Ageing in Commercial Li-ion Batteries: Lifetime Testing and Modelling for Electrified Vehicle Applications

In this thesis, ageing in a commercial pouch cell for vehicle application is investigated through lifetime testing and modelling. The lifetime tests investigate the impact of temperature, current, depth of discharge (DOD) and state of charge (SOC). Results from lifetime tests are used in the development of an empirical ageing model, to study the ageing as a result of different user cases in a vehicle application. The results showed that the battery lifetime in a vehicle application could be prolonged, without interfering with the driving itself, by better planning of the charging.Lifetime test results and ageing analysis of the tested cells are used as guidance for the development of a physics-based ageing model. The model includes capacity loss and resistance increase due to resistive film formation and loss of active material.The lifetime test results showed that for the studied cell, staying below 40% SOC level will improve the lifetime considerably. This was seen in both the calendar ageing tests and the cycling ageing tests. The lifetime tests performed in small SOC intervals at different SOC levels showed that the ageing is separated into two groups, with more pronounced ageing at high SOC and less ageing at low SOC.\ua0\ua0 The main ageing contribution is the growth of Solid Electrolyte Interphase (SEI), which is also the case seen in the model when using parameters for the SEI growth extracted from calendar ageing data. The more pronounced ageing at higher SOC levels can partly be explained by the SEI growth that is increased at higher SOC level and partly from the contribution of manganese dissolution. This was confirmed by Post Mortem (PM) analysis and successfully captured by the model

A model platform for solving lithium-ion battery cell data gaps in life cycle assessment

With the advent of electromobility, life cycle assessment studies need to keep up with growing number of cell formats and chemistries being adopted for various vehicle applications. This often hindered by lack of data. A model platform is presented, starting with a cell design computation model which is used for calculating the mass of cell components and other design parameters. It also includes a cell performance model, which will link to a battery pack and vehicle model, both used for estimate losses caused by the cell during vehicle operation. Furthermore, the platform comprises a model generating inventory data for life cycle assessment of lithium-ion battery cell production. Together, these parts feed information to life cycle assessment calculations covering both production and use of lithium-ion battery cells. The aim is to support technology development and provide an understanding of how various design changes in cells link to environmental impacts. This conference paper explains model parts and provides exemplary results

Gender Aspects in Driving Style and Its Impact on Battery Ageing

The long and tiring discussion of who are the best drivers, men or women, is not answered in this article. This article, though, sheds some light on the actual differences that can be seen in how men and women drive. In this study, GPS-recorded driving dynamics data from 123 drivers, 48 women and 75 men, are analysed and drivers are categorised as aggressive, normal or gentle. A total of 10% of the drivers was categorised as aggressive, with an even distribution between the genders. For the gentle drivers, 11% of the drivers, the men dominated. The driving style investigation was extended to utilise machine learning, confirming the results from statistical tools. As driving style highly impacts a vehicle\u27s fuel consumption, while switching over to battery electric vehicles it is important to investigate how the different driving styles impact battery utilisation. Two Li-ion battery cell types were tested utilising the same load cycle with three levels of current amplitude, to represent accelerations for the three drive categories. While one cell type was insensitive to the current amplitude, the highly energy-optimised cell proved to be sensitive to higher current amplitudes, corresponding to a more aggressive driving style. Thus, the amplitude of the dynamic current can for some cells be a factor that needs to be considered for lifetime predictions, while it can be neglected for other cells

How the utilised SOC window in commercial Li-ion pouch cells influence battery ageing

In many lithium-ion battery (LIB) applications, e.g. hybrid vehicles and load-levelling storage systems, only part of the state-of-charge (SOC) range needs to be utilised. This offers the possibility to use an optimal SOC window to avoid LIB ageing. Here, a large test matrix is designed to study LIB ageing in a commercial 26 Ah pouch cell, in order to map the ageing behaviour at different SOC levels with respect to temperature and current. A quantification of the degradation modes, loss of lithium inventory (LLI), loss of active positive (LAM(PE)) and negative (LAM(NE)) electrode materials is made by analysing the change in the open circuit voltage (OCV). A key result is that lower SOC intervals significantly improved battery ageing. Even during harsh test conditions, such as high Crates and temperatures, the cells deliver more than three times the expected number of full cycle equivalents. High SOC combined with high C-rate increase ageing where the dominating ageing mechanisms are LLI, followed by LAM(PE)

Electrochemical Model-Based Fast Charging: Physical Constraint-Triggered PI Control

This paper proposes a new fast charging strategy for lithium-ion (Li-ion) batteries. The approach relies on an experimentally validated high-fidelity model describing battery electrochemical and thermal dynamics that determine the fast charging capability. Such a high-dimensional nonlinear dynamic model can be intractable to compute in real-time if it is fused with the extended Kalman filter or the unscented Kalman filter that is commonly used in the community of battery management. To significantly save computational efforts and achieve rapid convergence, the ensemble transform Kalman filter (ETKF) is selected and tailored to estimate the nonuniform Li-ion battery states. Then, a health- and safety-aware charging protocol is proposed based on successively applied proportional-integral (PI) control actions. The controller regulates charging rates using online battery state information and the imposed constraints, in which each PI control action automatically comes into play when its corresponding constraint is triggered. The proposed physical constraint-triggered PI charging control strategy with the ETKF is evaluated and compared with several prevalent alternatives. It shows that the derived controller can achieve close to the optimal solution in terms of charging time and trajectory, as determined by a nonlinear model predictive controller, but at a drastically reduced computational cost

Lithium ion Battery Aging: Battery Lifetime Testing and Physics-based Modeling for Electric Vehicle Applications

Electrification of vehicles is one solution in reducing emissions from the transport sector, and in particular to limit tail-pipe emissions in cities. In order to make the electrification successful and cost effective, there is a high demand for longer drive range and longer battery lifetime. By improving the understanding for the underlying aging mechanisms of vehicle lithium ion batteries, the utilization can be improved and thus cost effective. This thesis contributes with an extensive test matrix for lifetime testing, including calendar and cycling aging, on large commercial cells for automotive applications. The test matrix is constructed to investigate the effect of temperature, C-rate, SOC level, test procedure and DOD. A physics-based model including SEI formation, film resistance increase and loss of electrode volume fraction as a function of number of cycles is constructed. Aging parameters were determined through parameterizing the model using calibration experiments. Lifetime cycling data show that the expected temperature dependence can not be seen for 10% DOD for SOC levels below 50% SOC, it is first for SOC levels higher than 60% that this is observed. Also SOC levels higher than 30% is accelerating the aging, while SOC levels below this show very low aging effects, however, when using cycling in the whole SOC interval, the classical decreased lifetime with higher temperatures were obtained. For cells tested in different DODs, but with the same cut-off voltage, longer lifetime was observed for the small DOD, although the initial aging was similar up to 1200 FCE. After that, the aging decelerated for the small DODs while it accelerated for the large DODs

Extending battery lifetime by avoiding high SOC

\ua9 2018 by the authors. The impact of ageing when using various State of Charge (SOC) levels for an electrified vehicle is investigated in this article. An extensive test series is conducted on Li-ion cells, based on graphite and NMC/LMO electrode materials. Lifetime cycling tests are conducted during a period of three years in various 10% SOC intervals, during which the degradation as function of number of cycles is established. An empirical battery model is designed from the degradation trajectories of the test result. An electric vehicle model is used to derive the load profiles for the ageing model. The result showed that, when only considering ageing from different types of driving in small Depth of Discharges (DODs), using a reduced charge level of 50% SOC increased the lifetime expectancy of the vehicle battery by 44-130%. When accounting for the calendar ageing as well, this proved to be a large part of the total ageing. By keeping the battery at 15% SOC during parking and limiting the time at high SOC, the contribution from the calendar ageing could be substantially reduced

Accelerated lifetime testing in small SOC intervals on commercial pouch cells; challenges and countermeasures

A methodology for lifetime testing of lithium-ion batteries in small SOC intervals is presented. The specific test procedure and level of control is important in producing reliable data. The methodology offers high precision without increasing the test duration. The duration time can be reduced with up to 50%, by ensuring more continuous cycling and by reducing the number of performance tests

Inverter and Battery Drive Cycle Efficiency Comparisons of CHB and MMSP Traction Inverters for Electric Vehicles

This papers investigates the performance of several inverter types for electric vehicles. A standard two-level and two seven-level multilevel inverters, a cascaded H-bridge (CHB) and a modular multilevel series parallel (MMSP) inverter, are considered. Based on the AC impedance spectra measured on a single battery cell, the battery pack impedances of the multilevel and two-level inverter systems are modeled. The inverter losses are modeled using the semiconductors\u27 datasheets. Based on the loss models, the inverter and battery efficiency during different driving cycles are assessed. In comparison to the two-level inverter system, the multilevel inverter drivetrains show an increased drivetrain efficiency, despite increased battery losses. The MMSP topology showed the best result. In comparison to the CHB topology, the battery losses were reduced by the MMSP inverter system