E-Mobility Engineering 014 l InoBat Auto dossier l In Conversation: Brandon Fisher l Battery monitoring focus l Supercapacitor applications insight l Green-G ecarry digest l Lithium-sulphur batteries insight l Cell-to-pack batteries focus

points out is because they don’t rely on chemical reactions. “Every time you charge or discharge a battery, a chemical reaction takes place, using up some of the electro-active materials,” he explains. “How fast you charge/discharge, the temperature, and how old the battery is, can all have a major impact on how much damage is done to that battery for that cycle, and each cycle shortens the battery’s life.” That is why supercapacitors are so good for protecting batteries in high- power applications and improving performance at the same time. “Applications that require high energy/power in and out, such as acceleration and brake energy regeneration, are ideal for a capacitor,” Hall adds. “You can capture about 55% more energy with an ultracap during regen than with a lithium-ion battery – and do it a million times.” The impact on battery life can be substantial, as Jason McMillen, director of engineering at LiCAP Technologies, points out. “As an example, we are supporting major rail customers to replace batteries for emergency back-up track switching,” he says. “The switching is power- intensive but short in duration. That triples the life of the main lead-acid battery.” Dr Pohlmann notes that most of the operational examples of supercapacitors paired with batteries these days are in industrial settings rather than e-mobility, but he does expect the combination to play an important role in the automotive sector, again mostly in low-voltage applications. “If you combine a lithium-ion battery technology such as LFP with supercapacitors, you fix a big disadvantage of batteries – poor low- temperature performance,” he says. “That means you can reduce the size of the batteries, and at the same time add redundancy into the system.” cell vehicles. “They provide the power required for fast acceleration and the energy to support the fuel cell during longer periods of uphill travel or for warm-up purposes. Here we’re talking about periods of between 10 seconds and 5 minutes of use,” he explains. “In these applications, SuperBatteries are preferable to high- power lithium-ion batteries, because they provide higher safety as well as better low-temperature performance and better efficiency, especially at high C-rates. “However, we don’t see SuperBatteries rivalling batteries, but rather as a complementary technology. Batteries are not ideal for some applications, such as high-voltage hybrids, for which they lack power density, but SuperBatteries can help there.” One of the major contributors to a supercapacitor’s high power density compared with batteries is its low resistance. This is usually expressed as the equivalent series resistance (ESR), which is the internal resistance that appears in series with the device’s capacitance. Skeleton argues that ESR is the factor with the single strongest impact on system efficiency and working temperature. In turn, these determine the proportion of the energy stored in the supercapacitor that can be retrieved under real-world conditions. Further, the operating temperature is the strongest predictor of how long the device will last in service. Through innovations in materials, construction and electrode manufacturing, Skeleton says it has been able to reduce the ESR significantly. Ragone plots with W/kg on the horizontal axis and Wh/kg on the vertical axis allow the specific energy and specific power of devices with different capacitance ratings to be compared directly at different discharge rates, and consequently at different power levels. For example, at a modest power output – indicated by a complete discharge over 10 seconds – a 300 F Skeleton supercapacitor achieves 5.3 Wh/kg, while a commercial standard 310 F device with a higher ESR manages only 3.3 Wh/kg. The difference is even more marked as the discharge rates and power levels increase, and the device with higher resistance sees its specific energy drop sharply, while that of the lower-resistance Skeleton device falls in a much gentler curve. Chad Hall emphasises that supercapacitors have much longer service lives than batteries, which he Anode, separator and cathode layers are wound onto a cylinder at LiCAP’s production facility (Courtesy of LiCAP Technologies) 46 Summer 2022 | E-Mobility Engineering