Moderated by Jeff Shepard
EE World has organized this “virtual roundtable” bringing together three experts in supercapacitor technology to share with you their experience and practical insights into these important devices. Joining us for this virtual roundtable are: Chad Hall (CH), Co-Founder / Sr. Vice President Sales & Operations, with Ioxus; Eric DeRose (ED), Global Product Manager – SuperCapacitors, with AVX Corp.; and Jason Lee (JL), Global Product Manager for Supercapacitors, with Eaton Corp.
JS: What are the non-ideal characteristics of actual supercapacitors compared with theoretical devices and what are the sources of those non-ideal characteristics? How can those non-ideal aspects be minimized?
JL: Lifetime effects are the first. A lot of testing and characterization is modeled. However, manufacturing variation can affect the lifetime, so parts do not perform to the model. This is typically overcome by building in the margin to the design, either lower operating voltage per cell or designing in higher capacitance.
CH: The non-ideal characteristics of supercapacitors (a.k.a. ultracapacitors or EDLCs) is the lower voltage limit than other electronics and the lower energy density than batteries. The chemistry (electrolyte) chosen typically dictates a number of the characteristics of a cell for the maximum voltage, which in turn drives the energy density. The formula of E=1/2CV2 (energy = one-half of the capacitance X the voltage squared) shows a direct correlation of energy to voltage. The material chosen (electrolytes are typically acetonitrile based or propylene carbonate-based), including the conductive salts, the carbons (typically organic-based from carbonized coconut shells for example), and the separator (cellulosic of a PTFE for example) are used. The carbon is activated via steam (lower contaminates, but less activation which leads to lower capacitance and lower power) or using a chemical process such as KOH (higher capacitance and power). All of these material choices lead to the ability to function over the life differently, and there are capacitors that use lower-cost materials that result in shorter life (higher capacitance loss or faster resistance gain), or products that take advantage of highly engineered materials to provide the highest power and energy over a longer life. The lower-quality materials result in more chemical side reactions, that drive leakage current, and poor performance. The life of an ultracapacitor is typically described at end-of-life (EOL) by reaching a 20% capacitance loss or a 200% increase in resistance (ESR or equivalent series resistance).
JS: Conventional capacitors are offered in standard sizes with standardized electrical ratings. Are there industry-standard sizes (physical sizes or electrical capacities) for supercapacitors?
Eric DeRose (ED), Global Product Manager – SuperCapacitors, with AVX Corp.
ED: In general, the same thing applies for standard-sized supercapacitors. For example, a 10x30mm can supercapacitor is generally 10 Farads across the industry. You may come across certain suppliers offer it as an 11F or 12F with different capacitance tolerances possibly, and this same trend spans in other can sizes as well, but you can partially chalk that up as marketing strategy. What truly differs are the other electrical parameters such as DCL (leakage current) or ESR (equivalent series resistance) that directly impact performance in use of the application. Those are absolutely not standard across the industry based on size or capacitance.
JL: Many of the sizes mimic electrolytic capacitors for supercapacitors used in electronics applications. Up to 600F, they come in cans leveraged from electrolytic sizes. There are also slight changes to this for customer-specific applications. As you get to larger capacitances, the “standards” have been set by the early suppliers. Today, we have 60mm diameter cells with varying lengths to meet capacities from 650F to 3400F. Coin cell sizes are based on button cell batteries.
CH: The industry that produces supercapacitors (a.k.a. ultracapacitors) has developed standard sizes over time, with some standardization on termination. There are two types of cells produced; laminated pouch cells (where the cells’ electrodes are stacked or wound in a flat manner) or cylindrical cells. Of the larger cells used in most power industrial applications, the standard ratings for voltage are 2.7V or 2.85V. Standard capacitance sizes are 100F, 350F, 600F, 1200F, 2000F, and 3000F. Termination ranges from flat tabs on pouch cells to solderable terminals on the 100F-600F, and screw or weldable terminals on the 600F – 3000F cells. There is an IEC specification for testing cells and UL-810A for safety testing ultracapacitors. More industry standards should be developed for testing products to make it easier for customers to choose the right quality product.
JS: Are supercapacitors most-often used alone or in combination with other energy storage devices such as batteries? Or conventional capacitors?
CH: In most cases, supercapacitors are used without another energy storage product, but often there are advanced power electronics involved such as DC/DC converters, which may drive the need for filtering capacitors to reduce high frequencies across the supercapacitors. Due to the nature of ultracapacitors, being very high power (10-40kw/kg), long cycle life (1,000,000 charge/discharge cycles), the wide temperature range (-40C to +65C or even +85C), the low resistance (.02mOhms), or the high round trip efficiency (typically 95-99%), ultracapacitors should be paired with batteries (high energy, low power, low cycle life, narrower temperature window). Capacitors will take or give almost any current you want to give/take, and not care. A single 3000F cell is typically capable of providing 3,000A for 1 second. And it can do so with very little heat generation. This makes supercapacitors a wonderful product to use for large current demands (UPS, vehicle acceleration, cranes, automated guided vehicles, starting engines, wind turbine pitch control, etc.). The fast charge acceptance of a supercapacitor allows for much higher brake energy regeneration than advanced batteries, and they will happily perform this cycle a million times. Allowing the battery to handle the lower power, steady-state, energy demands, and the capacitor to handle the peak loads of acceleration or load shifts, makes an extremely efficient system design. Often, supercapacitors are looked at as too expensive, but that is usually because the designer is not sizing the system properly. Working with your ultracapacitor supplier in the early design phase often dramatically lowers system costs.
JL: In terms of number of cells, they are still used most often alone. However, there are many applications where they are combined with batteries such as water meters and electric busses. They are not typically combined with conventional capacitors.
Button cell supercapacitors from Eaton
ED: Supercapacitors are often used in conjunction with primary or secondary batteries. They are ideal for peak power assist applications where the supercapacitor provides the necessary current pulse(s) that would otherwise drain the battery, as well as power hold up applications. In general, they are a value add in extending lifetime of the battery and application. There are instances in which supercapacitors can completely replace a battery but those are usually much larger designs such as industrial applications.
JS: What is the most misunderstood aspect of supercapacitor technology/operation? How does that translate into challenges for design engineers using supercapacitors?
JL: Cost remains the most misunderstood aspect of supercapacitors. We still get many inquires which ask what the cost per watt-hour is. It’s really understanding the advantages and limitations so that they are applied to the right application.
CH: The most misunderstood aspect of supercapacitors is often the system sizing. This is a challenge because designers will assume the cost is too high, and look past supercaps. If the power electronics are allowed to use a wider voltage window (full rated V to ½ rated V) and allow the full energy level of the supercaps to be used, it helps reduce the capacitor costs. Also part of the cost is the ultracapacitor module design and how that fits into the system architecture. There are modules made specifically for standard racks, which allow for low-cost system building blocks. The designers should work with their ultracapacitor provided to fill out application worksheets and have discussions about the system needs. Lifetimes can be achieved for 20+ years of maintenance-free operation, but perhaps the system only needs to work for 7 years? If that is the case, the capacitor manufacturer should be able to use their life modeling, based on the system sizing inputs, and reduce size by increasing the derated of the volts per cell and achieve the proper life at a reasonable cost.
JS: What is usually the biggest challenge engineers face when first using supercapacitors?
ED: I will take this question and the one that precedes it as one and the same as I answer here. The biggest challenge or misunderstanding in my opinion would be properly sizing a supercapacitor solution for end of life due to its failure mechanisms. Supercapacitors are much more complex than conventional board-level capacitors, so in turn understanding how failure is influenced by voltage and temperature is never a “one size fits all” equation. All we can do is rely on decades of industry experience and internal captured test data to better characterize expected lifetimes. Taking a different view on this as well, specifically as it relates to design engineers first using supercapacitors, certainly be conscious of the amount of energy and current potential they are using or “playing around with.” Especially when designing for higher voltage applications where multiple cells are necessary to attain that voltage level with supercapacitors, the current potential can be dangerous or even dead.
JL: How to size supercapacitors and accounting for the voltage drop. Many engineers are used to working with batteries or a more constant voltage source. Understanding how the voltage drops as it powers the load and the effect of the current on this.
CH: Typically one of the biggest challenges engineers face when first using supercapacitors is to not fully understand their system needs. By really understanding the energy and power needs, the system design can be altered to ensure they are using the right size products. Supercapacitors can be used to deliver very high currents for a short period (starting an engine or lifting a container), or they can be used to deliver a small amount of current for a longer period, acting as more of a battery (powering LEDs for example). Knowing how much power is usable, and looking at the energy needs will allow for flexibility in choosing chemistries of energy storage.
JS: What should designers do to maximize supercapacitor performance and lifetimes?
JL: Lower the operating voltage per cell is the main “knob” designers have in order to maximize lifetime. The typical method is to put more cells in series, but this increases the ESR of the system. This can be overcome by adding capacitance as this typically correlates with lower ESR.
ED: Supercapacitor lifetime is directly tied to applied voltage and temperature. Make sure to adhere to recommended voltage deratings for high-temperature performance or long lifetime expectancy, and when in doubt seek advice from the manufacturer for their guidance & resources.
CH: Two things really affect the life of an ultracapacitor; heat and voltage. Proper module design will reduce the thermal and electrical effects, by employing thermally conductive materials to pull heat from the cell, or balancing and limiting the voltage to the cells. High rate duty cycling will often drive the internal temperature higher than ambient, and this temperature rise needs to be considered when sizing the system. Voltage is also typically de-rated on a per-cell basis to achieve a very long life. Lower temperatures (down to -40C) do not affect life but can cause an increase of up to 2x for the ESR. These should be considered when looking at life.
JS: Thank you to our three Virtual Roundtable participants for sharing their insights and experience! You might also be interested in reading, Supercapacitor ESR, and Optimal Performance – Virtual Roundtable (part 1 of 2).