On-board monitoring of 2-D spatially-resolved temperatures in cylindrical lithium-ion batteries: Part II. State estimation via impedance-based temperature sensing

Impedance-based temperature detection (ITD) is a promising approach for rapid estimation of internal cell temperature based on the correlation between temperature and electrochemical impedance. Previously, ITD was used as part of an Extended Kalman Filter (EKF) state-estimator in conjunction with a thermal model to enable estimation of the 1-D temperature distribution of a cylindrical lithium-ion battery. Here, we extend this method to enable estimation of the 2-D temperature field of a battery with temperature gradients in both the radial and axial directions. An EKF using a parameterised 2-D spectral-Galerkin model with ITD measurement input (the imaginary part of the impedance at 215 Hz) is shown to accurately predict the core temperature and multiple surface temperatures of a 32113 LiFePO$_4$ cell, using current excitation profiles based on an Artemis HEV drive cycle. The method is validated experimentally on a cell fitted with a heat sink and asymmetrically cooled via forced air convection. A novel approach to impedance-temperature calibration is also presented, which uses data from a single drive cycle, rather than measurements at multiple uniform cell temperatures as in previous studies. This greatly reduces the time required for calibration, since it overcomes the need for repeated cell thermal equalization.Comment: 11 pages, 8 figures, submitted to the Journal of Power Source

Redes de sensores de fibra ótica para monitorização in situ de baterias de ião de lítio

In this work, fiber optic sensor networks were developed to be integrated in commercially available lithium-ion batteries (cylindrical and prismatic) and pre-fabricated batteries in a laboratory environment (pouch cells), with the objective of monitoring in situ, operando and in real time, the internal and external variations of temperature and strain, under different environmental conditions and different charge and discharge rates. To this end, and in order to show the improved performance of fiber optic sensors in relation to the electronic ones, typically used in this type of application, their response time and resolution were compared. An improvement of 28% of the response time and a better resolution are attained with fiber optic sensors. Monitorization studies of the temperature and strain variations using fiber Bragg gratings (FBGs) in the cylindrical configuration have been made, as well as temperature and bi-directional strain variations in the prismatic configuration, under normal or abusive operating conditions, using the FBG method strain free. When the batteries were subjected to abusive operating conditions, it was evident that greater temperature and strain variations occur, being promoted by the rapid transport of lithium ions between the positive and negative electrodes. Due to the thermal expansion of the materials that compose the battery, its internal structure is an important parameter to consider and that can influence its behavior in terms of expansion and contraction. In order to monitor the thermal performance of lithium-ion prismatic batteries in different environmental conditions, studies were performed in which the battery operated at different discharge rates over different conditions of temperature and relative humidity, in order to simulate the performance of the battery in three distinct climates: cold, temperate, and dry. From these studies, the poor performance of this type of batteries in the cold climate, and consequent lower thermal performance was verified. A network of 37 FBG sensors has also been used to monitor the interfaces of a pack of 3 lithium polymer batteries connected in series. It was possible to perform a spatial and temporal thermal mapping under different discharge rates, and to identify areas that are more susceptible to the appearance of hot spots and that are capable of endanger its normal functioning. Hotter zones were detected near the current collectors, due to the higher density of lithium ions in this region. For the first time, the simultaneous discrimination of internal temperature and strain variations in lithium-ion batteries in the pouch cell configuration was carried out, through the incorporation of hybrid sensors, which combine the operational characteristics of the Fabry-Perot and FBG sensors. The evolution of the strain and temperature signals was followed by the proposed sensors and the largest strain variations were detected at the beginning of the discharge process, in the bottom position of the pouch cell. With the work developed in this Thesis, it is concluded that the integration of optical fiber sensors into lithium-ion batteries contributes to a better internal and external knowledge of the thermal performance and volume variations under different operating conditions. This might improve the safety conditions and optimize the design of the next generation of lithium-ion batteries.No presente trabalho, desenvolveram-se redes de sensores em fibra ótica para integrar em baterias comerciais de ião de lítio (cilíndricas e prismáticas) e em baterias pré-fabricadas em ambiente de laboratório (pouch cells), com o objetivo de monitorizar in situ, em funcionamento e em tempo real variações internas e externas de temperatura e deformação, sob diferentes condições ambientais e diferentes taxas de carga e descarga. Para tal, e de maneira a mostrar o melhor desempenho dos sensores de fibra ótica em relação aos eletrónicos tipicamente usados neste tipo de aplicação, os seus tempos de resposta e resolução foram comparados, obtendo-se uma melhoria de 28% do tempo de resposta e uma resolução superior com os sensores em fibra ótica. Foram feitos estudos da monitorização de variações de temperatura e deformação através de redes de Bragg em fibra (FBG) na configuração cilíndrica e variações de temperatura e deformação bidirecional na configuração prismática, aquando do seu funcionamento em condições normais e abusivas, através do método FBG strain-free. Quando as baterias foram submetidas a condições operacionais abusivas, ficou evidente que ocorrem maiores variações de temperatura e de deformação, sendo promovidas pelo rápido transporte dos iões de lítio entre os elétrodos positivo e negativo. Devido à expansão térmica dos materiais que compõem a bateria, a sua estrutura interna é um importante parâmetro a ter em consideração e que pode influenciar o seu comportamento em termos de expansão e contração. A fim de monitorizar o desempenho térmico de baterias prismáticas de ião de lítio em diferentes condições ambientais, realizaram-se estudos nos quais a bateria operou sob diferentes taxas de descarga em diferentes condições de temperatura e humidade relativa, por forma a simular o desempenho da bateria em três climas distintos: frio, temperado e seco. Destes estudos, constatou-se o fraco desempenho deste tipo de baterias no clima frio, e consequente inferior desempenho térmico. Uma rede de 37 sensores FBG foi ainda usada para monitorizar as interfaces de um pack de 3 baterias poliméricas de lítio, conectadas em série. Foi possível realizar um mapa térmico espacio-temporal para diferentes taxas de descarga, e identificar as zonas mais suscetíveis ao aparecimento de pontos quentes e capazes de colocar em risco o seu normal funcionamento. As zonas mais quentes foram detetadas próximas dos coletores de corrente, devido à superior densidade dos iões de lítio nesta região. Pela primeira vez, foi realizada a discriminação simultânea de variações internas de temperatura e deformação em baterias de ião lítio na configuração pouch cell, através da incorporação de sensores híbridos, que combinam as características operacionais dos sensores Fabry-Perot e FBG. A evolução da deformação e temperatura foi seguida pelos sensores propostos e as maiores variações de deformação foram detetadas no início do processo de descarga, na posição inferior da pouch cell. Com o trabalho desenvolvido nesta Tese, conclui-se que a integração de sensores em fibra ótica em baterias de ião de lítio contribui para um melhor conhecimento, interno e externo, do desempenho térmico e de variações de volume sob diferentes condições de funcionamento. Assim, poder-se-á melhorar as condições de segurança e otimizar o design da próxima geração de baterias de ião de lítio.Programa Doutoral em Engenharia Físic

On battery recovery effect in wireless sensor nodes

With the perennial demand for longer runtime of battery-powered Wireless Sensor Nodes (WSNs), several techniques have been proposed to increase the battery runtime. One such class of techniques exploiting the battery recovery effect phenomenon claims that performing an intermittent discharge instead of a continuous discharge will increase the usable battery capacity. Several works in the areas of embedded systems and wireless sensor networks have assumed the existence of this recovery effect and proposed different power management techniques in the form of power supply architectures (multiple battery setup) and communication protocols (burst mode transmission) in order to exploit it. However, until now, a systematic experimental evaluation of the recovery effect has not been performed with real battery cells, using high accuracy battery testers to confirm the existence of this recovery phenomenon. In this paper, a systematic evaluation procedure is developed to verify the existence of this battery recovery effect. Using our evaluation procedure we investigated Alkaline, Nickel-Metal Hydride (NiMH) and Lithium-Ion (Li-Ion) battery chemistries, which are commonly used as power supplies for WSN applications. Our experimental results do not show any evidence of the aforementioned recovery effect in these battery chemistries. In particular, our results show a significant deviation from the stochastic battery models, which were used by many power management techniques. Therefore, the existing power management approaches that rely on this recovery effect do not hold in practice. Instead of a battery recovery effect, our experimental results show the existence of the rate capacity effect, which is the reduction of usable battery capacity with higher discharge power, to be the dominant electrochemical phenomenon that should be considered for maximizing the runtime of WSN applications. We outline power management techniques that minimize the rate capacity effect in order to obtain a higher energy output from the battery

In-situ instrumentation of cells and power line communication data acquisition towards smart cell development

The internal core temperature of cells is required to create accurate cell models and understand cell performance within a module. Pack cooling concepts often trade off temperature uniformity, vs cost/weight and complexity. Poor thermal management systems can lead to accelerated cell degradation, and unbalanced ageing. To provide core temperature an internal array of 7 thermistors was constructed; these in conjunction with cell current, via bus bar mounted sensors, and voltage sensor measurements, we have developed instrumented cells. These cells are also equipped with power line communication (PLC) circuitry, forming smart cells. We report upon data from these miniature sensors during cell cycling, demonstrating successful operation of the PLC system (zero errors compared to a reference wired connection) during typical cell cycling (C/2 discharge, C/3 charge) and the application of automotive drive cycle, providing a transient current test profile. Temperature variation within the cell of approximately 1.2 °C gradients, and variation of >2.8 °C during just 30 min of 2C discharging demonstrate the need for internal sensing and monitoring throughout the lifetime of a cell. Our cycling experimental data, along with thorough cell performance tracking, where typically <0.5% degradation was found following instrumentation process, demonstrate the success of our novel prototype smart cells

A compatibility study of protective coatings for temperature sensor integration into sodium-ion battery cells

Instrumented battery cells (i.e. those containing sensors) and smart cells (with integrated control and communication circuitry) are essential for the development of the next-generation battery technologies, such as Sodium-ion Batteries (SIBs). The mapping and monitoring of parameters, for example the quantification of temperature gradients, helps improve cell designs and optimise management systems. Integrated sensors must be protected against the harsh cell electrolytic environment. State-of-the-art coatings include the use of Parylene polymer (our reference case). We applied three new types of coatings (acrylic, polyurethane and epoxy based) to thermistor arrays mounted on flexible printed circuit board (PCBs). We systematically analyse the coatings: (i) PCB submersion within electrolyte vials (8 weeks); (ii) analysis of sample inserted into coin cell; (iii) analysis of sensor and cell performance data for 1Ah pouch SIBs. Sodium-based liquid electrolyte was selected, consisting of a 1 M solution of sodium hexafluorophosphate (NaPF6) dissolved in a mixture of ethylene carbonate and diethylene carbonate in a ratio of 3:7 (v/v%). Our novel experiments revealed that the epoxy based coated sensors offered reliable temperature measurements; superior performance observed compared to the Parylene sensors (erroneous results from one sample were reported, under 5 d submersed in electrolyte). Nuclear magnetic resonance (NMR) spectroscopy revealed in the case of most coatings tested, formation of additional species occurred during exposure to the different coatings applied to the PCBs. The epoxy-based coating demonstrated resilience to the electrolytic-environment, as well as minimal effect on cell performance (capacity degradation compared to unmodified-reference, within 2% for the coin cell, and within 3.4% for pouch cell). The unique methodology detailed in this work allows sensor coatings to be trialled in a realistic and repeatable cell environment. This study demonstrated for the first time that this epoxy-based coating enables scalable, affordable, and resilient sensors to be integrated towards next-generation Smart SIBs

Sensores de fibra ótica para meios desafiantes

With the present work, the development of fiber optic sensor solutions for the application in challenging media was intended. New sensor structures based on the post-processing of optical fibers were addressed, taking into account their sensitivity to variations in the external environment. In a first stage, fiber Bragg gratings were embedded in lithium batteries, to monitor temperature in situ and operando. Due to the harsh chemical environment of the battery, fiber optic sensors revealed to be the most advantageous alternative, when comparing to the electronic sensors. Fiber sensors exhibited good sensitivities and fast responses, besides being less invasive, thus they did not compromise the battery response. Furthermore, they were chemically stable. Still in the framework of this theme, and with the objective of monitoring possible strain and pressure variations inside the batteries, new sensors based on in-line Fabry-Perot cavities have been proposed. These sensors were characterized in lateral load, strain, and temperature. In a later stage, the study focused on the development of configurations that allowed to obtain high-resolution and/or sensitivity sensors. One of such configurations was obtained by creating a hollow microsphere at the fiber tip. The sensor was used to detected concentration variations and refractive index of glycerin and water mixtures. The influence of the diaphragm size in the sensor response was also studied, as well as the temperature response. New sensors based on multimode interference have also been characterized, using a coreless silica fiber tip. First, the influence of different parameters, such as length and diameters were analyzed. The sensors were tested in different solutions of glucose and water. It was observed that the sensor diameter is a decisive factor in obtaining devices that are more sensitive to refractive index and, consequently, to concentration. The determination of the thermo-optic coefficient of water/ethanol mixtures was also addressed using a multimode fiber interferometer sensor. Finally, a multimode interferometer sensor was functionalized by depositing agarose throughout the structure, allowing to optimize the response of the sensors to the external environment.Com o presente trabalho pretendeu-se explorar soluções de sensores em fibra ótica para a aplicação em meios desafiantes. Novas estruturas sensoras baseadas em pós-processamento de fibra ótica foram abordadas, tendo em consideração a sua sensibilidade a variações do meio externo. Numa primeira etapa, foram embebidas redes de Bragg no interior de baterias de lítio, para monitorizar variações de temperatura in situ e operando. Devido ao complexo meio químico da bateria, os sensores em fibra ótica revelaram ser uma alternativa mais vantajosa em relação aos sensores elétricos, não só pela sensibilidade e rápida resposta, mas também pelo fato de não afetarem o desempenho da bateria. Além disso, os sensores usados revelaram ser pouco invasivos e quimicamente estáveis. Ainda no âmbito deste tema, e com o objetivo de monitorizar possíveis deformações e variações de pressão no interior da bateria de lítio, foram desenvolvidos novos sensores baseados em cavidades de Fabry-Perot do tipo in-line. Esses sensores foram caraterizados em pressão lateral, deformação e temperatura. Numa fase posterior, o estudo centrou-se no desenvolvimento de configurações que permitissem a obtenção de sensores com elevada resolução e/ou sensibilidade. Uma das configurações consistiu na formação de uma microesfera oca na ponta de uma fibra ótica. Esse sensor foi utilizado para detetar variações de concentração e índice de refração de misturas de glicerina e água. A influência do tamanho do diafragma na resposta do sensor também foi estudada, assim como a resposta em temperatura. Em seguida, desenvolveram-se novos sensores baseados em interferência multimodo, utilizando para tal uma ponta de fibra de sílica sem núcleo. Numa primeira abordagem analisou-se a influência de diferentes parâmetros, como o comprimento e o diâmetro dos sensores. Os sensores foram expostos a diferentes soluções de glucose e água. Verificou-se que o diâmetro do sensor é um fator decisivo para a obtenção de dispositivos mais sensíveis ao índice de refração e, consequentemente, à concentração. Foi também desenvolvido um sensor baseado em interferência multimodo que permitiu determinar o coeficiente termo-ótico de misturas de etanol e água. Por fim, procedeu-se à funcionalização de um sensor baseado em interferência multimodo através da deposição de agarose ao longo da estrutura, permitindo assim otimizar a sua resposta a variações do meio externo.Programa Doutoral em Engenharia Físic

Predictive maintenance of induction motors using ultra-low power wireless sensors and compressed recurrent neural networks

In real-world applications - to minimize the impact of failures - machinery is often monitored by various sensors. Their role comes down to acquiring data and sending it to a more powerful entity, such as an embedded computer or cloud server. There have been attempts to reduce the computational effort related to data processing in order to use edge computing for predictive maintenance. The aim of this paper is to push the boundaries even further by proposing a novel architecture, in which processing is moved to the sensors themselves thanks to decrease of computational complexity given by the usage of compressed recurrent neural networks. A sensor processes data locally, and then wirelessly sends only a single packet with the probability that the machine is working incorrectly. We show that local processing of the data on ultra-low power wireless sensors gives comparable outcomes in terms of accuracy but much better results in terms of energy consumption that transferring of the raw data. The proposed ultra-low power hardware and firmware architecture makes it possible to use sensors powered by harvested energy while maintaining high confidentiality levels of the failure prediction previously offered by more powerful mains-powered computational platforms

Historical Building Monitoring Using an Energy-Efficient Scalable Wireless Sensor Network Architecture

We present a set of novel low power wireless sensor nodes designed for monitoring wooden masterpieces and historical buildings, in order to perform an early detection of pests. Although our previous star-based system configuration has been in operation for more than 13 years, it does not scale well for sensorization of large buildings or when deploying hundreds of nodes. In this paper we demonstrate the feasibility of a cluster-based dynamic-tree hierarchical Wireless Sensor Network (WSN) architecture where realistic assumptions of radio frequency data transmission are applied to cluster construction, and a mix of heterogeneous nodes are used to minimize economic cost of the whole system and maximize power saving of the leaf nodes. Simulation results show that the specialization of a fraction of the nodes by providing better antennas and some energy harvesting techniques can dramatically extend the life of the entire WSN and reduce the cost of the whole system. A demonstration of the proposed architecture with a new routing protocol and applied to termite pest detection has been implemented on a set of new nodes and should last for about 10 years, but it provides better scalability, reliability and deployment properties

In-situ instrumentation for smart energy storage

Lithium-ion technology is an increasing choice for battery powered systems, offering long-lasting, reliable and efficient energy storage. However, significant safety and performance challenges within the technology are still apparent. The current state of the art for monitoring cells performance is typically based on observing full cell voltage and occasional temperature sensor on the skin of a cell. Consequently, it is extremely difficult to track cells’ health within complex, especially high-performance, battery systems. Therefore, a new way of characterising cells’ is required. Here we show the design and manufacturing methods of transforming normal cells into smart systems. The sensor topologies embedded into the cells were electrical temperature, electro-chemical and optical temperature sensors. This enabled in-situ and operando thermal and electrochemical data collection during cells’ real-life operations. In this work, the impact of the sensors upon the cells performance has been shown to be negligible, with over 100 cycles conducted, versus unmodified cells for both pouch and cylindrical formats. This was validated using time and frequency domain analysis. A significant temperature difference was identified between the cell’s core and can temperatures of up to 6 °C during discharge and 3 °C during charge phase. Therefore, this work illustrates the necessity of internal cell temperature measurements for thermal management and safety validation. Lastly, with the aid of the in-situ measurement tools, certain cells can be further optimised without compromising thermal safety limits, while under particular scenarios safety limits can be breached earlier than the external sensors would indicate, showing how paramount in-situ data is to the operational safety