Lithium-ion batteries have seen increased usage as electrochemical energy storage systems for electronic devices and vehicles. It is important to accurately estimate the state-of-charge (SoC) of a battery management system to control battery operation to optimize performance, lifetime, and safety. This study experimentally leverages ultrasonic diagnostic technology to investigate the SoC of lithium-ion batteries during the charge/discharge processes. A cylindrical-type 2500mAh/20A battery was used for ultrasonic measurements with various charge/discharge rates of C/10.4, C/5.2, and C/1.3. The ultrasonic signals were analyzed to extract wave velocity and attenuation. For all of the testing rates, wave velocity increased/decreased in the charge/ discharge process. Velocity profiles corresponding to C/10.4 and C/5.2 exhibited primary peaks at the maximum SoCs, whereas the absolute wave velocity of C/1.3 showed primary peaks that occurred slightly after the SoC peak, indicating a delayed maximum Young's modulus. The wave attenuation of C/10.4 had local maxima in the charge/discharge processes and depicted negative correlations with SoC, within 0% to 18%, and positive correlations with SoC, within 18% to 85%. The wave attenuation curves of the C/1.3 showed no local peaks and had negative correlations with SoC, within 0% to 28%, and positive correlations with SoC, within 28% to 53%.