Showing papers by "Andrew McGordon published in 2017"

The effect of external compressive loads on the cycle lifetime of lithium-ion pouch cells

Abstract: In application, lithium-ion pouch-format cells undergo expansion during cycling. To prevent contact loss between battery pack components and delamination and deformation during battery operation, compressive pressure is applied to cells in automotive battery modules/packs by way of rigid cell housing within the modules. In this paper, the impact of such compressive pressure on battery degradation is studied. Samples of commercial, 15 Ah LiNiMnCoO2/Graphite electrode pouch-type cells were cycled 1200 times under atmospheric, 5 psi and 15 psi compressive loads. After 1200 cycles, the capacity fade for 0, 5 and 15 psi loads was11.0%, 8.8% and 8.4%, respectively; the corresponding power fade was found to be 7.5%, 39% and 18%, respectively, indicating power fade peaks between 0 and 15psi. This contrasting behaviour is related to the wettability increase and separator creep within the cell after compressive load is applied. The opposing capacity fade and power fade results require consideration from automotive battery engineers at the design stage of modules and packs. In addition to capacity fade and power fade results, the study identified the evolution of compressive pressures over multiple cycles, showing that pressure increases with cycling.

Structural Identifiability of Equivalent Circuit Models for Li-Ion Batteries

Abstract: Structural identifiability is a critical aspect of modelling that has been overlooked in the vast majority of Li-ion battery modelling studies. It considers whether it is possible to obtain a unique solution for the unknown model parameters from experimental data. This is a fundamental prerequisite of the modelling process, especially when the parameters represent physical battery attributes and the proposed model is utilised to estimate them. Numerical estimates for unidentifiable parameters are effectively meaningless since unidentifiable parameters have an infinite number of possible numerical solutions. It is demonstrated that the physical phenomena assignment to a two-RC (resistor–capacitor) network equivalent circuit model (ECM) is not possible without additional information. Established methods to ascertain structural identifiability are applied to 12 ECMs covering the majority of model templates used previously. Seven ECMs are shown not to be uniquely identifiable, reducing the confidence in the accuracy of the parameter values obtained and highlighting the relevance of structural identifiability even for relatively simple models. Suggestions are proposed to make the models identifiable and, therefore, more valuable in battery management system applications. The detailed analyses illustrate the importance of structural identifiability prior to performing parameter estimation experiments, and the algebraic complications encountered even for simple models.

Transportation Safety of Lithium Iron Phosphate Batteries - A Feasibility Study of Storing at Very Low States of Charge.

Abstract: In freight classification, lithium-ion batteries are classed as dangerous goods and are therefore subject to stringent regulations and guidelines for certification for safe transport. One such guideline is the requirement for batteries to be at a state of charge of 30%. Under such conditions, a significant amount of the battery’s energy is stored; in the event of mismanagement, or indeed an airside incident, this energy can lead to ignition and a fire. In this work, we investigate the effect on the battery of removing 99.1% of the total stored energy. The performance of 8Ah C6/LiFePO4 pouch cells were measured following periods of calendar ageing at low voltages, at and well below the manufacturer’s recommended value. Battery degradation was monitored using impedance spectroscopy and capacity tests; the results show that the cells stored at 2.3 V exhibited no change in cell capacity after 90 days; resistance rise was negligible. Energy-dispersive X-ray spectroscopy results indicate that there was no significant copper dissolution. To test the safety of the batteries at low voltages, external short-circuit tests were performed on the cells. While the cells discharged to 2.3 V only exhibited a surface temperature rise of 6 °C, cells at higher voltages exhibited sparks, fumes and fire.

Impact of Vibration on the Surface Film of Lithium-Ion Cells

Abstract: Cylindrical 18650-type lithium-ion cells are being utilized more often for automotive applications. This introduces error in calculating expected lifetime due to varied usage conditions accelerating or reducing material damage. One such usage condition is vibration, which has been shown to impact the electrical performance over extended periods. Within this study X-ray photoelectron spectroscopy (XPS) has been performed on nickel manganese cobalt (NMC) cells subjected to vibration. This study found that vibration causes the removal of the selectively-formed surface film created during a cell’s first cycles and replaces it with the surface film from electrolyte decomposition. The surface films formed by vibration are composed of much higher concentrations of organic electrolyte decomposition products than the film from the control cell. The impact of this chemical mechanism is an increased level of cell degradation. This is exhibited in increased capacity fade and cell impedance. This is the first study presented within the academic literature which has identified an electro-mechanical mechanism responsible for the performance degradation in lithium-ion cells from vibration.

Adaptive behaviour selection for autonomous vehicle through naturalistic speed planning

Abstract: As autonomous technologies in ground vehicle application begin to mature, there is a greater acceptance that they can eventually exhaust human involvement in the driving activity. There is however still a long way to go before such maturity is seen in autonomous ground vehicles. One of the critical limitations of the existing technology is the inability to navigate complex dynamic traffic scenarios such as non-signalised roundabouts safely, efficiently and while maintaining passenger drive comfort. The navigation at roundabouts has often been considered as either a problem of collision avoidance alone or the problem of efficient driving (reducing congestion). We argue that for any autonomous planning solution to be accepted for replacing the human driver, it has to consider all the three objectives of safety, efficiency and comfort. With human drivers driving these complex and dynamic scenarios for a long time, learning from the human driving has become a promising area of research. In this work, we learn human driver's longitudinal behaviours for driving at a non-signalised roundabout. This knowledge is then used to generate longitudinal behaviour candidate profiles that give the autonomous vehicle different behaviour choices in a dynamic environment. A decision-making algorithm is then employed to tactically select the optimal behaviour candidate based on the existing scenario dynamics. There are two important contributions in this paper, firstly the adaptive longitudinal behaviour candidate generation algorithm and secondly the tactical, risk aware, multi-objective decision-making algorithm. We describe their implementation and compare the autonomous vehicle performance against human driving.

Internal temperature prediction of Lithium-ion cell using differential voltage technique

Abstract: The performance of a Lithium-ion cell is strongly dependent on cell operating temperature. However, the measured temperature is often obtained from thermocouples attached to the surface of the cell. These measurements may not be representative of the internal temperature of the cell especially for lower ambient temperatures and high C-rates. A novel method utilizing differential voltage to predict the internal temperature of a 40 Ah Lithium-ion pouch cell is proposed. The difference between internal and measured external temperatures depends upon the C-rate and ambient temperature. For a continuous-rate discharge, the difference, between surface and measured temperatures, rises at beginning of discharge before peaking in the middle region and reducing towards the end-of-discharge. The outcome of this study could positively support control strategies within a battery management system (BMS).

Improving the Performance Attributes of Plug-in Hybrid Electric Vehicles in Hot Climates through Key-Off Battery Cooling

Abstract: Ambient conditions can have a significant impact on the average and maximum temperature of the battery of electric and plug-in hybrid electric vehicles. Given the sensitivity of the ageing mechanisms of typical battery cells to temperature, a significant variability in battery lifetime has been reported with geographical location. In addition, high battery temperature and the associated cooling requirements can cause poor passenger thermal comfort, while extreme battery temperatures can negatively impact the power output of the battery, limiting the available electric traction torque. Avoiding such issues requires enabling battery cooling even when the vehicle is parked and not plugged in (key-off), but the associated extra energy requirements make applying key-off cooling a non-trivial decision. In this paper, a representative plug-in parallel hybrid electric vehicle model is used to simulate a typical 24-h duty cycle to quantify the impact of hot ambient conditions on three performance attributes of the vehicle: the battery lifetime, passenger thermal comfort and fuel economy. Key-off cooling is defined as an optimal control problem in view of the duty cycle of the vehicle. The problem is then solved using the dynamic programming method. Controlling key-off cooling through this method leads to significant improvements in the battery lifetime, while benefiting the fuel economy and thermal comfort attributes. To further improve the battery lifetime, partial charging of the battery is considered. An algorithm is developed that determines the optimum combination of key-off cooling and the level of battery charge. Simulation results confirm the benefits of the proposed method.

Developing and testing of control software framework for autonomous ground vehicle

Abstract: Automation in ground vehicles has been gaining momentum in recent years highlighted by the significant number of public demonstrations in the last two decades This momentum has created an urgency within research organizations, vehicle manufacturers and academia to solve existing problems with autonomous vehicle technology to make it usable in the real world As autonomous ground vehicles operate in close proximity to one another, the margin of error for navigation is smaller than in other domains such as aerospace and marine application The real-world driving scenarios for the autonomous ground vehicle can sometimes be predictable and unpredictable at other times, demanding different behaviours from the autonomous vehicle for successful navigation To satisfy such as requirement, the autonomous vehicle should exhibit the capability to adapt to through deliberative planning in predictable environments and reactive planning in unpredictable environments In this paper, we describe a hybrid control software framework designed to incorporate behaviour planning algorithms that are capable of both deliberative and reactive planning The paper describes the development of this novel adaptive autonomous control software framework and validates it through both virtual testing and real world testing environments

Use of a Thermal Battery with a Heat Pump for Low Temperature Electric Vehicle Operation

Abstract: Below 10oC, electric vehicles suffer from reduced range, which can be as severe as a 70% reduction (at -26oC). This is due to reduced battery performance at low temperatures and increased cabin heating demand. Heat pumps have been shown to have good steady state performance, but suffer slow and inefficient transients, while thermal storage has been shown to provide large heat flows and reduced warm up times, but space for such thermal storage is limited. Here a heat pump is combined with an optimally designed thermal battery and the simulated results are presented with improvements to energy consumption demonstrated.

Development of a very light rail vehicle

Abstract: The collaborative very light rail project involves the development of a novel railcar designed to revolutionise the rail industry: a self-powered, very light rail vehicle Each of the two bogies contains a complete diesel–electric series-hybrid drive system, while the whole vehicle has undergone significant lightweighting activity to realise a target weight of less than 18 t, or 1 t per linear metre The target cost is £500 000, which is to be achieved through the use of standardised, modular components, and appropriate materials and structural design methodologies The research covers several aspects of the GB Rail Technical Strategy chapter relating to rolling stock Lightweighting leads to a reduction in the propulsion requirements and reduces the infrastructure installation and maintenance costs The use of higher-efficiency drive systems achieved through on-board energy systems enables a reduction in carbon dioxide emissions These hybridisation activities improve the passenger experience through qui

Automotive to rail : can technologies cross the gap?

Abstract: There are significant drivers of change in the automotive industry today. Not only is legislation forcing manufacturers to meet ever more stringent emissions standards (particularly in terms of CO2), but customers are also demanding more efficient, safer and more electronically advanced vehicles (both in terms of performance features and interfaces). Manufacturers have responded with dramatic improvements in engine and powertrain efficiencies which have helped address legislative requirements to date. Furthermore they have rapidly moved away from standard steel bodies to multi-material solutions including various advanced grades of steel, aluminium, magnesium and polymer-based materials. Indeed there is currently significant research in the field of composite use for automotive bodies where there are pressing questions about manufacturing times for high volume production, costs and recyclability. The rail industry faces similar pressures as those seen in the automotive sector, driven by needs for lower costs, increased capacities, reduced carbon emissions and higher customer expectations. This paper will discuss the current state-of-the-art in automotive technologies and consider if and how they can be translated into the rail sector. It will consider current research within the WMG High Value Manufacturing Catapult towards implementation of automotive-style technologies in a light rail context.

Innovative Method for Simultaneous Electrical and Thermal Parametrisation of Automotive Batteries

Abstract: Coupled electro-thermal models are essential for the battery management system (BMS) to accurately predict the electrical and temperature dynamics of automotive batteries. Currently, an electrochemical model and a thermal model are developed and parametrised separately before being coupled together. A new procedure known as pulse- multisine (PM) has been developed that combines a multisine and a pulse signal, which allows for concurrent electrochemical and thermal parametrisation from the same experiements. A non- linear Equivalent Circuit Model and an analogous lumped parameter thermal model are parametrised and validated against automotive drive cycles. Despite the procedure requiring fewer experiments, the model has increased accuracy.

Evaluation of cyclic battery ageing for railway vehicle application

Abstract: Mobile transportation systems rely heavily on hydrocarbon based internal combustions engines (ICE) as the prime mover of vehicles. For rail applications electrification of the route provides an opportunity to improve efficiency and eliminate local emissions at point of use. However, route electrification is not always cost effective for secondary routes which see lower passenger volumes and less frequent trains; there is therefore an increasing interest in railway vehicles being equipped with energy storage based propulsion systems. Most of the railway vehicles that use an electrical traction energy storage system are at a prototype stage. Therefore, long-term real life data for the behaviour of traction batteries is not available up to now. The study presented in this paper describes ageing characterisation of two battery chemistries (Nickel-Manganese-Cobalt (NMC) and Lithium-Iron-Phosphate (LFP)) for representative rail duty cycles. Test bench trials are performed to represent roughly 1500 h of battery operation. A Battery Only and a Hybrid Energy Storage System case are considered.