This work presents an approach to monitoring the State-Of-Charge of Lithium-Ion battery cells via piezo disc-based ultrasonic Time-Of-Flight measurement by measuring the traveling time of a mechanical pulse through the cell between two surface-mounted sensors. The main advantage of this approach is the simplicity and the resulting low cost, which makes it suitable for future application in battery management systems. In detail, the excitation of the piezo actuator is done using a single semiconductor switch instead of a power amplifier, and the received signal is processed with an amplifier and Schmitt-trigger combination to condition the signal for the microprocessor, which is part of a battery management system. Both the functionality and the limits of the design are evaluated with a high energy density Lithium-Ion pouch cell under different operational scenarios. Several parameters such as temperature, current rates, and excitation frequency are varied to prove the design concept. For validation purposes, an estimation function is generated and a real-world driving cycle applied. An estimation with an error of 1.29 % of the Time-Of-Flight total value or 16.85 % of the State-Of-Charge value under challenging conditions is achieved with the current setup.INDEX TERMS Battery management system, diagnostics, lithium-ion battery, piezoelectric transducers, state-of-charge, time-of-flight, ultrasonic.

During the charge and discharge process, lithium-ion batteries change their mechanical properties due to internal structural changes caused by intercalation and de-intercalation of the ions in the anode and cathode. Furthermore, the behavior changes over the lifetime of the battery due to several degradation mechanisms. The mechanical properties of the cell hold valuable information for monitoring these changes and additionally provide data for mechanical construction and further optimization of battery systems. Hence, in this manuscript, the mechanical frequency response function is investigated as a non-destructive method to determine parameters such as stiffness and damping of pouch cells and their correlation with the state of charge (SOC), the state of health (SOH), and the temperature of the cell. Using a mechanical shaker and an impedance head, it is shown that low amplitude forces of only a few Newton and a low frequency region of several hundred Hertz already suffice to show differences in the state of charge and state of health as well as in mechanical properties and the dependencies on temperature. Also the limitations of the method are shown, as the frequency response is not distinct for each parameter and thus, at the moment, does not allow absolute determination of a single value without prior system knowledge.

Temperature is a crucial factor for the safe operation of lithium-ion batteries. During operation, the internal temperature rises above the external temperature due to poor inner thermal conductivity. Various sensors have been proposed to detect the internal temperature, including fiber Bragg grating sensors. However, to the authors’ knowledge, there is no detailed description of the encapsulation of the fiber Bragg grating sensor in the literature to shield it from strain. In this study, different encapsulation methods for strain compensation were compared to find the encapsulation material most compatible with the electrolyte. For this, we stored the proposed sensors with different encapsulation methods in ethylene carbonate:ethyl methyl carbonate (EC:EMC) 3:7 with LiPF6 (lithium hexafluorophosphate) electrolyte and applied temperature changes. After evaluating the sensor encapsulation methods in terms of handling, diameter, uncertainty, usability, and hysteresis behavior, the most suitable sensor encapsulation was found to be a fused silica capillary with polyimide coating.

A typical method for measuring the radial thermal conductivity of cylindrical objects is the pipe method. This method introduces a heating wire in combination with standard thermocouples and optical Fiber Bragg grating temperature sensors into the core of a cell. This experimental method can lead to high uncertainties due to the slightly varying setup for each measurement and the non-homogenous structure of the cell. Due to the lack of equipment on the market, researchers have to resort to such experimental methods. To verify the measurement uncertainties and to show the possible range of results, an additional method is introduced. In this second method the cell is disassembled, and the thermal conductivity of each cell component is calculated based on measurements with the laser flash method and differential scanning calorimetry. Those results are used to numerically calculate thermal conductivity and to parameterize a finite element model. With this model, the uncertainties and problems inherent in the pipe method for cylindrical cells were shown. The surprising result was that uncertainties of up to 25% arise, just from incorrect assumption about the sensor position. Furthermore, the change in radial thermal conductivity at different states of charge (SOC) was measured with fully functional cells using the pipe method.