Enhanced Equivalent Circuit Model for High Current Discharge of Lithium-Ion Batteries with Application to Electric Vertical Takeoff and Landing Aircraft

TL;DR

This paper introduces an enhanced equivalent circuit model for lithium-ion batteries that accurately predicts high current discharge behavior, crucial for eVTOL aircraft safety during fault conditions.

Contribution

A novel data-driven extension to conventional ECMs that captures rapid voltage drops at high discharge rates with minimal added complexity.

Findings

Model predicts landing hover duration with 2.9 seconds mean error.

Validated against 150+ flight data points across diverse conditions.

Accurately captures voltage behavior during high discharge scenarios.

Abstract

Conventional battery equivalent circuit models (ECMs) have limited capability to predict performance at high discharge rates, where lithium depleted regions may develop and cause a sudden exponential drop in the cell's terminal voltage. Having accurate predictions of performance under such conditions is necessary for electric vertical takeoff and landing (eVTOL) aircraft applications, where high discharge currents can be required during fault scenarios and the inability to provide these currents can be safety-critical. To address this challenge, we utilize data-driven modeling methods to derive a parsimonious addition to a conventional ECM that can capture the observed rapid voltage drop with only one additional state. We also provide a detailed method for identifying the resulting model parameters, including an extensive characterization data set along with a well-regularized objective function formulation. The model is validated against a novel data set of over 150 flights encompassing a wide array of conditions for an eVTOL aircraft using an application-specific and safety-relevant reserve duration metric for quantifying accuracy. The model is shown to predict the landing hover capability with an error mean and standard deviation of 2.9 and 6.2 seconds, respectively, defining the model's ability to capture the cell voltage behavior under high discharge currents.