A Mathematical Model for a Lithium-Ion Battery/Electrochemical Capacitor Hybrid System

A one-dimensional model for predicting the performance of a battery/electrochemical capacitor-hybrid system has been developed. Simulation results are presented for a ethylene carbonate/dimethyl carbonate∣carbon battery system and a Maxwell PC 10F carbon double-layer electrochemical capacitor. The current shared between the battery and the electrochemical capacitor at very short times depends on the ohmic resistances of the battery and the capacitor. As the discharge proceeds, the operating conditions such as frequency, duty ratio, and peak pulse discharge current control the current shared among parallel circuits. These parameters also determine the extent of the run time increase of the hybrid system as compared to the battery system. The inclusion of a number of identical series/parallel capacitors is considered in the present model by introducing the parameter, capacitor configuration index. Ragone plots are simulated for a battery-alone and a hybrid system. A substantial improvement in the available energy density is observed while operating hybrid systems under high power densities. Finally, a general optimization approach is presented.