| Li-ion batteries are increasingly widespread with the energy problem becoming theconstraints of sustainable development for social and economy. Therefore, it is vital topredict and evaluate the performance and safety of Li-ion batteries accurately andquickly. Thus, this paper proposed an electrochemical-heat coupled model for the lowpower Li-ion batteriesbased on the rigorous electrochemical modeland reasonablesimplification strategies. Then, numericalsimulationswere conducted for differentconditionson MATLAB environment.First of all, the equations of ion current densityor electrochemical overpotentialdistribution in porous electrode under electrochemical polarization control system weredeveloped separately. Based on the results, the ion current density governing equationsunder linear and strong polarization conditions were solved analytically, while theover-potential governing equation was solvednumerically due to its complexity. Theanalysis revealed that the ion current density could be treated as linear under a specificrange of electrode parameters and discharge rate. On the basis,combining thecharacterizations of the low power cells,the simplification strategieswere developedfrom3aspects, transform the elliptical2-order partial differential equations to commonalgebraic equations, replace numerical solution for nonlinear equations with analyticalsolution, and reduce the dimension of the solid concentrationcalculation. At the sametime, the energybalance equation was inserted considering theinfluence of the thermaleffect to discharge and the potential of the single cell used in battery stacks.On the basis of the above model, the solvingprogram was developed on MATLABenvironment mainly based on the finite difference and the4-order R-K method. Therelative error will reduce sharply with the discharge time increase and below1%after50s. In further, the necessaryparameters and empirical functions for solving the modelwere acquired by experiment and published papers. Thus, the program could simulatethe dischargeand temperature curves of different ambient temperatures and differentdischarge rate. Also, in order to compare the simulation results with the experimentaldata of the14450type LiFePO4cylinder cell objectively, several quantitativeindicatorssuch as the discharge capacity, the output voltage, slope of the platformvoltage, the ending temperature and so on were set up. All simulations resultsindicatethat the model can simulate the voltage, the capacity, the temperature data and corresponding change regulations correctly. Besides, the model can also give thechanges of some parameters, which are difficult to get information from insituexperiments, such as concentration, diffusioncoefficient, electrochemical reactionconstant and so on. In addition, it is believed that the program has a good simulationefficiency because of the time consuming for all conditions varied from3-60s on the laptop computer with a Pentium dual-core1G CPU and2.00GHz.In summary, the electrochemical-thermal coupled model and correspondingprogram can conduct godprediction and efficient simulation for the low power LiFePO4Li-ion batteries. The work in this paper can also enrich the researches aboutelectrochemical model of Li-ion cells and provide a useful example for simplifiedmodel development. |
