Performance analysis of lithium-ion battery/electrochemical capacitor hybrid systems

Electrochemical double layer capacitors are the most suitable power sources for high powered applications such as electric vehicles, power distribution systems, uninterrupted power supply, hybrid vehicles and other electronic devices due to their high power densities. However, their energy densities are considerably lower than those of high energy battery systems such as Lithium-ion. Although advanced battery systems and double layer electrochemical capacitors contrast with regard to energy-power relationship, in combination they can be utilized as an effective power source for various applications. So a systematic study of the performance of the combination of these energy sources (hybrid system) is indispensable.; In this thesis, a hybrid system consisting of a lithium-ion battery coupled with a network of electrochemical capacitors was constructed and investigated in detail under pulse type of discharge. The impact of various operating parameters such as duty ratio, frequency, pulse current amplitude, number of capacitors in the capacitor network on the performance of the hybrid system was studied. To further understand and optimize the hybrid system a mathematical model for a lithium-ion/electrochemical capacitor network hybrid was developed from first principles. The prominent features of the model were its capability to predict the current shared by the battery and the capacitor network during discharge and its versatility to include any number of identical capacitors/batteries in series/parallel configuration. Specific energy and power relationships were simulated to identify the regime where the performance of the hybrids was better than the battery on a mass basis. The validity of the model was also tested against experimental data obtained from a Sony US 18650 lithium-ion battery/Maxwell PC100F electrochemical capacitor hybrid system. Finally a case study on the performance of the battery-alone system against a hybrid system was done for two different high power pulse profiles encountered in modern applications.