Development of an Electrical Model for Lithium-Ion Ultracapacitors

Accurate knowledge of the electrical characteristics of an energy storage device is of high importance when utilized in any power system to achieve proper charging and discharging modes without inflicting any damage to its structure. In this paper, a Li-ion ultracapacitor, a hybrid type of energy storage, is thoroughly studied. This type of ultracapacitors has high energy density, high power density, high efficiency, long cycle life, and superior performance under high temperatures. Testing, analysis, and modeling of this energy storage device are presented in details. A battery-based model and a capacitor-based model are proposed. A series of dc and ac tests has been conducted at various temperatures to develop an electrical model spanning, the entire domain of temperatures under which the ultracapacitor can normally and safely operate. These models have been verified for model accuracy via simulation and comparison with the test results for both a single ultracapacitor cell and a 360 V ultracapacitor module built in the laboratory. Both coulombic and energy efficiency have also been investigated at different temperatures and current ratings.