Temperature and dynamics problems of ultracapacitors in stationary and mobile applications

Ultracapacitors as a powerful energy storage systems are used in various areas of power electronics. Depending on the application, temperature and dynamics properties of these components have to be considered. These properties strongly depend on the characteristics of basic materials of the capacitors.The frequency and temperature dependence of the capacitance as well as of the internal resistance, ESR, is manly affected by the electrodes of activated carbon and the electrolyte. Under operating conditions differences of 15% and more of the capacitance due to a different structure of the electrodes are observed. Due to the reduced solubility of the conducting salt and the increased viscosity of the solvents for temperatures below freezing point the conductivity of the electrolyte drops drastically with decreasing temperatures. Thus, increases of the ESR between 200 and 700% between room temperature and −30 °C depending on the electrolyte are registered. Due to the slightly different selfdischarge of the single capacitors equivalent to a voltage drop of 4–12% within 3 days, the individual cell inside a module has to be protected by a cell voltage balancing unit. By an active cell voltage balancing unit connected in parallel to the capacitors a voltage drop will be leveled out after 1 h.In addition to the electrical characteristics of the ultracaps also the thermal properties of the single cell as well as of the modules have to be considered for the design in of these storage devices. By cooling elements integrated in the surface of the module casing and forced cooling the effective current load can be nearly doubled. Based on this know how ultracap modules were designed, which fulfill all the requirements of the applications in automotive and industrial electronics.