Properly Lumped Lithium-ion Battery Models: A Tanks-in-Series Approach

Abstract

This article introduces a lumped electrochemical model for lithium-ion batteries. The governing equations of the standard ‘pseudo 2-dimensional’ (p2D) model are volume-averaged over each region in a cathode-separator-anode representation. This gives a set of equations in which the evolution of each averaged variable is expressed as an overall balance containing internal source terms and interfacial fluxes. These quantities are approximated to ensure mass and charge conservation. The averaged porous domains may thus be regarded as three ‘tanks-in-series’. Predictions from the resulting equation system are compared against the p2D model and simpler Single Particle Model (SPM). The Tanks-in-Series model achieves substantial agreement with the p2D model for cell voltage, with error metrics of <15 mV even at rates beyond the predictive capability of SPM. Predictions of electrochemical variables are examined to study the effect of approximations on cell-level predictions. The Tanks-in-Series model is a substantially smaller equation system, enabling solution times of a few milliseconds and indicating potential for deployment in real-time applications. The methodology discussed herein is generalizable to any model based on conservation laws, enabling the generation of reduced-order models for different battery types. This can potentially facilitate Battery Management Systems for various current and next-generation batteries.