A nonlinear equivalent circuit model for lithium ion cells

A nonlinear equivalent circuit model for lithium ion cells is developed where the circuit elements are based on electrochemical processes like charge transfer reaction, and variable resistances that are functions of the state variables. The potential drops are the natural state variables rather than cell state-of-charge (SOC). A commercial pouch cell consisting of composite positive and meso-carbon microbeads (MCMB) negative electrodes is analyzed. The state space model developed consists of the current balance which forms the state variable evolution equation and voltage balance which forms the output equation. The global set of parameters for a two pair resistor capacitor circuit is calculated by solving the model equations consistently and minimizing the error with HPPC (Hybrid Pulse Power Characterization) pulse voltage at few SOCs. The entire HPPC protocol is predicted by the model accurately within an error of 50 mV. The model is validated using the full depth charge discharge experiments. The model is used to resolve the overall cell potential into potential drops due to individual processes, yielding insights into controlling mechanisms. The variable resistor formalism enables cell voltage prediction without parameter re-estimation and hence can be developed as an on-board algorithm. (C) 2012 Elsevier B.V. All rights reserved.