Effects of temperature inhomogeneity in lithium-ion batteries connected in series: experiments and modelling

Abstract

In serial battery packs a mismatch in cell capacity, resistance and state of charge prevents the full utilization of the capacity of each of the cells. Cell utilization is also constrained by temperature effects as these affect the internal resistance of the cells and determine the onset of degradation. All these effects directly affect the capacity and the energy of serial battery packs and voltage imbalance is a symptom of discrepancies within the pack. The aim of this work is to understand the effect of temperature on voltage imbalance and propose mitigation solutions to the problem. To give the necessary background to the study, the electrochemical processes within a single lithium-ion battery and their temperature dependency are explained. The analysis is then expanded to describe the effects of temperature distribution on serial and parallel battery packs. To justify the topic of the present study, the causes of temperature distribution within battery packs are explained and existing mitigation methods, involving pack design and pack management, are discussed. The emerging voltage imbalance is experimentally studied in a pack of four cells in series. The effects of imbalance are experimentally quantified by comparing the performance of a battery pack under thermal equilibrium with that of a battery pack under temperature distribution. It is found that, when a battery pack is subject to a temperature distribution, cell operating voltage is a misleading indicator of State of Charge. The effects of balancing strategies on the thermally imbalanced pack are studied via a validated, thermally-coupled physics-based model created in Comsol. The instantaneous imbalance caused by temperature-dependent cell overpotentials is quantified and provides the basis for recommendations of effective balancing strategies for improved pack performance.