Thesis (MTech (Electrical Engineering))--Cape Peninsula University of Technology, 2011 / The purpose of this thesis was to design and develop a measurement system that would allow accurate measurement of individual cell voltages in a series cell stack. The system was initially proposed to be used in conjunction with an active cell balancer. This would allow for the efficient equalising of cells as well as provide detailed information on the cell stack and how the stack operates over time. Having a system that measures voltages accurately, with which the active cell balancer can be controlled would allow for peak cell lifetime and performance. Current battery management systems are large, complex and inefficient and a new way of battery management had to be investigated. To accurately measure individual cells in a series stack, the high common mode voltage must be negated. Different techniques that are currently used to create galvanic isolation were reviewed; circuits were designed and were simulated to find the most suitable design. The traditional methods used to create galvanic isolation did not provide adequate results. The methods were too inefficient and not accurate enough to be used. The methods that had the required accuracy were too complicated to connect in a useable system. This led to the investigation of integrated circuits created to measure voltages in large cell stacks. An integrated circuit from Linear Technology was chosen and a system was built. A system was thus designed that fulfilled the most desirable design specifications while delivering excellent results. The system allowed accurate, individual voltages to be measured in the presence of high common mode voltages. Accuracies and measurement time were well below the required system specification. Power consumption was high, but different component choice will lower power consumption to within specification. Excellent results were obtained overall with most, although not all results well below the design specifications. By including current measurements, as well as other technologies such as wireless communication, USB connectivity and a better data processor, this system will be at the forefront of current battery management technology.

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A Thesis for The Department of Electrical Engineering in fulfilment of the requirements for the Magister Technologiae degree in electrical engineering, at the Cape Peninsula University of Technology, 2011 / The purpose of this thesis was to design and develop a measurement system that would allow accurate measurement of individual cell voltages in a series cell stack. The system was initially proposed to be used in conjunction with an active cell balancer. This would allow for the efficient equalising of cells as well as provide detailed information on the cell stack and how the stack operates over time. Having a system that measures voltages accurately, with which the active cell balancer can be controlled would allow for peak cell lifetime and performance. Current battery management systems are large, complex and inefficient and a new way of battery management had to be investigated. To accurately measure individual cells in a series stack, the high common mode voltage must be negated. Different techniques that are currently used to create galvanic isolation were reviewed; circuits were designed and were simulated to find the most suitable design. The traditional methods used to create galvanic isolation did not provide adequate results. The methods were too inefficient and not accurate enough to be used. The methods that had the required accuracy were too complicated to connect in a useable system. This led to the investigation of integrated circuits created to measure voltages in large cell stacks. An integrated circuit from Linear Technology was chosen and a system was built. A system was thus designed that fulfilled the most desirable design specifications while delivering excellent results. The system allowed accurate, individual voltages to be measured in the presence of high common mode voltages. Accuracies and measurement time were well below the required system specification. Power consumption was high, but different component choice will lower power consumption to within specification. Excellent results were obtained overall with most, although not all results well below the design specifications. By including current measurements, as well as other technologies such as wireless communication, USB connectivity and a better data processor, this system will be at the forefront of current battery management technology.

With an increasing demand for sustainable transport solutions, there is a demand for electrified vehicles. One way to store energy on board an electrified vehicle is to use a lithium-ion battery (LIB). This battery technology has many advantages, such as being rechargeable and enabling reasonably high power output and capacity. To ensure reliable operation of LIB:s, the battery management system (BMS) must be designed with regards to the electrochemical dynamics of the battery. However, since the battery ages over time, the dynamics changes as well. It is possible to predict ageing, but some ageing mechanisms can occur randomly, e.g. due to variations of circumstances during manufacturing, and variations of battery user choices. Hence, by monitoring ageing mechanisms in situ, the BMS can adapt accordingly, similar to a closed loop control system. One ageing mechanism in LIB:s is lithium plating. This mechanism signifies when Li ions are electrochemically deposited as metal onto the negative electrode of the LIB during charging, and can induce other ageing mechanisms, such as gassing or electrolyte reduction. The present project has investigated a method for detecting Li plating in situ after its occurrence by both analysing the voltage change over time during open-circuit voltage (OCV) periods after charging and monitoring battery swelling forces. Results show a correlation between a high probability of Li plating and the appearance of a swelling force peak and an OCV plateau. However, results also show a possible correlation between the onset of Li plating and the onset of the swelling force peak, while also showing a greater detectability of the force signal compared to the electrochemical signal. Furthermore, the present results show that the magnitudes of both signals are probably related to the amount of plated Li. The amount of irreversibly lost Li from plating is shown to have a possible correlation with accumulation of swelling pressure. However, to further validate the feasibility of these two signals, more advanced analysis is required, which was not available during this project. / Med en ökande efterfråga på hållbara transportlösningar så finns det ett behov av elektrifierade fordon. Ett sätt att lagra energi ombord ett elektrifierat fordon är att använda et litium-jon-batteri. Denna batteriteknologi har många fördelar: t.ex. är dessa batterier återladdningsbara, och de kan leverera höga uteffekter samtidigt som de kan ha ett stort energiinnehåll. för att säkerställa en säker drift av litium-jon-batterier måste batteriets styrsystem vara designat med hänsyn till den elektrokemiska dynamiken inuti batteriet. Dock åldras batteriet med tiden, vilket innebär att denna dynamik ändras med tiden, vilket innebär att styrningen av batteriet måste anpassa sig till denna föråldring. Det är möjligt att förutspå åldring av batterier, men vissa åldringsmekanismer kan ske slumpartat, t.ex. via slumpmässiga förändringar i tillverkningsprocessen av batteriet, eller variationer i användningen av batteriet. Genom att därmed bevaka dessa åldringsmekanismer in situ så kan styrsystemets algoritm anpassa sig utmed batteriåldringen, trots dessa slumpartade effekter. En åldringmekanism hos litium-jon-batterier är s.k. litiumplätering. Denna mekanism innebär att litium-joner elektrokemiskt pläteras i form av metalliskt litium på ytan av litium-jon-batteriets negativa elektrod. Mekanismen kan också inducera andra åldringsmekanismer, t.ex. gasutveckling eller elektrolytreduktion. Detta projekt har undersökt en metod för att detektera litiumplätering in situ efter att plätering har skett, genom att både analysera öppencellspänningens (OCV) förändring med tiden direkt efter uppladdning samt analysera de svällande krafterna som uppstår under uppladdning av batteriet. Resultaten visar på en korrelation mellan en hög sannolikhet för litiumplätering och observationen av en topp i svällningskraft och en platå i OCV-kurvan. resultaten visar också en möjlig korrelation mellan påbörjandet av litium-plätering och påbörjandet av toppen i svällningskraft. Vidare visar även resultaten ett troligt samband mellan signalernas magnitud och mängden pläterat litium. Slutligen visar resultaten också ett möjligt samband mellan irreversibelt pläterat litium och ett svällningstryck som ackumuleras med varje uppladdningscykel. Dock krävs det en validering med mer avancerade analysmetoder för att säkerställa användningsbarheten av dessa två signaler, vilket ej var möjligt inom detta projekt.