wise, most capacitors were very close to zero – mostly below 100mV with some minor negative readings as well. However, the voltages for the LICs were all over the place – some had charge, others didn’t … is this a problem?

Safety Aspects with Lithium-Ion Hybrid Supercapacitors

Due to the similarities between lithium-ion batteries and LICs, there may be some hesitation about the safety of such products. However, it seems that the replacement of just the anode of an EDLC with that from a lithium-ion battery results in a structure that is immune to thermal runaway even with a dead short-circuit via a nail puncture test even when the cell reaches above 100°C in the process.

image

From Taiyo Yuden’s Whitepaper - TAIYO YUDEN Lithium Ion Capacitors: An Effective EDLC Replacement

In fact, reading up on the topic of safety, it seems that supercapacitors with less than 0.3Wh are not regulated and are exempt from dangerous goods or HAZMAT shipping regulations which was interesting to know

Unfortunately, there is one safety aspect which seems to have already been violated – namely that some of the lithium-ion supercapacitors have a voltage below the minimum voltage. After digging through several other manufacturer’s documentation, I have come to the consensus that this is not good for the capacitor and may even result in failure or be dangerous.

image

For example, this excerpt from VINATech’s User Manual for their LIC product makes it clear that it shouldn’t be stored for long below the minimum voltage or else, risking permanent damage.

image

This excerpt from CDA Capacitors indicates that short circuiting is forbidden, suggesting as well that the product is charged to a constant voltage.

image

This was taken from Tecate Group’s Handling Instructions for their LIC product which clearly indicates it is shipped in a charge state, forbidding short-circuits during processing including working with leads individually, banning wave-soldering, not permitting water washing. Perhaps the biggest indication of potential safety issues is that after discharging below minimum voltage, they claim “the cell should be immediately removed from service”.

image

Finally, this graph comparing EDLCs with LICs from the Taiyo Yuden whitepaper seems to make it clear that EDLCs are happy to operate from 0V up to working voltage, whereas LICs are cycled within their working voltage window.

It’s a shame I didn’t find any firm guidance from Cornell Dubilier’s site on this, but given the general consensus from other vendors selling components of the same type, I think the conclusion is that LICs should not be discharged below their minimum voltage!

I suspect the packaging of capacitors such that the legs could touch in shipment was probably the cause of the capacitors being over-discharged which is a shame, but perhaps still partially recoverable.

Conclusion

The activated carbon electrode’s surface area and the double-layer formed by the interface to the electrolyte separator is what makes supercapacitors work. While this improves the capacitance manyfold due to the increase in surface area, it also does make it take longer to fully charge up. This is where the familiar electric double-layer capacitor (EDLC) name comes from. By taking the anode from a lithium-ion battery and replacing the anode from the EDLC, you get a hybrid lithium-ion capacitor (LIC) which inherits a mixture of advantageous traits including higher voltage, lower self discharge, higher energy density, high cycle life, lower ESR than batteries and improved safety.

Looking around, it was possible to determine a list of manufacturers of supercapacitors – the leading ones would include Eaton, Cornell Dubilier and VINATech. By collating the data from datasheets for the supplied supercapacitors, it was possible to see the range of values in terms of performance as well as the key benefit of LICs in terms of energy density.

It is important to realise LICs are not like EDLCs in that they have a minimum voltage beyond which they could be damaged, irrecoverable or even unsafe to use. They ship in a charged state and are not allowed to be shorted, thus also restricts the type of processing and tooling that can be used in manufacturing. Despite their similarities with lithium-ion batteries, they appear immune to thermal runaway failures. In the case of all supercapacitors included, they appear to be exempt from dangerous goods/HAZMAT regulations in shipping.

In checking the weights of the supercapacitors, some notable deviations in weight were found on some DSF-series components. Some of the LICs were also found to be below minimum voltage, likely due to the packaging in shipment causing leads to short out, thus discharging the capacitors. Perhaps they are still recoverable in some way (an experiment for later, it seems).

In future sections, I hope to measure capacity, AC and DC ESR and leakage current, although the latter can take a very long time to do properly based on the various vendor published test procedures (72h) due to the time required to fully charge the capacitor. Nevertheless, hopefully by the next blog, the solar panel will have arrived and some testing of that might be able to take place as well.

[[What’s Super about Supercapacitors: Blog Index]]