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 determines the ESR of supercapacitors?
JL: The primary effects for ESR are cell design and materials. Most of the ESR in supercapacitors is determined by how the manufacturer makes connections from the active electrode to the terminals that the customer sees. The next biggest effect are materials – electrolyte and electrode materials and how the electrode is processed.
CH: ESR in the supercapacitor cell comes from two things; chemistry and cell design. The chemistry used, depending on the carbons, conductive carbons, binders, separator, and electrolyte all lead to an ESR level. Thickness of the electrode will also affect the ESR. The cell design in how the electrode foils are attached to, and how much of the foils are attached to, to terminals, drive the cell ESR numbers as well. The lowest ESR is achieved by using a properly designed chemistry, with good connections to the terminals. Often, however, you need to look at how the ESR increases over time, rather than the initial rating. The increase in ESR is typically used to determine the system performance at end of life, and just because a cell as low ESR out of the box, does not mean it will stay low. Cycling, time spent at temperature and voltage, all play a role in the increase in ESR. Some manufactures have improved this ESR gain significantly by using chemistry. Lastly, since almost no ultracapacitor is used alone, the module construction and design drive a significant part of the ESR seen at a system level. Some companies screw/bolt bus bars to the cell terminals (higher ER), some weld bus bars to the cell terminals better), and one welds the cells directly together, end-to-end (best) to reduce ESR.
JS: Is there a trade-off between supercapacitor ESR and other performance specifications?
ED: Achieving lower ESR comes at the expense of DCL (leakage current) specification, and vice versa. They are inversely proportional.
JL: Typically, higher capacitance means lower ESR. There is also the effect of temperature on ESR. The lower the temperature, the higher the ESR, especially as you get below freezing. However, ESR does not come down much as temperature is increased from room temperature.
CH: There is a definite trade-off between ESR and energy density of a supercapacitor. To me, an ultracapacitor should have lower ESR / high power – that is what it is chemically designed to be. You can increase the energy, but it causes increased ESR / lower power, and often lower cycle life. The proper system should have a high-power capacitor with a high energy battery, as that is what both chemistries were made to be.
JS: What factors impact supercapacitor ESR in real-world applications – operating temperature? Discharge rate? If the ESR varies, is it a linear or non-linear characteristic?
CH: Temperature will affect ESR. Low temperatures (from approximately -20C to -40C) will see the ESR increase approximately 2x. The ESR stays fairly flat on the upper-temperature end. Over time, high temperatures will lead to degradation of the cell, generating gas and causing a reduction of capacitance and an increase in ESR. The variance ESR effect for temperature will show a fairly flat line from +65C to approximately -20C, where a noticeable increase occurs due to the conductivity of the electrolyte at lower temperatures. Self-heating will typically correct this within a few cycles.
ED: Since rise of ESR is considered a failure mechanism of supercapacitors, failure rate of supercapacitors, in general, are non-linear. Operating temperature and applied voltage have the biggest impacts on aging in the application. Current profile could come into play as it generates internal heat buildup depending on the frequency and depth of discharge.
JL: Temperature and voltage are the primary effects on ESR over time. The higher the temperature and higher operating voltage at the cell level will cause the ESR to increase faster. Temperature can also affect the ESR as the electrolyte mobility decreases in cold temperatures. Additionally, ESR variation over the operating life depends heavily on the manufacturer. Do they do burn-in prior to shipping, how pure are the materials, how clean is the manufacturing process.
JS: How much does supercapacitor ESR vary between various device models? and from manufacturer to manufacturer?
ED: Supercapacitor ESR will certainly vary between models, series, and from manufacturer-to-manufacturer. In general, ESR decreases as capacitance increases. Furthermore, AVX manufacturers special low ESR designed supercapacitors (SCC LE Series) that feature low ESR characteristics compared to our own standard series (SCC Series) or in the industry. These can be critical in an application or for solutions that require multiple cells in series as ESR is additive. However, the sacrifice made to achieve that low ESR performance comes at the expense of DCL as they are typically inversely proportional. What I think would be certainly expected – you will definitely see ESR specifications vary throughout the industry. Design engineers should also be mindful of the specific wording used in supplier datasheets that could cause added confusion when comparing two products or consider what is truly a side-by-side comparison.
CH: ESR varies tremendously between cell types and constructions and from manufacturer to manufacturer. The lower-cost products often have a higher ESR to start with, and it increases significantly in a short period. Mid-range products will start with a low ESR, but that ESR will also increase quickly and faster as it ages. The best products on the market start with very low ESR and that ESR rise over time stays low, across a wide temperature range and throughout its life in cycling applications.
JL: On datasheet specifications, the ESR does not appear to vary a lot between manufacturers. There is some, but the actual as shipped ESR varies quite a bit. In addition, the ESR change as the part changes varies widely between manufacturers. As noted above, ESR is largely affected by the interconnects. Thus, the device model and form factor have a large effect on ESR. So, coin cells are orders of magnitude higher than cylindrical cells which are orders of magnitude higher than large axial cells. Typically, higher capacitance also relates to lower ESR as the electrode area is higher.
JS: Thank you to our three Virtual Roundtable participants for sharing their insights and experience! You might also be interested in reading, Supercapacitor Specifications and Lifetimes – Virtual Roundtable (part 2 of 2).