Series-Parallel Testing of Laboratory Lithium-Ion Cells: Not All Coin Cells Are Created Equal

Abstract This work evaluates parallel battery testing as a research tool for assessing both the reproducibility of laboratory-fabricated cells and the effects of cell-to-cell variation during pack-relevant operation. Eight hand-assembled CR2032 Li||LFP half-cells were tested using both parallel differential battery testing and conventional linear battery testing. Although all cells were prepared from the same nominal materials and procedures, SEM/EDS and electrochemical results showed measurable variability arising from microscale heterogeneity in the hand-cast cathodes and lithium metal anodes. These differences became more pronounced at higher C-rates, where weaker cells delivered lower capacity and contributed less effectively during coupled operation. Parallel testing revealed unequal current sharing, post-discharge equalization behavior, and broader performance differences than linear testing, while impedance measurements confirmed that intrinsic cell-to-cell differences persisted throughout cycling. Overall, the results show that parallel battery testing provides information beyond conventional single-cell measurements and is an effective method for evaluating reproducibility and pack-relevant cell behavior in laboratory-scale batteries.