Machine Learning-based Identification, Prediction, and Control of Lithium-ion Batteries

This presentation will describe advances in machine learning-based techniques for addressing systems problems that arise for lithium-ion batteries. The specific systems problems include the prediction and classification of battery cycle lifetime (aka remaining useful life), the determination of optimal charging protocols, and the identification of fundamental physicochemical expressions for electrochemical kinetics, thermodynamics, and mass transfer from real-time video imaging. The development of the techniques and their application are in collaboration with materials science, applied physics, and computer science researchers at Stanford University, Toyota Research Institute, and MIT.

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An Electrochemical-Thermal Coupled Battery Thermal Runaway Model for Battery Safety Design

Thermal runaway is always a troublesome problem that hinders the safe application of high energy lithium-ion batteries. There is an urgent need to interpret the voltage and temperature changes and their underlying mechanisms during thermal runaway, in order to guide the safe design of a battery system. This research is dedicated to building a coupled electrochemical-thermal model that can well predict the voltage drop and temperature increase during thermal runaway. The model can capture the underlying mechanism of 1) the capacity degradation under high temperature; 2) the internal short circuit caused by the thermal failure of the separator; and 3) the chemical reactions of the cell components that release heat under extreme temperature. The model is validated using by experimental data, therefore the modeling analysis has high fidelity. We employ the model to analyze 1) the capacity degradation under extreme temperature; 2) the influence of the SEI decomposition and regeneration on the thermal runaway behavior; 3) the heat generation by internal short circuit in the thermal runaway process. The discussions presented here help extend the usage of lithium-ion batteries at extreme high temperature (>80◦C), and guide the safe design of lithium-ion batteries with less hazard level during thermal runaway.

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Data-Driven Battery Health Diagnosis in Real-World Applications

Accurate diagnostics and prognostics of battery health improves overall system performance. This allows industry to unlock value by detecting faults and improving maintenance, extending operational range, and understanding asset depreciation. However, battery aging is complex and caused by many interacting factors. Two key questions arise: first, how to handle modelling challenges, including parameter variability and nonlinearities, in methods for online estimation of state of health. Second, how to develop validated predictions of future health, where key issues include coping with variable usage scenarios, and cell-to-cell behavioural differences. This talk will discuss recent approaches to tackle some of these exciting topics, including the use of Bayesian non-parametric models for health prediction, and the combining of non-parametric and parametric models to allow flexibility in model fitting from data, whilst retaining the benefits of equivalent circuit and physical models.

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Data Analysis and Optimization for Battery State and Parameter Estimation

Estimation of battery internal states and parameters is one of the most important tasks of battery management. Estimation typically involves 3 basic elements, i.e. model, algorithm and data (e.g. current, voltage, and temperature measurements). While substantial efforts have been devoted to model development and algorithm design, our interest focuses on exploring the role of data in estimation, e.g. what are the optimal data pattern that would yield the best estimation accuracy? We will first demonstrate data optimization using the Fisher information criterion, which is the traditional “gold standard” for optimal experiment design, based on our analytically derived sensitivity expression of battery parameters. We will then show that such approach suffers major limitations including inability to address system uncertainties. We will then present our recently discovered data structures that relate estimation errors to system uncertainties. By incorporating these data structures for data optimization, the accuracy of estimation can be dramatically improved under system uncertainties (by up to 1 order of magnitude). Based on these data structures, we are developing a new set of criteria and framework applicable to a variety of data-optimization-for-estimation problems, including optimal experiment design and parameter management for system identification, and data selection/mining (from passive data stream) for online estimation.

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Unleashing the Full Potential of Batteries: Learning and Control

Batteries are ubiquitous. However, today’s batteries are expensive, range-limited, power-restricted, die too quickly, charge too slowly, and susceptible to safety issues. For these reasons, advanced model-based battery management systems (BMS) are of extreme interest. In this talk, we discuss eCAL’s recent research electrochemical-based BMS, which are modeled by nonlinear partial differential equations (PDEs). Specifically, we discuss the safe-fast charging control problem. Two methods are reviewed: (i) reinforcement learning-based methods, and (ii) reference governors. Finally, we close with exciting new perspectives for next-generation battery systems.

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Pushing the Envelope in Battery Estimation Algorithms

Battery Management Systems (BMSs) manage the charging and discharging of the battery providing estimates of state of charge and health during its use to protect the battery from misuse, ensure safety and longer duration. In this talk, we discuss the Stanford Energy Control lab’s recent research on advanced BMS using control theory tools, electrochemistry and hardware validation. Specifically, we will discuss state of health estimation using advanced observer design and its battery-in-the-loop validation.

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Thermal and Flow Actuation for High Performance Battery Systems

The opportunities for control in battery systems have been limited by a lack of actuators that can change the battery response in real-time. Recently, thermal and flow actuation have become interesting new tools in the battery systems engineers toolbox. Temperature has long been known to influence battery performance, including aging, power, and safety. Flow has been used to enhance metal electroplating surface finish, a process similar to charging in lithium metal batteries where dendrite growth is a safety hazard. This talk will address the potential and opportunities for thermal and flow actuation to improve the performance and lifespan of lithium ion chemistries.

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Simultaneous Identification and Control for Hybrid Energy Storage System using Model Predictive Control and Active Signal Injection

Online State of Charge (SoC) and State of Health (SoH) estimation is essential for efficient, safe, and reliable operation of Lithium ion batteries. The previous research on this topic mainly focuses on the development of battery models and estimation algorithms. However, the nature of the battery excitations also significantly influences the estimation performance. The impact of data on the estimation accuracy is investigated using Fisher information matrix and Cramer-Rao bound, considering measurement noise. A sequential algorithm, which uses the frequency-scale separation and estimates the parameters/states sequentially by actively injecting current signals with different frequencies, has been proposed and developed, it will be presented at this workshop. By incorporating a high-pass filter, the high-frequency and medium-frequency currents are injected to facilitate estimation of SoC, battery parameters, and SoH in a sequential form. Experimental results show that the estimation accuracy of the proposed sequential algorithm is much better than the concurrent algorithm where all parameters/states are estimated simultaneously. As a case study, the sequential algorithm is applied to the battery/supercapacitor hybrid energy storage system (HESS), which is over-actuated in the sense that there are two power sources providing power to the load. This over-actuation feature is exploited to achieve accurate identification of the battery states/parameters and high system efficiency simultaneously. To resolve the conflict between providing rich signal content for identification and achieving energy efficient operation, a model predictive control (MPC) strategy is used to incorporate both objectives to determine the optimal power distribution between supercapacitor and battery. The results showing the benefits of the Simultaneous Identification and Control framework will be discussed in the context of managing the competing requirements for estimation accuracy and energy efficiency.

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Cell Swelling for SOC, SOH, and SOS

In this presentation, we’ll show you advances in identifying the electrode-specific state of health (SOH) parameters by observing the reversible cell expansion and the intrinsic shift of phase transitions in the battery material as its State of Charge (SOC) and age change. These shifts create an “aging signature” like a wrinkle that can be observed in the measured force more clearly than in the measured voltage, especially at relevant discharge ranges and rates. Data collected from aged cells and at higher discharge rates will further address the critical real-world considerations in estimating the evolution of the electrode “age wrinkles” in the electrical and mechanical domains. electrode-specific state of health (SOH) can be used to manage behind the meter, vehicle to grid, and 2nd life applications that can provide value and lower the total cost of ownership of EVs. We will conclude by highlighting the benefits of measuring cell swelling in estimating gas venting as an onset of thermal runaway and an indicator of the battery state of safety (SOS).

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Model-Based BMS and the Design of Efficient Algorithms for Current and Next-Generation Batteries

In order to significantly expand the PHEV/ BEV market, and to increase the use of lithium-ion batteries in electric grids, there is a need to develop optimal charging strategies to utilize the batteries more efficiently and enable design for longer life. Advanced battery management systems (BMS) that can calculate and implement such strategies in real-time are expected to play a critical role for this purpose. This talk will present approaches for determining model-based optimal charging profiles for batteries, and experimental validation of the same. Model-based BMS enables > 100% improvement in cycle life, 30% reduction in charging time, and >50% reduction in temperature rise. Validation of the same for 6 different batteries/sizes/chemistries/form-factors will be presented.

In addition, the design and relevance of efficient algorithms for enabling model-based BMS will be discussed. To simulate solid-phase diffusion inside the electrode particles efficiently (along the radial direction), the concept of volume-averaged flux (with quartic polynomials) was introduced by us in 2005 and has been successfully adopted by the battery modeling and control community worldwide. To simulate the concentration, temperature, and potential fields along the electrode thickness (x-direction, measured from the cathode current collector to the separator) model-reformulation approach based on spectral methods was developed. This enables efficient simulation of lithium-ion battery models (15 ms for a discharge curve prediction) without losing fidelity. To address the frequent failures of standard DAE solvers (integrators in time) a robust fail-safe approach for simulating index-1 nonlinear differential-algebraic equations was developed. (As of today, this is the most robust approach for solving battery models and fails 0% of the time). To enable real-time simulation and control of 100s of cells in series/parallel configuration, a novel tanks-in-series approach was developed. This method has the same computational complexity as simple flowsheet models. Application (and the inherent presence) of control theory in enabling the development of some of these efficient algorithms will be highlighted.

Finally, challenges in simulating multiscale-multidomain-multiphysics-multiphase models for next-generation batteries will be presented.