States observability is a crucial aspect in the system control and optimization. In the battery modelling context, the equivalent circuit model (ECM) observability is often taken for granted without addressing the impact of input, uncertain parameters, and the choice of the open circuit potential (OCP) fitting function. In this work, a non-linear interval-based approach is adopted to obtain guaranteed observability conditions for the ECM. The analysis shows that different OCP functions, with comparable fit, lead to different result in terms of model observability and there exist state-input combinations potentially non-observable, when the input time derivative and uncertain parameters are considered.
