## 1 Introduction
### Literature Review  
Plenty of works have been done in the field of battery thermal management system for commercially sold EVs and HEVs in the market, such as direct air cooling, liquid cooling, phase change material, heat pipes, hybrid cooling system and other emerging cooling technologies. The air-cooling system uses air as the heat exchange medium to let the air sweep across the battery surface to take away the heat generated by the battery. The air-cooling system is the most widely used cooling mode with simple structure, light weight, low cost and convenient maintenance. [PERSON] et al. [13] showed that the thermal conductivity of Li-ion battery was low, and air cooling alone could not meet the requirements of thermal management. [PERSON] et al. [14] and [PERSON] et al. [15] analyzed the thermal balance of li-ion batteries and found that when the wind speed reached a certain degree, increasing the wind speed had little effect on the heat dissipation effect of the batteries. [PERSON] et al. [16] have shown that the cooling effect of an air-cooling system is poor when the ambient temperature is high. In addition, the air-cooling system has a large volume and poor protection level, which limits its application in battery packs with a large scale. The liquid-cooling system uses the circulating flow of the coolant to take away the heat generated in the battery. The coolant has higher specific heat capacity and higher heat exchange efficiency, which makes it easier to meet the cooling and heating requirements of the battery comparing with air. The research of [PERSON] et al. [7] and [PERSON] et al. [16] confirmed this point of view. The liquid-cooling system has been widely studied and applied in recent years because of its good cooling effect and reasonable cost, which has allowed the technology to develop rapidly. [PERSON] et al. [17] showed that the number of channels, flow rate and liquid flow direction have an important impact on the cooling effect of the straight cooling channel, and gave suggestions on the number of channels and the requirement flow rate. [PERSON] et al. [18] studied the influence of different serpentine structures on the temperature uniformity and average temperature of the cooling plate. However, their research only focuses on a single cooling plate corresponding to several cells, not on the real, large-scale battery thermal management system, which comprehensively considered the flow distribution, pressure drop and temperature difference. Moreover, the effect of heat generation and heat dissipation under different current ratio is worthy of further study. The heat pipe is a technology of thermal management based on the principle of phase change heat transfer. Flexible geometry, low maintenance requirements and good thermal conductivity make it attractive as a BTMS option [19; 20; 21], however, the complex manufacturing process and high cost limit the large-scale use of heat pipe [22]. The PCM cooling system uses phase change materials to absorb the heat generated by the battery in the form of latent heat, which has the advantages of excellent uniform temperature effect and large heat storage capacity [23; 24], but with the disadvantages of low thermal conductivity and volume change affecting the application and promotion of PCM [25]. The independent passive phase change material system and heat pipe system are only conducive to heat transfer and storage. For large-scale or high-power battery systems, it is difficult to transfer the heat to the outside of the battery pack by itself in the natural state. It is necessary to combine independent cooling technology into a hybrid heat management system [26], for example, PCM and air-cooling combination [27], PCM and liquid cooling combination [28], PCM, HP and air-cooling combination [29], etc. The hybrid thermal management system combines the advantages of various cooling technologies, but the design cost and material cost are too expensive, and the implementation of the system is very complex.