Correlation between different impedancemeasurement methods for battery cells

Abstract: Stricter regulations concerning emissions from road traffic and increasing fuel prices has lead to an interest in hybrid electric vehicles (HEVs). Today even manufacturers of heavy duty vehicles are introducing hybrid alternatives. Batteries are expensive and a complex part in HEVs, and ways of determining a battery’s capacity is a current research topic. When a battery is used it ages, i.e. the capacity decreases and the impedance rises. Since battery cost is high, it is important to be able to determine battery ageing properly. The focus of this master thesis has been on impedance measurement methods for Li-ion batteries. The work has been carried out in cooperation with Scania CV AB. When a battery is aged, the impedance increases. Monitoring ageing mechanisms could enable increased lifetime of the batteries through optimized usage in for example heavy duty hybrid vehicles. In this work, Hybrid Pulse Power Characterization (HPPC) has been compared with Electrochemical Impedance Spectroscopy (EIS). A major difference between these methods is that HPPC uses pulses of high direct current, whereas a small alternating current perturbation is used in EIS. EIS give information about different mechanisms influencing the battery impedance, e.g. internal resistance and charge transfer resistance, but requires expensive and complex laboratory equipment. HPPC gives less detailed information about the impedance, but is more similar to field applications for a vehicle. A literature survey showed that much research is conducted on in-situ impedance measurements of batteries. One example is the long-term demonstration of an Impedance Measurement Box (IMB), which is currently carried out at Idaho National Laboratory. The method uses a sum-of-sines signal consisting of octave harmonics for a fast impedance measurement with good precision. The results showed a good correlation with laboratory EIS measurements. The experimental part of this project suggest that a linear correlation  exists between the discharge resistance from HPPC measurements and the sum of internal resistance and charge transfer resistance from EIS measurements. The linear fitting did not have very good R-squared value but a residual analysis showed that the residuals were randomly scattered around zero, indicating that a linear fitting is suitable. However, the precision of the results is too poor for the correlation to be useful in a real HEV application. Additional work to improve the linear fitting is recommended. Furthermore, it was showed that AC-components have to be used as a measurement signal in order to measure the complex impedance of a battery. A paired t-test was conducted in order to study if noise could be used as that signal for a battery under load. The impedance at 100 Hz was calculated, which corresponds to the second harmonic of the power grid. The difference between this impedance and the impedance measured at 100 Hz with EIS was statistically tested. For shorter times pans (in this case 20 milliseconds) after applying the DC pulse, using noise cannot be ruled out for measuring a battery’s impedance under load. But for longer time spans after applying the DC pulse (in this case 1.3 seconds), there was a significant difference between the two methods. Concentration gradients caused by mass transfer limitations could be causing this effect.