## 1 Introduction  
The performance of an electric vehicle (EV) depends strongly on its battery pack performance. Due to its better characteristics in terms of capacity, energy densities, charge retention, life cycles, and competitive cost when compared with other rechargeable batteries, lithium-ion (Li-ion) battery technology has been considered to be one of the most promising long-term advanced battery technologies and hence is widely used in modern EVs [1]. However, the battery pack is encountering more and more thermal problems with the demands of long-drive and huge-power EVs. Presently, researchers have come to a consensus that the best range of work temperature for a Li-ion battery is 10-40 \({}^{\circ}\)C, and once its temperature is above 50 \({}^{\circ}\)C, battery efficiency and life will deteriorate quickly. Especially when a battery module works at 80-119 \({}^{\circ}\)C but without in-time heat dissipation, solid electrolyte interfaces will dissolve, which can lead to batter over-charging and internal short circuiting, or even pose a safety hazard [2; 3]. Mathematical simulation of heat transfer within large batteries is an effective tool to obtain knowledge about whether excessive heat generated during the battery charge or discharge process can be removed, and how operating temperature can be controlled. [PERSON] et al. [4] built a two-dimensionalmodel to study the effect of various cell components, stack size, and cooling conditions on the performance of Li-polymer electrolyte batteries under different discharge C-rates (C-rate means charge and discharge current with respect to its nominal capacity). [PERSON] [5] particularly analyzed the performance of four main types of thermal management systems (TMS): free convection, forced convection, liquid cooling, and phase-change-material cooling. [PERSON] et al. [6,7] developed a numerical model to simulate a mini-channel cold plate and found that increased discharge rates and increased operating temperatures resulted in increased temperatures at cold plates, as experimentally measured. Meanwhile, many experiments were carried out on TMS to validate simulation results, such as serial and parallel air cooling [8], integrated liquid TMS [9], and expanded graphite based composite phase change materials [10]. These studies have produced many findings on the highest temperature control of battery packs. In theoretical and experimental studies on thermal problems, researchers gradually discovered that temperature uniformity can also greatly influence battery performance as well as maximum temperature. That is because temperature variations cause electrochemical reactions to proceed at different C-rates in different regions of the cell, which thereby lead to incomplete energy utilization, and inefficient management of battery life. As a result, an outstanding TMS for EV should not only maintain the battery pack to work below maximum temperature limits, but should also maintain the battery temperature as uniform as possible. The temperature uniformity can be affected by the C-rate [11], cooling strategy [12], battery module arrangement [13], and so on. With the wide application of liquid cooling systems in modern EVs, coolant mass maldistribution in flowing channels will play a key role on temperature uniformity. For liquid cooling, the coolant is used to absorb approximately 80-95% of the heat from a battery pack so that the performance of TMS strongly depends on coolant flowing characteristics. In order to maximally elevate its heat dissipation capacity, the cooling plate used in the system is usually manufactured with different internal flowing channels, where the coolant flows through and takes heat away from the battery pack. Regardless of flowing parameters and status, mass maldistribution in flowing channels will become an important force on temperature uniformity and system performance, as the maldistribution phenomenon can greatly induce an uneven liquid convection in the channels and consequently lead to a non-uniform heat transfer in the plate surface.