The current energy situation is dominated by fossil fuels, especially oil. This dependency is turning critical due its reducing reserves, political and economical ramificationsto and environmental impact. This situation has caused an increase in the research and development of electrochemical energy systems, such as fuel cells, batteries and ultracapacitors and its application to stationary or transport applications. This book presents the nonlinear dynamic per-unit modelling of electrochemical systems through frequency domain tests. The per-unit approach is a useful tool for comparison and design, due to the great disparity of voltages and capacities, e.g. in batteries, which can be found. The book also includes the description of the experimental setup needed to allow electrochemical impedance spectroscopy on supercapacitors during high current loads. The modelling results are applied to a hardware-in-the-loop simulation, which allows to test complex systems in an easy way. This book can be useful to electric, electronic and electrochemical researchers who wish to model and simulate energy systems.