The quest for new and more sustainable battery systems is leading us to frontiers of less visited and less understood battery chemistries. While many of these systems offer high theoretical energy densities, actual electrochemical performance is often quite poor, as well as plagued by high overpotential and side reactions, resulting in a limited understanding of their practical significance. In this work, we start from evaluating the Ca storage potential of three commercially available aromatic anhydrides. The best-performing building blocks were selected for polymerization. Practical performance of obtained polymers was investigated through a combination of 3-electrode and symmetric cell electrochemical testing. These tests revealed the intrinsic performance of polyimides and significant differences among perylene and naphthalene building blocks. The electrochemical mechanism was investigated through a combination of ex situ IR and SEM-EDS and revealed good reversibility as well as the presence of calcium cation–anion pairs. Experimental results are supported by density functional theory (DFT) calculations. The study demonstrates the importance of comprehensive testing of organic materials in multivalent batteries, reveals their potential for high energy density Ca metal–anode organic batteries, and outlines further necessary steps for their practical development and improved understanding.