Date of Award
Doctor of Philosophy (PhD)
Electrical and Computer Engineering
Efficient battery management systems (BMSs) in rechargeable battery-based systems require precise measurements of various battery parameters including state of charge (SOC), state of health (SOH) and charge capacity. Presently, SOC, charge capacity and SOH can only be indirectly inferred from long-term measurement of current, open circuit voltage (OCV), and temperature using multiple sensors. These techniques can only give an approximation of SOC and often require knowledge of the recent battery history to prevent excessive inaccuracy.To improve the performance of the BMS, an alternative method of monitoring the internal state of Li-ion batteries is presented here. Theoretical analysis of Li-ion batteries has indicated that the concentration of active lithium ions in the cathode is directly related to the magnetic susceptibility of the electrode materials. While charging/discharging, due to the change in the oxidation and/or spin state of metal atoms, the magnetic moment in the cathode varies. This indicates the potential for directly probing the internal state of the Li-ion batteries during charging/discharging by monitoring the changes in magnetic susceptibility via an appropriately designed magnetic sensor. In this research, a highly sensitive micromagnetic sensor design is investigated consisting of a single interdigital transducer (IDT) shunt-loaded with a magnetically sensitive Giant Magnetoimpedance (GMI) microwire. This design takes advantage of the coupling of the impedance characteristics of the GMI microwire to the IDT transduction process. The initial GMI-IDT sensor design is further developed and modified to maximize sensitivity and linearity. The sensor can detect magnetic field in the range of 900 nT and minute changes less than 1 μT when operated at or near its peak sensitivity. In addition, an appropriate procedure for preconditioning the GMI wire is developed to achieve sensor repeatability. Furthermore, using the identified optimum geometry of the experimental setup, the proposed sensor is implemented in monitoring the internal state of two types of Li-ion cells used in electric vehicles (EVs). The initial characterization results confirm that the GMI-IDT sensor can be used to directly monitor the charge capacity of the investigated Li-ion batteries. Other possible applications also include energy storage for renewable energy sources, and portable electronic devices.