Thermal management of parallel cells — Methodology based on non-linear dynamics

Battery thermal management is crucial to ensure an efficient, safe, and durable use of battery. Yet despite various efforts devoted, there is still few methodologies available to guide either the design or the control of the battery thermal management system (BTMS) to maintain predefined limit on the temperature difference (△T) among cells subjected to various forms of non-uniformity. Our work aims to develop such methodology using the theory of non-linear dynamics. The theoretical framework is introduced using the preheating of two parallel-connected cells under simplified conditions as an example. Two types of portraits with zero and non-zero △T are revealed in phase plane. The one with a single stable focus in the vicinity of the diagonal is preferred as it represents a small △T. A methodology to determine the region of control parameters so as to maintain △T within a prescribed limit is developed using bifurcation analysis, and validated with the preheating experiments. The restriction of factors, including the frequency, acceptable △T, target temperature and preheating time, on such region are examined, thereby establishing an optimal region capable of simultaneously meeting the multiple design criteria for fast and uniform preheating. Utilizing the optimal region, engineer can identify the cells' steady-state temperature, estimate the power requirements of heating equipment, and determine the heat removal capability of BTMS, etc. in design stage, and optimize the control strategy in usage stage.