The electrical double layer capacitor (EDLC) — most often called a “supercapacitor” and sometimes an “ultracapacitor” — is an amazing passive energy-storage component. As a result of its high capacitance of multiple farads and small size, it provides high-density energy storage by both volume and weight. In some remote sensing, IoT, and energy harvesting-sourced applications, supercaps are an alternative to rechargeable batteries; in other situations, they are used in conjunction with batteries to overcome some of the weaknesses of those electrochemical-based energy-storage components. It’s not that one is inherently better than the other; instead, supercaps and rechargeable batteries (regardless of chemistry) each have their relative strengths and weaknesses. The priorities of the application determine which one makes the most sense, or both are needed in some sort of tandem arrangement.
There is another interesting alternative to choosing just one or even both as two discrete components: the hybrid supercapacitor. This energy-storage device is not just an obvious co-packaging of a rechargeable battery and a supercap. Instead, it uses a unique construction in which the single assembly is both a supercap and a Li-ion battery at the same time, Figure 1 (see References for more details).
Among the vendors of these hybrid supercaps are Taiyo Yuden (the company calls them lithium-ion supercapacitors, which is technically quite correct), Eaton, and Maxwell Technologies, Inc. (now part of Tesla).
There are many posted tables providing comparisons between standard supercaps and lithium-ion rechargeable batteries (table 1). Keep in mind that each resource and vendor has a different perspective, as you would expect, and the technology itself is evolving at a rapid rate.
Despite the apparent virtues of these hybrid supercaps, I’ve always had mixed feelings about hybrid devices and structures in general. On one hand, the combining of two technologies or materials often allows us to retain the best aspects of each while overcoming some weaknesses. This does not apply just to electronics: think of concrete reinforced with bars, or the carbon-fiber-reinforced polymers (CFRP) used as the skin of the latest generation of aircraft bodies and appendages.
At the same time, these combinations sometimes have new shortcomings. For example, multifunction test equipment may have reduced specifications or some flexibility limits compared to single-purpose, optimized units. The widely known “Swiss Army knife” is a non-electrical example: each of its individual tools may be “OK enough” yet is definitely not as good as a dedicated tool; nonetheless, the overall blade/accessory combination and packaging brings benefits in size, weight, and cost.
For hybrid supercaps, there’s also a management issue. Li-ion rechargeable batteries have their specific needs with respect to oversight of charge and discharge rates, coulomb counting, and temperature (to cite a few factors) — and supercaps have their own comparable list. So, how is the hybrid supercap to be managed? Will the tactics be in conflict, or are they similar enough that a single approach can work for the two-terminal hybrid?
I think about the tunnel diode: despite its attractive performance characteristics, as a two terminal-device without distinct input-output-ground connections it was fairly difficult to actually use and so fell into disfavor; the same holds true for the PIN diode (just look at some of its application-circuit schematics). Perhaps ICs such as the recently introduced Maxim MAX38889, a 2.5V to 5.5V, 3A reversible buck/boost regulator optimized for supercap backup applications, work well enough for both? (Figure 2.)
Deciding whether to use a hybrid solution for a given problem often involves weighing hard-to-assess tradeoffs. In addition to the obvious advantages where each constituent overcomes one or more shorting of the other, there are also many cases where new weaknesses are introduced.
Does it make sense to use the supercap hybrid? The answer is simple: it depends. In some cases, a new shortcoming is unacceptable in the application, while in others, the new benefits outweigh the drawbacks. Quantitatively, the model must not only solve the equation “is 1 +1 <, =, or > 2?” but must also assess any gaps the solution creates as well.
What has been your experience with hybrid — combined or merged — solutions (and not just hybrid supercaps)? Was the overall gain more important than any added downside? How do you make the judgment on the balance between the advantages versus disadvantages of the hybrid approach?
Eaton, “Hybrid supercapacitors explained”
Eaton, “HS Hybrid supercapacitor white paper”
Battery University, “BU-209: How does a Supercapacitor Work?”
Taiyo Yuden, “Lithium Ion Capacitors: The Ultimate EDLC Replacement”
Battery progress reality check: good and bad news