Ultracapacitor based energy storage systems are becoming increasingly popular for applications requiring short-term support. This paper investigates the impact of non-idealities on and control of ultracapacitor-based energy storage systems. The impact of two non-idealities, namely, the equivalent series resistance and the voltage-dependent capacitance, has been studied in detail. The typical variation of capacitance as a function of voltage is experimentally determined, and its impact on the controller design is studied. Further, the challenges arising due to equivalent series resistance in adopting a dual loop control, especially during the charging mode, are reported. The oscillatory response resulting due to equivalent series resistance is demonstrated experimentally using a 24 V laboratory prototype comprising of 10 series connected ultracapacitors. © 2022 IEEE.