A Characterization of the Uncertainties Associated With an Automated System for the Study of Lithium-Ion Cells: A Case-Study of a Domestic Grid 24-h Scenario

In the last years, the use of domestic renewable energy solutions has increased, leading to a more widespread use of electrochemical energy storage systems due to their unpredictable nature. Lead acid cells are still the most employed kind of cells in this field. However, lithium-ion cells are considered the most promising approach, and their use is increasing considerably. In this context, we have developed an automated system for characterizing lithium-ion cells, able to reproduce any protocol the cells are subjected to during real applications with renewable energy solutions. Here, we focused on the uncertainty study, particularly determining the relative difference in the measurement of the most critical parameters involved in charging and discharging cycles and the combined uncertainty associated with the measurement of capacity, energy, and energy density. Unlike previous studies, we assess repeatability uncertainty by testing two cells with very different chemistry compositions: one more suitable for renewable energy solutions usage, the lithium iron phosphate (LFP) cell, and one more indicated for automotive applications, the polymer ion lithium (POLI) cell, performing three consecutive repetitions for each battery. The combined uncertainty was then calculated by combining the repeatability uncertainty with the instrumentation one. After that, we reproduced a 24-h real scenario of a home solar-based storage system integrated with the grid. We reported an accurate discrimination in the transition between the constant current (CC) and constant voltage (CV) charge phases and the correct interruption during the CV charge and the CC discharge cycles. In addition, the system accurately measures the capacity and energy with combined uncertainties generally lower than 2.4%, regardless of battery type. This study makes an important contribution to the battery energy storage field by providing a combined study between the uncertainties analysis and practical implementation of lithium-ion cells for real conditions related to renewable energy systems.