Ultracapacitor-based energy storage systems are becoming increasingly popular as a secondary power source in Renewable energy and Electric Vehicle applications. The design of a well-tuned ultracapacitor-based energy storage system requires accurate characterization of its simplified electrical equivalent circuit. The existing approaches for estimating parameters of the simplified equivalent circuit rely on the initial slope of the ultracapacitor response, which can lead to inaccurate estimation. This work focuses on developing a simple and accurate linear regression-based approach for estimation of equivalent circuit parameters. We highlight some critical observations related to the equivalent circuit parameters of a simplified equivalent circuit of an ultracapacitor. In particular, this work indicates the non-uniqueness of capacitance during the charging and discharging modes. The characterization and analysis reported in this study are based on a 24 V laboratory prototype comprising 10 series-connected ultracapacitors. © 2023 Elsevier Ltd